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Department of Biological Sciences, Columbia University, New York, NY, USA

Explanations in evolutionary theory1

W. J. Bock

AbstractThe theory of biological evolution is defined in many ways, leading to considerable confusion in its application and testing against objective

empirical observations. Evolutionary change is usually defined as genetic which would exclude both cultural and template evolution; hence the

qualifying adjective genetic should not be included in the definition of biological evolution. Darwins theory, always described by him in the

singular, is actually a bundle of five independent theories about evolution as advocated by Mayr. Furthermore only one of these theories, that of

common descent, is historical, and the other four evolution as such, gradualism, processes of phyletic evolution and of speciation, and causes of

evolution are nomological. Hence not all evolutionary theory is historical. Biological comparisons can be divided into horizontal and vertical

ones and valid conclusions from one type of comparisons cannot be automatically extrapolated to the other. All phyletic evolutionary change, no

matter how extensive it may be, never crosses species taxa boundaries; hence it is not possible to distinguish trans-specific evolution( evolution

beyond or above the level of the species) from evolution within the species level. Macroevolution does not differ from microevolution except in the

scale of the overall change; no special causes or processes of macroevolution exist.

Key words: Definitions of evolution Darwins five evolutionary theories nomological and historical theories of evolution nomological-deductive and historical-narrative explanations functional explanations horizontal versus vertical comparisons in biology species limits

macroevolution

Introduction

Countless people have followed the recent debates and trial in

Harrisburg, Pennsylvania, USA on the teaching of biological

evolution versus intelligent design in public schools in the

United States as well as the earlier one in the state of Arkansas,

USA on teaching biological evolution versus scientific crea-

tionism. In both cases the courts declared against the teaching

of intelligent design and scientific creationism as being

religious, not scientific, and hence violating the separation of

church and state in the United States Constitution. Most

interesting in these two trials was the spectrum of meanings

given to the Theory of (Biological) Evolution which varied somuch that one could be surprised that any decisions could be

reached at all in these trials. Most likely these court decisions

were reached, not on the basis of understanding what is meant

by biological evolutionary theory, but on a conclusion that

intelligent design and scientific creationism are not scientific

theories (Brockman 2006).

A major foundation for this diversity in the meanings of the

theory of evolution goes right back to 1859 and the publication

of Darwins On the Origin of Species, namely that:

(1) Darwin always referred to his ideas as my theory in the

singular;

(2) the impression that all aspects of evolutionary theory are

historical;

(3) the failure to distinguish between horizontal and vertical

comparisons in biology.

Over the decades as more and more was learned about

evolution, many evolutionists stated that Biological Evolution

was no longer a theory, but was factual or a fact ( an

objective empirical observation) which confused the issue even

more as a sharp difference exists between scientific theories and

objective empirical observations. In most of the statements

that evolution is a scientific fact, the author actually meant

that historical evolutionary theory ( the general notion that

living organisms descended with modifications from a com-

mon ancestor) is so exceedingly well tested (well corrobor-

ated, Popper 1959; 1968: 3234) that it can for all intense

purposes be accepted as factual. However, it is still better to

state that historical evolutionary theory is an exceedingly well

corroborated theory and that massive counter tests supported

by strong empirical objective observations are needed to

disprove it. Facts, as used in science, are quite different from

theories and the two are best kept strictly separated.

The major themes to be addressed in this essay have beennicely summarized a quarter-century ago by Mayr (1982: 8) in

the introductory chapter of his The Growth of Biological

Thoughtwhere he wrote: As a consequence, some exceedingly

confused accounts of the history of biology have been

published by authors who did not understand that there are

two biologies, that of functional and that of evolutionary

causations. Similarly, anyone who writes about Darwins

theory of evolution in the singular, without segregating the

theories of gradual evolution, common descent, speciation,

and the mechanism of natural selection, will be quite unable to

discuss the subject competently. Today the problems are

much the same as when Mayr penned this passage in the late

1970s with the addition that most scholars do not comprehend

the distinction between nomological-deductive (N-DE) andhistorical-narrative explanations (H-NEs) and hence that

major differences exist between nomological and historical

theories of evolution. Mayr hinted at this last difficulty when

he wrote (Mayr 1982: 27): Every evolutionist who has had a

discussion with lay people has been asked: Has evolution been

proven? or How do you prove that man descended from

apes? But he did not emphasize that these represent quite

different scientific questions and require different modes of

explanation, the first being nomological-deductive and the

second historical-narrative. Later in this introductory chapter,

Mayr argued (pp. 7176) for the importance of historical

narratives and for the need of a philosophy of biology separate

1

It is with great pleasure that I dedicate this paper to my teacher,

mentor and friend, Ernst Mayr (19042005) whose writings on evo-

lutionary biology and the philosophy of biology formed the founda-

tion for my thinking on the ideas presented herein.

2007 The AuthorJournal compilation 2007 Blackwell Verlag, Berlin

Accepted on 10 December 2006J Zool Syst Evol Res doi: 10.1111/j.1439-0469.2007.00412.x

J Zool Syst Evol Res (2007) 45(2), 89103


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from that of the physical sciences. Yet he never pushed these

ideas to the logical conclusion that a distinction must be made

between nomological and historical theories of evolution, most

likely because of his strong aversion to law-like statements in

science and hence to nomological explanations. Possibly Mayr

equated functional explanations with nomological-deductive

and evolutionary explanations with historical-narrative which

is largely true although this relationship does not hold quite so

simply (Bock 2004a). Or possibly because Mayr did very little

work in functional biology, he was not sufficiently versed in the

nuances of nomological-deductive explanations (N-DEs) and

overlooked their role in evolutionary biology. Or possibly, he

restricted his view of biology to evolutionary explanations and

considered all other aspects of biological study ( strictly

functional explanations) as physical sciences.

Because Darwin always referred to his ideas as my theory

(Mayr 1982: 8; 1985: 757) always in the singular almost all

workers accepted the existence of only single theory of

biological evolution which was regarded as being strictly

historical. Most specialists and laity alike have been and still

are mainly interested in the historical evolutionary theory, that

is, in the history of life, or in modern terms The Tree of Life

.

This is nicely shown by the history of evolutionary studies in

which biologists accepted rapidly the theory of common

descent ( historical evolutionary theory), but ignored or

rejected Darwins theory of natural selection as well as

gradualism and speciation ( nomological historical theory;

see Mayr 1985). Many biologists, then as well as now, do not

care about nomological theories of evolution and the relation-

ship between them and historical evolutionary theory. And if

both nomological ( process) and historical ( pattern)

evolutionary theories are mentioned, emphasis is always

placed on the latter with little consideration on how the two

are connected. Even Popper (1977) focused so strongly on the

historical aspect of evolutionary theory that he concluded that

all evolutionary biology is historical and hence according to his

approach to science, evolutionary theory is not scientific (see

Hull 1999, for a detailed analysis). Caplan (1977, 1978, 1979),

on the other hand, provided a convincing argument that

[nomological] evolutionary theory is not circular and is

deductive. A close examination of his papers reveals that

Caplans position applies only to some of the set of Darwins

theories about organic evolution (Mayr 1985) and in particular

to the one dealing with evolutionary mechanisms (theory D,

see below). Caplans analysis definitely does not apply to the

last of these theories (theory E, or common descent) which is

the aspect of evolutionary theory foremost in most peoples

mind when they use the term Darwinism or evolutionary

theory. Futumya in his text books (1998:1112; 2005:1115)

implies that a distinction exists between nomological and

historical evolutionary theories, but does not develop these

ideas fully.

In this essay four major points have been addressed, namely:

(1) Examining the effect of horizontal versus vertical compar-

isons in biology on evolutionary concepts; (2) providing a

definition of evolution which will serve as the foundation on

which to examine the scope of evolutionary theories and to

show which of these theories are nomological and which are

historical; (3) arguing that Darwin proposed a set of at least

five different evolutionary theories, not just one, as advocated

by Mayr (1985) which can still be used today to characterize

the major, independent evolutionary theories; and (4) showing

which of these evolutionary theories are nomological and

which are historical, following Bock (2004a), and what is the

relationship between the two types of evolutionary theory.

Horizontal and vertical comparisons

Before proceeding into the main discussion on evolutionary

theory, the consequences of horizontal and vertical compar-

isons in biology (Bock 1989a) on evolutionary thinking have to

be considered. In the middle of the 19th century almost all

biologists believed that only single type of comparisons

existed that is, all comparisons between all organisms are

the same because at that time no general concept had yet been

advocated of organisms changing over historical time. This

belief was excusable in early Darwinian thinking but not

thereafter, although most biologists still accept today that a

single type of comparison exists and that conclusions can be

readily extrapolated from any comparison to another.

It must be stressed that the idea of horizontal versus vertical

comparisons is absolutely different from the concepts of

horizontal versus vertical evolution. The latter set of concepts

implies an absolute difference in the time needed for each type

of evolutionary change, but these terms refer to speciation (orsplitting of phyletic lineages horizontal) versus phyletic

evolution ( vertical); both types of evolutionary change

require time, albeit different amounts. Use of the terms

horizontal versus vertical evolution is misleading to the extent

of being wrong and is best not used.

Horizontal comparisons are those between individual organ-

isms of the same species taxon or between members of different

species taxa ( members of different phyletic lineages) whether

or not the organisms exist at the same time level. Most

biological comparisons are horizontal. In horizontal compar-

isons, not only can homologous features be compared but also

nonhomologous ones depending on the nature of the compar-

ison. Differences observed in horizontal comparisons can never

be evolutionary modifications because they do not represent

changes between different stages within a phyletic lineage (see

Bock 1979, 1986, 1995, 2004b, for the distinction between the

species concept and the phyletic lineage concept, and between

the species concept, the species category, and the species taxa.).

Because, interspecific horizontal differences are between dif-

ferent species between organisms of different phyletic

lineages great care must be used when extrapolating results

of horizontal comparisons to represent phyletic evolutionary

changes (Bock 1979).

Vertical comparisons are those between members of the

same phyletic lineage existing at different times, that is,

between ancestral and descendent groups. Because vertical

comparisons are between organisms in different time slices ofthe same phyletic lineage, these comparisons are never

between different species. The features being compared

vertically are generally homologous ones, although it might

be possible to have useful vertical comparisons between

nonhomologous features. Differences noted between homol-

ogous features in vertical comparisons represent evolutionary

modifications. Empirical vertical comparisons are difficult to

make because one generally does not know whether the

organisms being compared are members of the same phyletic

lineage even when dealing with fossils. Hence such compar-

isons are largely hypotheses and are indicative of whether the

organisms are believed to be members of the same phyletic

lineage at different points in time which are almost always

theoretical.

90 Bock

2007 The Author J Zool Syst Evol Res (2007) 45(2), 89103Journal compilation 2007 Blackwell Verlag, Berlin


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Horizontal species boundaries are between species taxa

existing at the same time and same place (synchronous and

sympatric) and represent the barriers to gene flow between

species; these boundaries are considered to be unbridgeable,

although in reality they may be somewhat leakybut not to the

extent of resulting in definite gene flow between the two

species.

Vertical species boundaries are postulated to be similar to

horizontal boundaries between species taxa along the same

phyletic lineage, but their exact nature has always been left

rather vague. Most workers advocating vertical species

boundaries have assumed that they are the same as the

unbridgeable horizontal boundaries (barriers to gene flow),

and hence special evolutionary processes are required for this

trans-specific evolutionary change which are different than

those existing for evolution within the species limits. Yet if

phyletic evolutionary change is gradual, then such vertical

species boundaries cannot exist, and that there is no difference

between evolution within the species limits and evolution

beyond the limits of the species( trans-specific evolution).

Hence, horizontal comparisons are between members of the

same species taxon or between members of different speciestaxa ( different phyletic lineages), while vertical comparisons

are between members of the same phyletic lineage at different

times. Valid conclusions reached on the basis of a horizontal

comparison cannot automatically be extrapolated to a vertical

comparison and vice versa. In the following discussion about

species, I follow the biological species concept (Mayr 1942,

1963) and not the evolutionary or the phylogenetic species

concepts (see Wa gele 2000 for a good modern discussion of

these concepts). The biological species concept is character-

ized, quite correctly by Mayr as non-dimensional, as distin-

guished from the multi-dimensional species taxon. The

evolutionary and phylogenetic species concepts do not distin-

guish between the species concept and the phyletic lineage

concept, causing confusion. Most philosophers of biology do

not make a distinction between the species concept and the

phyletic lineage concept, often leading to confusion in their

analyses. Furthermore, distinctions must be made between the

species concept, the species category and the species taxon

(Mayr 1963; Bock 1995, 2004b), to avoid the ambiguity arising

when using only the term species.

For Darwin and other biologists of his time, different species

taxa of organisms were considered to be separated from one

another by unbridgeable gaps a horizontal comparison. That

is, definite boundaries or limits existed around each species

taxon. A major problem facing Darwin and other early

evolutionists was how this boundary can be overcome in the

evolution of one species taxon to another. The concept of thehorizontal barrier between species taxa had been extended

automatically to the idea ofa vertical boundaryseparating an

ancestral species taxon from a descendent one a vertical

comparison. This is what Darwin had in mind when he

concluded that with sufficient (phyletic) evolutionary change, a

new species taxon would arise. Or to put this another way, a

foremost question for biologists at that time was how much

evolutionary change was necessary before a variety of a species

taxon (a particular breed, such as the breeds of the domesti-

cated dog, or a subspecies or geographic race), reached the

status of a new species taxon this question still exists for

many workers. Darwin failed to realize that two distinct

evolutionary processes existed phyletic evolution and speci-

ation and concentrated on phyletic evolution which led to his

famous disagreement with Moritz Wagner (see Mayr 1982:

562566). Wagner had stressed the role of geographic isolation

in evolution and especially in the origin ( multiplication) of

new species taxa. It is impossible to be precise on how most

evolutionists (including Darwin and Wagner) viewed the origin

of new species taxa until the period of the Evolutionary

Synthesis (19371948), because of vagueness in stated posi-

tions on both sides.

Yet it was clear well before 1859 biologists realized sharp

and definite boundaries existed between sympatric and syn-

chronous species taxa. In modern terms, these horizontal,

unbridgeable boundaries around species taxa are formed by

intrinsic genetic isolating mechanisms (Bock 1995, 2004b),

preventing the flow of genetic information from species to

species. With the acceptance of evolutionary ideas and the

realization that species taxa had a history, biologists automat-

ically extended this horizontal boundary around species taxa

to a vertical boundary between species in the same phyletic

lineage. This extension was to be expected at that time because

Darwin and most other biologists did not appreciate the

distinction between the species and varieties (including sub-

species or geographical races) within species. By 1860 conceptsabout geographic races were developing in the United States

and Imperial Russia (Haffer 1986, 1992, 1994), mainly in

ornithology, but these ideas were rejected by most systematists

and evolutionists in the United Kingdom until after the end of

the nineteenth century. Further Darwin and most other

evolutionists did not understand the role of external barriers

in the speciation process and believed that new species arose by

continuous phyletic evolution.

Although in 1859 biologists automatically extended this

horizontal boundary around species taxa to a vertical bound-

ary between ancestral and descendent species, no factual

evidence exists for this vertical boundary. Nor did any need

exist to continue the idea of vertical species taxa boundaries as

evolutionary theory became better understood. A major source

of this problem came from the exceedingly clever and catchy

title of Darwins book On the Origin of Specieswhich stuck in

the minds of biologists, and was used in much later books such

as Dobzhanskys 1937 Genetics and the Origin of Speciesand

Mayrs 1942 Systematics and the Origin of Species. These

latter authors borrowed the title of Darwins book to claim

proper kinship with it, but the phrase the origin of speciesin

these later titles should be read as evolution. Yet titles, e.g.,

Systematics and Evolution, would not have been as attractive

as Systematics and the Origin of Species. And it is most

difficult to think of a better title than the one chosen by

Darwin for his 1859 volume although species taxa do not

originate as implied in the title of his book and the text.The concept of a vertical boundary as well as a horizontal

boundary between species taxa remains until today in the

thinking of almost all evolutionists and lay-persons. Until the

middle of the twentieth century, this notion still implied, at

least for some evolutionists (e.g., Schindewolf 1950; Herre

1951), that a distinct jump existed a macroevolutionary

change from one species taxon to a descendent one, the

so-called evolution beyond the level of a species or trans-

specificevolution. And they argued that a special, and usually

unknown or mysterious, macroevolutionary mechanism exists

that cannot be reduced to the known mechanisms of micro-

evolutionary change evolution within the bounds of the

species taxon (but see Bock 1979 for a reductionistic

approach). Even after the period of the evolutionary synthesis,

Explanations in evolutionary theory 91



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become irrational to call it a theory. To be sure, there are

particular evolutionary theories such as those of common

descent, origin of life, gradualism, speciation and natural

selection, but scientific arguments about conflicting theories

concerning these topics do not in any way affect the basic

conclusion that evolution as such is a fact. It has taken place

ever since the origin of life. Earlier Mayr (1991: 112) wrote:

Weismanns attitude toward evolution as such was close to

that of the modern evolutionist, for whom evolution is not a

theory but an accepted fact. Hence for Mayr, evolution is a

fact (an objective empirical observation).

Gerd von Wahlert (2002), following Professor Vernadsky of

Leningrad, USSR who developed the concept of the bio-

sphere in the 1920s (see also Hutchinson 1965: 1), defined

evolution as development (change) in the characteristics of

the biosphere over time. Whether this definition differs from

that of Mayr (above) depends on whether the term living

worldpossesses the same meaning as biosphere; I suspect not

quite. The latter includes not only living organisms but all

environmental aspects (including all physical factors) of the

Earth necessary to maintain these organisms. At least some of

these physical factors, such as the generation of an atmosphererich on oxygen, appear to be the consequence of the action of

living organisms. The important aspect of von Wahlerts

definition of evolution is that it includes a clear mention of the

external environment of living organisms which is an integral

factor of evolutionary causes and which is lacking in most

other definitions. The biosphere, in von Wahlerts approach,

includes not only the living organisms, but the physical

characteristics of the Earth. Therefore his expression history

of the biosphere includes transformational evolution of the

earth which is change over time of the same object and

biological ( variational) evolution which is the observed

change in characteristics of living organisms from one genera-

tion to a descendant one. No reason exists why these two types

of change cannot be included in a single definition of

evolution, but one must be careful in any further development

of causes and processes involved in the historical modification

of the biosphere. Von Wahlerts definition does not include

any claim that evolution is factual or that it has to be genetic.

In his well-known textbook on evolution, Futuyma (1979

503; 1986: 551; 1998, Glossary; 2005: 547) provided the

complex statement: Evolution in a broad sense, the origin of

entities possessing different states of one or more character-

istics, and changes in their proportions over time. Organic

evolution, or biological evolution, is a change over time of the

proportions of individual organisms differing genetically in

one or more traits; such as changes transpire by the origin and

subsequent alteration of the frequencies of alleles or genotypesfrom generation to generation within populations, by the

alterations of the proportions of genetically differentiated

populations of a species, or by changes in the numbers of

species with different characteristics, thereby altering the

frequency of one or more traits within a higher taxon. His

genetic definition of evolution also excludes cultural evolution.

In the very beginning of the text Futuyma (1986: 13) stated:

As is indicated above, evolutionary biology consists of two

principal endeavors: inferring the history of evolution and

elucidating its mechanisms.which clearly appreciates both the

historical and the nomological theories of evolution. And he

wrote further (1979: 14; 1986: 16): Evolution, a fact, rather

than a hypothesis, is the central unifying concept of biology.

and hence agreed with Mayr on this point. In the next edition

of this textbook, Futuyma (1998: 11) retained this position and

wrote that: In the light of the preceding discussion, evolution

[? historical change] is a scientific fact. But it is explained by

evolutionary theory. He repeated this position (Futuyma

2005: 13), saying that: Given these definitions, evolution is a

fact. But the fact of evolution is explained by evolutionary

theory.(italics his). He added the explanation (Futuyma 2005:

13) that: What we call facts are hypotheses that have acquired

so much supporting evidence that we act as if they were true.

These are what Popper and many other philosophers of science

consider as very well corroborated theories and are still best

called theories to distinguish them from objective empirical

observations which can be considered as facts within the limits

of observational abilities and underlying theories.

In another well-known text, Strickberger (2000: 640) defined

evolution in his glossary as: Evolution Genetic changes in

populations of organisms through time that lead to differences

among them. Again cultural evolution is excluded. Further

Strickberger (2000: 636) defined: Darwinism the concept,

proposed by Charles Darwin, that biological evolution has led

to many different highly adapted species through natural

selection acting on hereditary variations in populations.

Scotts (2004: 2345) position on evolutionary ideas are

presented in her Chapter 2 Evolution.where she wrote under

the major heading EVOLUTION BROAD AND NAR-

ROW: In biology, evolution is the inference that living things

share common ancestors and have, in Darwins words,

descended with modification from these ancestors. The main

but not the only mechanism of biological evolution is

natural selection.(pp. 2324). And just below, she stated that

Biological evolution is defined as the descent of living things

from ancestors from which they differ. (p. 27).

Under the major heading BIOLOGICAL EVOLUTION,

she wrote: Descentconnotes heredity, and indeed members of

species pass genes from generation to generation.(p. 27), and

at the end of this section, that: Evolutionary biologists are

concerned both with the history of life the tracing of lifes

genealogy and the processes and mechanisms that produced

the tree of life. This distinction between the pattern of

evolution and the process of evolution is relevant to the

evaluation of some of the criticisms of evolution that will

emerge later in this book. First lets look briefly at the history

of life.

Although Scott made a distinction between discovering the

process of evolutionary change and working out the history of

life, and does not claim that evolution is a fact, the order in

which she presented her ideas is unclear.

Denton (1986: 3668) in his Chapter 2 on THE THEORY

OF EVOLUTION

did not provide a clear definition ofevolution but he was primarily concerned with historical

evolutionary theory ( the history of life) and with difficulties

of major evolutionary change in the same way as was Behle

(1996). Denton was concerned almost exclusively to historical

evolutionary theory. And although he did not use this term, he

advocated intelligent design as his primary disagreement in

dealing with macroevolution.

A committee was established in the 1990s by the American

Society of Naturalists and published its report under the title

Evolution. Science and Society (Meager and Futuyma 2001),

which is well worth reading as it covers thoroughly a broad

spectrum of aspects of evolutionary biology, its teaching and

its significance to many aspects of human society. I would like

to present several quotes from this report.




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Under, II. WHAT IS EVOLUTION? they wrote (p. 3):

Biological evolution consists of change in the hereditary

characteristics of groups of organisms over the course of

generations. Groups of organisms, termed populations and

species, are formed by the division of ancestral populations or

species, and the descendant groups then change independently.

Hence, from a long-term perspective, evolution is the descent,

with modification, of different lineages from common ances-

tors. Thus, the history of evolution has two major compo-

nents: the branching of lineages, and changes within lineages

(including extinction). Initially similar species become ever

more different, so that over the course of sufficient time, they

may come to differ profoundly.

And later in the same section (p. 4) they continued:

Evolutionary theory is a body of statements about the

processes of evolution that are believed to have caused the

history of evolutionary events. Biological (or organic) evolu-

tion occurs as the consequence of several fundamental

processes. These processes are both random and nonrandom.

(italics theirs).

And they clarified further (p. 5), saying: It is important to

distinguish between the history of evolution and the processheld to explain this history. Most biologists regard the history

of evolution the proposition that all species have descended,

with modification, from common ancestors as a fact that is,

a claim supported by such overwhelming evidence that it is

accepted as true. The body of principles that describe the

causal processes of evolution, such as mutation, genetic drift,

and natural selection, constitutes the theory of evolution.

Next under, III. WHAT ARE THE GOALS OF EVOLU-

TIONARY BIOLOGY? they wrote (p. 5): Evolutionary

biology is the discipline that describes the history of life and

investigates the processes that account for this history.

Evolutionary biology has two encompassing goals:

(1) To discover the history of life on earth: that is, (1) to

determine the ancestor-descendant relationships among all

species that have ever lived their phylogeny; (2) to determine

the times at which they originated and became extinct; and (3)

to determine the origin of and the rate and course of change in

their characteristics.

(2) To understand the causal processes of evolution: that is, to

understand (1) the origins of hereditary variations; (2) how

various processes act to affect the fate of those variations; (3)

the relative importance of the many co-acting processes of

change; (4) how rapidly changes occur; (5) how processes such

as mutation, natural selection, and genetic drift have given rise

to the diverse molecular, anatomical, behavioral, and other

characteristics of different organisms; and (6) how populations

become different species. Virtually all of biology bears on thisvast project of understanding the causes of evolution, and

reciprocally, understanding the processes of evolution informs

every area of biology. (italics theirs).

After pursuing the comments in the scientific literature, it is

always useful to consult dictionaries; as for example, referring

to the Concise Oxford English Dictionary (Pearsall 2002) and

the Websters Third International Dictionary of the English

Language. Unabridged (Gove 1963). Other dictionaries would

provide much the same definitions.

First in Pearsall (2002), the following definitions are offered:

evolution the process by which different kinds of

living organisms are believed to have developed from

earlier forms, especially by natural selection.(p. 494)

Darwinism the theory of the evolution of species,

advanced by the English natural historian Charles

Darwin (180982). (p. 364)

phylogeny another term for phylogenesis. (p. 1078)

phylogenesis the evolutionary development and

diversification of a species or group of organisms.

(p. 1078)

Next Gove (1963) presents the following definitions:

evolution the development of a race, species or

other group Phylogeny

the process by which through a series of changes or

steps any living organism or group of organisms has

acquired the morphological or physiological charac-

ters that distinguish it.

the theory that the various types of animals and

plants have their origin in other preexisting types, the

distinguishable differences being as a result of mod-

ifications in successive generations. (p.789)

phylogeny the racial history of a specified kind of

organism.

the evolution of a race or generically related group

of organisms (such as a species, family or order) as

distinguished from the development of the individual

organism. (p. 1706)

Many evolutionists and philosophers writing texts on

evolution or philosophical analyses on evolution never define

just what they mean by biological evolution. Dennett (1995:

21) wrote: Let me lay my cards on the table. If I were to give

an award for the single best idea anyone ever had, Id give it to

Darwin, ahead of Newton and Einstein and everyone else.As

far as I can determine Dennett does not provide a definition of

evolution. He did say (1995: 39) that: Darwins project in

Origin can be divided in two: to prove that modern species

were revised descendants of earlier species species had

evolved and to show how this process of descent with

modificationshad occurred.(italics his). Although numerous

aspects of evolutionary theory are discussed in the excellent

volume The Darwinian Heritage (Kohn 1985), none of the

many authors presented a definition of evolution.

With just this diversity of definitions, of lack of definitions

of evolution and whether evolution is a theory or a fact, it is a

bit surprising that any decisions could have been reached in the

two most recent court cases in the USA on the teaching of the

theory of evolution as advocated by most evolutionarybiologists versus alternative approaches, such as scientific

creationism and intelligent design.

Two points should be mentioned about this series of

definitions of evolution. Although several authors (Dobzhan-

sky, Futuyma, Scott, Meager and Futuyma, and Dennett)

distinguished between the evolutionary history of organisms

and the mechanisms (causes and processes) of evolution by

which these organisms changed over time, they almost always

placed discussion of the mechanisms after presentation of the

history of the organisms, rather than the more logical reverse

order. The major reason delaying Darwin until 1859 on the

publication of his ideas about transformation of organisms

was that he believed he could not present a valid case for the

94 Bock



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evolutionary history of organisms without first providing a

strongly supported set of mechanisms. He was completely

correct nomological theory before historical theory. Darwin

was well aware of the criticism and abuse piled upon the head

of the then unknown author ofVestiges of a natural history of

creation (Chambers 1844), largely because of the unrealistic

method Chambers suggested for modification of organisms

over time (see Secord 2000). Darwin resolved that he would

not fall into the same pit; he wanted to present a solidly

supported mechanism of natural selection when he advocated

his novel concept of evolution of living organisms.

Second, some of these workers used the expression the

theory of evolution to denote the set of mechanisms of

evolutionary change and considered the evolutionary history

of organisms so well supported that it is factual, not a theory.

(Here one could point out the position of most physicists

towards the end of the nineteenth century on the absolute

truthfulness of Newtons laws of motion.) These workers

overlooked the point that almost all anti-evolutionists are

uninterested in evolutionary mechanisms or may even accept

them for change within the limits of a species. The anti-

evolutionists

major dispute is with evolutionary history oforganisms which they argue is a theory only and not a very

well supported one at that. They certainly do not consider the

evolutionary history of organisms so well supported that it can

be regarded as factual in contrast to the discussion in Meagher

and Futuyma (2001: 43).

Having presented these other definitions of evolution, I will

reiterate the one advocated above:

Evolution: change in organisms over time with the minimum

time being one generation. Hence evolutionary change is that

observed between organisms of one generation and their

descendants. This is variational or Darwinian evolution

(Lewontin 1983: 63; Mayr 1988: 1516, 1991: 4344, 1997:

176). It differs sharply from transformational evolution, i.e.,

change of the same object, such a star or the earth, over time.

Hence change in the characteristics of an individual organism

during its life is not an evolutionary change, but an onto-

genetic modification ( transformational evolution; Lewontin

1983; Mayr 1988). Aside from one very vague point in

common that change over time occurs, there is nothing similar

in transformational and variational evolution the evolution

of the planet earth and the evolution of life on earth differ

completely. In a similar vein, there in nothing causally similar

between ontogeny (embryologic development during the life of

an individual organism transformational evolution) and

Haeckelian phylogeny ( variational evolution), although

the latter is the basis for the former (the foundation for Mayrs

concept of dual causation in biology, Mayr 2004: 30, andelsewhere).

Note that the definition of evolution advocated herein does

not include any statement of heredity or genetic change.

Several types of variational evolution exist depending on how

information is transmitted from one generation to the next.

These are:

(1) Genetic evolution in which transmission of information

from generation to generation is via genes or other elements in

the gametes and which is the form of biological evolution

proposed by Darwin and specified in almost all definitions of

biological evolution ever since.

(2) Cultural evolution in which transmission of information

from generation to generation is via learning by the individual

from adults (e.g., its parents) or from other individuals which

are usually conspecific, but do not have to be conspecific; this

transmission is non-genetic and depends only on the ability of

organisms to learn the phenotypic attribute (such as song or

the trail from the summer to the winter ranges) that can be

further transferred by learning, from generation to generation.

Cultural evolution is far commoner than most workers

consider although forgotten about in almost all definitions of

biological evolution. Modifications occur when there are

errors in the learning process.

(3) Template (or perhaps epigenetic) evolution in which trans-

mission from generation to generation is by non-learning the

copying of a phenotypic characteristic in the offspring from a

template in the phenotype of the previous generation. Mod-

ifications occur if there are accidental changes in the pheno-

typic feature of the parental generation which are hence copied

by the offspring when the new phenotype is developed as

shown by the formation of the teeth (notches) around the

mouth of the protozoa, Difflugia corona, in the sandy shell

formed around the body of this animal (Jennings 1937;

Nanney 1968). Template evolution is perhaps quite rare and

is overlooked in almost all discussions of evolution.

Perhaps one could also include in this classification of typesof biological evolution modifications of the phenotype that

occur because of changes in the external environment and/or

the internal interactions between parts of the body. The ability

for these modifications in the phenotype is genetic, although

the phenotypic change is not so and will reverse in the next

generation if the environmental/internal interaction reverses.

These modifications have been listed under a series of terms

from physiological adaptation to somatic modifications (Bock

and von Wahlert 1965). These changes can take place during

ontogenetic development and/or during the adult stage, and

may be reversible during the life of the organism; the

modifications are determined by alterations in the external

environment or in the internal interactions between parts of

the body (such as muscles affecting the shape of the bony

skeleton); no modifications occur in the genetic basis for the

observed phenotypic changes. Although this type of evolu-

tionary change is exceedingly common, I do not include it in

the above list as types of evolutionary change as it does not

depend on a different form of information transmission from

an organism in one generation to the next. Unfortunately until

recently there has been little consideration of this non-genetic

evolutionary change (Bock and von Wahlert 1965; West-

Everard 2003; Starck 2005) in spite of its extreme commonness

in evolutionary change.

Evolution (here I will restrict myself, only for simplicity, to

genetic evolution and to sexually reproducing organisms) can

be subdivided into two major processes, which are:(A) Phyletic evolution or anagenesis is evolutionary change

in a single phyletic lineage. Phyletic evolution occurs without

any speciation; hence no matter how much modification occurs

in phyletic evolution, no species boundaries are crossed. The

major causes of phyletic evolution are: (1) the generation of

individual phenotypic variation; and (2) Selective demands

arising from the external environment. I use selective demands

arising from the external environment because the term

natural selection is usually defined by evolutionists as an

outcome, not a cause (Bock 1993). Although Darwin used

natural selection as a cause in most of his 1859 book, his

clearest definition of this term (Darwin 1859: 61, see below) is

definitely an outcome definition which was adopted by R.A.

Fisher (1930) and J.B.S. Haldane (1932) see also Huxley




8/15

(1942) who stated in his Preface that his treatment of

evolutionary mechanisms are based strongly on the earlier

books of Fisher and Haldane). Most evolutionists (e.g.,

Futuyma 1986: 554; 1998: Glossary; 2005: 550) cite a definition

of natural selection which is basically nonrandom differential

reproduction of genotypes(an outcome) that is adopted from

the analyses of Fisher (1930) and Haldane (1932) and then use

this term as a cause, leading to considerable confusion (Bock

1993: 1115). Other workers (e.g., Lewontin 1970: 1) labeled

natural selection as a motive force(? cause) and still others,

such as Endler (1986: 4) stated that: Natural selection can be

defined as a process in which... (italics his). Interestingly the

three conditions given by these two workers for natural

selection are basically the same.

(B) Speciation or cladogenesis is the multiplication of species

from an ancestral species (Mayr 1942, 1963, 2001) and can be

thought as the splitting of a single phyletic lineage into two or

more. (Speciation occurs only in sexually reproducing organ-

isms and hence could be considered as a narrower process than

cladogenesis which is general to all organisms.) Complete

speciation always involves phyletic evolution in at least one of

the phyletic lineages, and generally in both. No special causesexist for speciation other than those operating for phyletic

evolution. But a definite initial condition is needed in the form

of an external barrier separating two populations of the

original species during which time intrinsic isolating mecha-

nisms for genetic isolation evolve by phyletic evolution.

Darwins five theories

Darwin always referred to his ideas published in his On the

Origin of Speciesasmy theory, always in the singular which has

been a primary source of confusion ever since. Mayr (1982: 8)

noted in the introductory chapter of his The Growth of

Biological Thought: Similarly, anyone whom writes about

Darwins theory of evolution in the singular, without segre-

gating the theories of gradual evolution, common descent,

speciation, and the mechanism of natural selection, will be

quite unable to discuss the subject competently. And in a most

important, but apparently little known paper, Ernst Mayr

(1985; see also 2004) elaborated on this point and demonstra-

ted that Darwins theory of evolution as originally advocated

in his On the Origin of Species (1859) was actually a bundle of

five separate but interrelated theories. Mayr showed that these

theories in various combinations, but not all, were differen-

tially advocated by diverse biologists before Darwin or by

most of his contemporaries. Only after the evolutionary

synthesis of 19371948 were all of these theories accepted by

most evolutionists. But even after the evolutionary synthesis,almost all biologists and philosophers still considered evolu-

tion as an undivided, historical theory.

The five separate theories found in Darwins 1859 book can

still characterize the major areas within evolutionary biology

today; they are:

(1) Evolution as suchis thetheory that statesthat allpopulations

of organisms are changing over time, with the minimum time

period being one generation (Mayr 1985: 757758).

(2) Gradualism is the idea that evolutionary change takes place

in steps of the magnitude seen between parents and offspring

and never in large sudden saltations or jumps. Evolutionary

jumps do not take place between species or taxa of higher

levels such as expressed in the idea that the first bird hatched

from a reptilian egg (Mayr 1985: 761764).

(3) Multiplication of species states that there is splitting of

phylogenetic lineages in addition to transformational change

within lineages. Hence evolutionary change includes two

processes phyletic evolution or transformation and speci-

ation. Although, Darwin appreciated the need for speciation in

his general ideas about evolution, he didnt provide any clear

discussion on how speciation took place or how it differed

from phyletic evolution (Mayr 1985: 764767).

(4) Natural selection is Darwins mechanism for phyletic

transformation. Today this would be expressed as the complete

causes, initial and boundary conditions, or mechanisms of

evolutionary change, regardless of the diverse causes that

different evolutionists would include (Mayr 1985: 767771).

Here one must be careful because Darwin used the term natural

selection interchangeably as a cause of evolution, as the process

of evolution and as the outcome. (see Darwin 1859: 61, where

he wrote that: Owing to this struggle for life, any variation,

however slight and from whatever cause proceeding, if it be in

any degree profitable to an individual of any species, in its

infinitely complex relations to other organic beings and to

external nature, will tend to the preservation of that individual,

and will generally be inherited by its offspring. The offspring,also, will thus have a better chance of surviving, for, of the

many individuals of any species which are periodically born,

but a small number can survive. I have called this principle. by

which each slight variation, if useful, is preserved by the term of

Natural Selection, in order to mark its relation to mans power

of selection.) This is clearly an outcome definition of natural

selection and is the one advocated by Fisher (1930) and by

Haldane (1932) in their analyses, and has been broadly

accepted by population geneticists and evolutionists.

(5) Common descent which implies that all species or popu-

lations of organisms have descended with modification from

common ancestors; this descent includes both modification

and branching (Mayr 1985: 758671). Darwinian common

descent ( historical evolution although Darwin did not use

this term in 1859) is equivalent to Haeckelian phylogeny

(Haeckel, 1866). Hennigian phylogeny is equal only to the

branching aspect of Haeckelian phylogeny (Mayr and Bock

2002). Common descent is expressed in Darwinian classifica-

tions, Haeckelian dendrograms and Hennigian cladograms,

which are all clearly theoretical statements. In recent years,

common descent has been labeled by some workers as The

Tree of Life.

Because most biologists and lay-persons are mostly interes-

ted in the last of these five theories which is clearly historical

and because most people still consider evolution to be a single

theory, the overwhelming conclusion by most biologists and

philosophers of science is that the theory of evolution is onlyhistorical as did Mayr (2004: 3233) and Ghiselin (2005: 133)

who stated that: Evolutionary biology is an historical science.

Unfortunately this is not completely true. This belief is the

source for much of the erroneous analysis in the philosophy of

evolutionary biology. Mayr (2004: 33) mentioned historical

narratives, but did not provide details for this approach.

Evolutionary explanations

What scientists do is to provide explanations of phenomena,

but not any type of explanations. The approach used by

scientists is to formulate theoretical statements that might

apply to the phenomenon, generate deductions from these

theories, and finally test these deductions against objective

96 Bock



9/15

empirical observations. What is absolutely essential in the

scientific methodology is not that empirical observations are

made, but that these observations are objective as opposed to

subjective hence the term objective science. Objective

empirical observations in the philosophy of science means

that the same observations can be made by any person having

the abilities to do so. Abilities means having the proper sense

organs and training. People with certain types of color vision

deficiencies cannot make the necessary observations depending

on color in many fields of science. Subjective observations are

ones that only certain persons can make but not everyone.

In an early paper commenting on whether only the choice of

words separated the ideas of G.G. Simpson and H.O.

Schindewolf, von Wahlert and I argued that evolutionary

theory had to be considered both as nomological ( causes,

mechanisms) and as historical that a major difference existed

between these two approaches (Bock and von Wahlert 1963). I

continued to muse on this distinction between approaches to

evolutionary biology and published my ideas in a series of

papers (Bock 1973, 1978, 1988, 1991, 1994, 1999, 2000a,b,

2004a), but they were either not fully formed or they were

published in rather specialized symposium volumes. Herein Iwould like to describe and contrast two major explanatory

systems in science and to show their relationship to one

another. These systems are N-D Es and H-N Es. Although the

latter are probably general for science, they appear to be of

most significance in fields such as biology, geology and

astronomy. Almost all philosophers of science have concen-

trated exclusively on N-D Es as I know of no general treatment

of H-N Es. An additional system of explanations exists in

biology the dichotomy of Functional Explanations versus

Evolutionary Explanations (see below). Hence it is essential

not only to characterize carefully the properties of N-D Es and

H-N Es, but to show which of these are functional and which

are evolutionary explanations.

Explanations in biology deal with phenotypic attributes of

organisms and their genetic basis, including interactions

between phenotypic attributes found in the same individual

organisms, correlations between phenotypic attributes and

diverse aspects of the external environment, and the relation-

ships among the features in different organisms, be they

conspecific or of different species. Hence, given any phenotypic

attribute, a complete ( full) explanation includes resolving:

(1) all existing physical-chemical properties (form, function,

biological role, etc.) which are functional explanations; (2) its

ontogenetic development resulting from interactions between

the existing genotype and the external environment (pro-

grammed systems, Mayr 1974, 1988, 1997) which are also

functional explanations; and (3) its evolutionary origin and,therewith, the evolutionary origin of the genotype (constituting

the programmed systems), which constitutes evolutionary

explanations (Bock and von Wahlert 1963). Although the

topic of full versus partial explanations in biology is an

important one, I will not consider it further herein except to

say that partial biological explanations can be most important

and are all that can be achieved and/or desired in most cases.

Mayr (1961, 2004) has argued strongly that the formation of

all biological phenotypic attributes depends on two different

set of causes working simultaneously which he has termed

proximal and ultimate causes and which form the basis of his

seminal conclusions about dual causation in biology and the

autonomy of biology from the physical sciences. His proximal

and ultimate causes are better called functional ( proximal)

and evolutionary (genetic; ultimate) causes with the latter

result from the evolutionary history of the organism.

Throughout, I will use functional explanations in the general

sense of functional analyses in biology (Mayr 1982), not in the

sense of functional explanations in philosophy (see Nagel

1961) as the latter do not appear to be of any value in scientific

explanations and are best omitted from scientific explanations.

In addition, almost all discussions of function in biology by

philosophers of science are presented in a historical evolution-

ary sense which places them in sharp contrast to explanations

by most functional biologists; this distinction should be noted

carefully when comparing the conclusions of papers on

function and functional explanations by most biologists versus

those by most philosophers. A further problem is that many

biologists in the mid-nineteenth century and some even today,

equated functional explanations with teleological explana-

tions. When many biologists realized that Darwinian evolution

eliminated teleology from biology, they also eliminated

discussions about functional properties of biological features

(as well as functional explanations) from their evolutionary

analyses a good example of throwing the baby out with the

bath water. This thinking resulted in serious problems forevolutionary biology as most, if not all, evolutionary expla-

nations depend on well established functional explanations

and on a full understanding of the interactions of organismic

features with the external environment (Hutchinson 1965;

Bock 2002, 2003; von Wahlert 2002).

Nomological-deductive explanations

Nomological-deductive explanations are the standardform of

explanation in science covering law explanation and deal

with general explanations of a class of phenomena, asking how

has each occurred? This is performed by formulating a

deduction from an appropriate set of laws (be they causes,

processes, or outcomes) and a set of initial and boundary

conditions (observations of the conditions in which the

phenomenon exists), both of which form the explanatory

sentence, or explanans, and from these reach a particular

conclusion or deduction, the explanandum. (Hempel and

Oppenheim 1948; Hempel 1965: 335338). The deduction is

then compared with objective empirical observations factual

observations of the phenomenon. If the observed phenomenon

agrees with the deduction (the degree of error is dependent on

what would be considered admissible), the explanation is

accepted. If an explanandum, resulting from the conjunction

of the set of facts invoked (initial and boundary conditions)

and the set of general laws, disagrees with objective empirical

observations, then that N-D E is not valid (has been falsified),and one must search for the reasons underlying this falsifica-

tion. Falsification means only that the explanandum does not

agree with independent, objective, empirical observations. If

the explanation is not accepted, it is necessary to investigate

the source of the error be it the set of law-like statements,

or the set of initial and boundary conditions used to formulate

the deduction, or the objective empirical observations used to

test the deduction. Falsification of an explanation does not

automatically imply that the general laws used in the explan-

ation are in error, although this is a possibility. Possibly, the

initial or boundary conditions used in the empirical test were

wrong, or the empirical observations were incorrect.

Because they are general, one nomological scientific theory

can be used to test another, and two or more nomological




10/15

theories can be included under the umbrella of an over-

arching theory. And as will be discussed below, nomological

scientific theories are required in the testing of any historical

scientific theories.

Nomological-deductive explanations answer the question:

how has a particular phenomenon [explanandum] occurred?

N-D Es apply to universals (non-limited number of phenom-

ena), do not depend on the past history of the objects or the

phenomena being explained, and their premises (the nomo-

logical statements) are assumed to be always true. In saying

that N-D Es apply to universals, these explanations are not

temporally-spatially restricted within the proper region of the

phenomena, which for biology is the earth and more specif-

ically the surface( the upper part of the crust) of the earth.

Examples of N-D Es include clarification of oceanic tides using

gravity and of phyletic evolution evoking natural selection

(nonrandom, differential survival and reproduction of organ-

isms). Explanation of how a certain feature of an organism is

an adaptation to selective demands arising from the external

environment is a N-D E, not a H-N E; the origin of the

adaptation, which is an entirely different question, is a

historical-narrative explanation (see below).For almost all philosophers of science (e.g., Popper 1959;

Nagel 1961) scientific methods apply only to N-D E. This is

interesting because Poppers approach has been accepted by

many systematists (e.g. cladists) as the foundation for their

analyses of the relationships among organisms; this cannot be

not correct because any explanations concerning the tree of

life, such as phylogenetic classification, deal with singulars (the

existing phylogeny of organisms) and not with universals (see

below, and Bock 2000a, 2004a).

Many philosophers of science and many biologists have

rejected the idea that law-like statements exist in science (e.g.,

Mayr 1997: 6063; but see Elgin 2006 for an argument for laws

in biology), sometimes substituting a vague idea of concepts in

place of law-like statements. The reasons for this rejection of

law-like statements by biologists and philosophers of biology is

not entirely clear, but may stem from the conclusion that

variation of phenomenon in biological organisms precludes the

existence of laws in biology rather than that this variation

arises from the nature of the initial and boundary conditions

which generally vary, often considerably, among biological

objects such as individual members of the same species and

will, of course, result in variation in the deductions. In this

connection, determinism is often raised as a critical factor

without the realization that the scientist must set the accept-

able limits of determinism in every case. Further, many of

these workers accept that only a single type of explanations

exists in science and hence confuse H-N Es with N-D Es. If thisis done, then the conclusion follows that no laws exist in

biological explanations, which include historical narratives,

because historical laws do not exist. At this point the argument

becomes circular.

Historical-narrative explanations

Contrary to the beliefs of many philosophers, not all science is

nomological. Some sciences, such as biology and geology, are

very largely historical. But little or no mention of these

explanations is found in the literature of the philosophy of

science (e.g., Feigl and Brodbeck 1953; Boyd, Gasper and

Trout 1991; Cornwell 2004). Indeed, considering numbers of

scientists and amounts of funding (which includes medicine

and agriculture as well as biology, geology, and some

astronomy), sciences with a historical aspect are now and

have been for a long time in the large majority. Why

philosophers failed to realize the importance of historical

science may be because the philosophy of science is little more

than one and a half centuries old and has been concentrated

very largely on physics as the ideal science. And physicists have

developed their science as a strictly non-historical inquiry,

which is certainly a completely valid thing to do and is

certainly the prerogative of physicists. But others do not have

to accept physics as the science on which to base all philosophy

of science. In view of the overwhelming acceptance that there

are no historical laws, the major difficulty for philosophers in

dealing with these historical aspects of science would be how to

extend the nomological-deductive approach to them (see

Hempel 2001, especially chapters 14 and 15). This was clearly

the reason why Popper (1977; see Hull 1999 for an excellent

analysis of this matter) said, or implied, that evolutionary

theory was not scientific. The difficulties of dealing with N-D

versus H-N Explanations and assigning different explanations

in evolutionary biology to one or the other of these two types

are well illustrate in the interesting analysis of adaptation byAmundson (1996).

Historical-Narrative Explanations provide an understand-

ing of the existing attributes of a particular set of objects or

phenomena at specified points in time; these explanations

absolutely depend on the past history of these objects and to be

scientific they must use pertinent N-D Es. The latter point is

essential!Any explanation of historical events that is not based

on pertinent N-D Es is not scientific. This includes not only

approaches such as scientific creationism and intelligent

design, but also claims by some scientists of the existence of

pure order and/or design in nature (including some approaches

to biological classification, see Brower 2000), self-determinism

and structuralism. Phenomena and objects explained by a

HN E are singulars, not universals, and have definite spatial-

temporal positions. HN Es are considered in a non-deductive

and probabilistic basis with the hope of reaching the most

reasonable and probable explanation for the objects studied.

Several aspects of H-N Es are stressed, the, first being the

most important:

(1) HN Es must be based on pertinent and well-tested N-D

Es, and these N-D Es, together with the pertinent empirical

observations testing them, form part of the chain of arguments

used in testing the H-N E. If no N-D Es exist or if the N-D Es

are poorly tested, then that H-N E lies outside of science.

(2) These explanations are historical in character, which means

that earlier events affect later events earlier events form the

initial conditions for explaining later events. Great care mustbe given to formulating the analysis within the presumed

correct chronological order of events and changes.

(3) H-N Es must be tested against objective empirical

observations, which may involve a chain of arguments,

including the underlying N-D Es and the objective empirical

observations used to test these N-D Es.

(4) Acceptance of a particular H-N E is always given on a

probability basis. This is necessary as these explanations

frequently employ some conflicting N-D Es and because of

generally considerable uncertainty over the initial and bound-

ary conditions involved in the explanation.

(5) H-N Es are not universal as are N-D Es, in that a successful

H-N explanation for one phenomenon (e.g. origin of

mammalian homoiothermy) need not hold for a similar

98 Bock



11/15

phenomenon (e.g. origin of avian homoiothermy) even if both

H-N Es include a number of the same N-D Es.

(6) Because of their complexity, the possible confusion between

competing explanations and the difficulty in identifying valid

confirming or falsifying tests, H-N Es must be stated clearly

and in the presumed proper chronological order. Failure to do

this may preclude meaningful tests or appraisal of rival H-N

Es.

(7) Generally the more precisely a H-N E is stated, the more

difficult it is to test and support it. The H-N E that humans

have evolved from a greater chimpanzee-like (Pan troglodytes)

ancestor is more difficult to support than that humans evolved

from an anthropoid ancestor which is more difficult to support

than that of humans evolved from a primate ancestor, etc.

Historical-narrative explanations in biology include the

evolution, phylogeny and classification of organisms or the

evolutionary history of their genetic characteristics or of their

phenotypic attributes that is, anything related to the history

of life, such as historical biogeography. All full explanations in

biology would include a historical-narrative portion which is

why full biological explanations are so difficult to formulate

and test.Because they deal with singular events, particular historical

scientific theories cannot be used to test any other scientific

theories, be they nomological or historical.

Both nomological-deductive and H-N Es are scientific under

the criterion of demarcation for scientific explanations advo-

cated by almost all philosophers of science in that they are

both available for testing against objective, empirical obser-

vations. N-D and H-N Explanations differ in the many ways of

how they are expressed, tested, and used to test other

theoretical statements, and must not be confused. The accu-

racy of most tests of H-N Es may be weak, and a distinction

must be made between valid tests and weak or unconvincing

tests (see Bock 1989b: 339342, discussing the concept of

homology). Many of the tests available for HN Es are valid,

but are relatively poor or non-robust and should not be

rejected as invalid tests. A robust, valid test is one that has a

high ability to distinguish between correct and an incorrect

hypotheses.

Being theoretical scientific statements, H-N Es are available

to tests against empirical observations, but such tests are often

difficult and inconclusive. Generally H-N Es are not tested by

falsification (in spite of numerous statements in the literature)

but usually by confirmation with the addition of more and

more corroborating support. This procedure is closely akin, if

not identical, to induction in the strict sense of that concept.

Objections cannot be raised to inductive testing of H-N Es

because they are theoretical statements about a singular,containing a finite number of objects, in contrast to N-D Es

which cover universals or an unlimited number of objects.

Testing of H-N Es depends on argument chains involving

pertinent N-D Es and often on a large number of background

assumptions (hypotheses, many being initial and boundary

conditions), and they must be finally tested against objective

empirical observations. One should proceed to the empirical

observations as directly as possible, although the argument

chain is often complex. The empirical observations and their

roles as tests, whether falsifying or confirming, should be

designated clearly.

What makes H-N Es scientific is point 1 (above) that H-N

Es must be based on pertinent and well-tested N-D Es,

and these N-D Es, together with the pertinent empirical

observations, form part of the chain of arguments used in

testing the H-N E.If no N-D Es exist or if these N-D Es have

not been well corroborated, then that H-N E lies outside of

science. The argument that scientific order exists in nature by

itself and in the absence of any N-DE is simply invalid as a

scientific statement.

Theories of evolutionAs some sciences contain very different types of explanations,

it seems reasonable to propose that in such sciences as biology,

geology and astronomy, two different types of theory exist

these being nomological theories and historical theories.

Therefore we should speak of nomological evolutionary theory

and historical evolutionary theory. Further, for historical

evolutionary theory, it is best to consider both general theory

and special theories. One can discuss general historical

evolutionary theory such as the Haeckelian phylogeny of

organisms and special theories such as the evolutionary history

of birds, of insects, of aquatic carnivores. etc. In geology, the

movement of continental plates over the earths surface would

be a general historical theory and the splitting, including thetime, of North and South America from Europe and Africa to

form the Atlantic Ocean would be a special theory.

Lets reconsider the five theories of Darwin as outlined by

Mayr (1985). The first four theories are clearly nomological

evolutionary theories, and are:

(1) Evolution as such is the theory that states that all

populations of organisms are changing over time, with the

minimum time period being one generation.

(2) Evolutionary change is gradualism in that it takes place in

steps of the magnitude seen between parents and offspring and

never in large sudden saltations or jumps.

(3) Evolutionary change includes two processes, namely

phyletic evolution or transformation and speciation. Multipli-

cation of species occurs by a splitting of phylogenetic lineages

as well as phyletic change within at lease one of the two

lineages.

(4) Evolutionary change takes place as the result of a small

number of causes, of which the most important are the origin

of (genetically based) phenotypically varying individuals in the

population and the action on these individuals of selective

demands arising from the external environment. Natural

selection was Darwins term for the overall mechanism of

phyletic transformation.

Recall that there are no special nomological causes restricted

to speciation. Although the process of speciation is nomolog-

ical, it depends on the nomological causes of phyletic evolution

plus the important initial condition of an external barrierseparating two populations for a sufficiently long period

during which Intrinsic Isolating Mechanisms for Genetic

Isolation evolve (Mayr 1963; Bock 1995, 2004b).

These four theories are all nomological albeit of different

types. Some could be further subdivided, especially 3 and 4,

but I wished to keep this list the same as the five theories of

Darwin from Mayr (1985).

These theories are law-like because they apply to all living

organisms on the Earth although they have been tested only

for a very small number of species. To my knowledge, none of

these nomological theories of evolution failed testing against

the appropriate objective, empirical observations. In a reason-

ably large number of cases, direct observations have been

made of all of these aspects of nomological evolutionary




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change, including phyletic evolution in domesticated animals

and plants, development of tolerance to pesticides by many

insects, and development of tolerance to antibiotics by many

bacteria. Phyletic evolution has been observed in the House

Sparrow, Passer domesticus, after its introduction into North

America. This species has spread rapidly over the continent

and differentiated into numerous local populations or races

(Johnston and Selander 1964; Selander and Johnston 1967).

Speciation has been observed by the re-creation of naturally

occurring as well as novel polyploid species of plants [Appa-

lachian ferns, Asplenium, Wagner 1954; Spartina townsendii

(the only naturally occurring species of plants whose origin has

been observedin the wild), Huskins 1931; the artificial radish-

cabbage hybrid Raphanobrassica Stebbins 1950; and many

others, Grant 1963, 1981]. Moreover some breeds of domes-

ticated animals that have become so different that they would

act as different species under natural conditions. If it was

possible to release a population of giant Irish Wolfhounds and

a population of dwarf Chihuahuas (these breeds were chosen

as the largest and smallest breeds of dogs, but the same would

happen with many other combinations of dog breeds) in a

region where both could survive and reproduce, one will findonly Irish Wolfhounds and Chihuahuas generation after

generation responding to one another as good species. The

large difference in size between these two groups of dogs serves

as the intrinsic isolating mechanism for genetic isolation

between these two taxa regardless of whether they are

technically placed in the same species, Canis familiaris, and

regardless of the fact that, with series of intermediate steps

involving different breeds of dogs, genetic material could be

transferred from Irish Wolfhounds to Chihuahuas. Moreover

large breeds of dogs and Northern Hemisphere wolves, Canis

lupus, readily interbreed with viable and fertile offspring

although the large and toy breeds of dogs cannot because of

their size difference, The same would be true if populations of

dwarf horses and the largest dwarf horses were released in a

region where both would survive and breed. If one wishes to

speak of any aspect of evolutionary theory being factual, it is

these direct observations of different aspects of nomological

evolutionary theory, not the history of life which remains

theoretical, but exceeding well corroborated with very exten-

sive testing without any falsification.

Only the last of Darwins five theories as listed by Mayr is a

historical evolutionary theory. This is:

(5) Common descent which implies that all species or popu-

lations of organisms have descended with modification from

common ancestors; this descent includes both modification

and branching. Darwinian common descent is equivalent to

Haeckelian phylogeny. Hennigian phylogeny is equal only tothe branching aspect of Haeckelian phylogeny (Mayr and

Bock 2002). Common descent is expressed in Darwinian

classifications, Haeckel dendrograms, Hennigian cladograms,

and the tree of life, all of which are theoretical statements.

Historical biogeography and any other biological theory,

that fits the characteristics of H-N Es outlined above, would

also fall into the class of historical theories. Most important is

that the historical evolutionary theory of common descent, as

well as any other historical biological theory, depends firmly

on a set of appropriate nomological theories to be scientific. In

the case of the historical evolutionary theory of common

descent, it is based on the preceding four nomological

evolutionary theories. Nomological evolutionary theory is

primary and historical evolutionary theory is secondary, and

must be considered and discussed in that order.It is incorrect to

speak first of the pattern of evolutionary change ( historical

theory) and then the process of evolutionary change

( nomological theory).

The fifth of Darwins theories that of common descent is

a general theory and deductions from it can be tested against a

number of objective empirical observations. If an evolutionary

history of living organisms (both fossil and recent) is obtained

from the nomological theory of evolution, then one can reach

three further deductions, namely:

(1) Groups of related organisms will be found non-stochasti-

cally over thesurface of theEarth depending on their degree

of evolutionary relationships and their abilities to disperse.

(2) The origins of groups of organisms will be found

chronologically in the fossil record depending on their

ancestral-descendent relationships.

(3) Vestigial structures will be found in descendent groups

with the homologous structure existing well developed in

ancestral groups.

These deductions had been tested successfully against a

exceedingly large number of empirical observations of the

spatial and chronological distributions of organisms and theoccurrence of well developed features in ancestral groups

versus vestigial features in descendent groups. Hence one can

conclude that general historical evolutionary theory is

extremely well corroborated, and that ultra-massive counter

observations would be needed to disprove Darwins historical

theory of common descent. But this well-tested and supported

historical evolutionary theory is still a theory as noted by

almost all philosophers of science and is not a fact or factual as

many evolutionists like to characterize it. The general histor-

ical theory of evolution, although exceedingly well corrobor-

ated, cannot be used for testing any special historical

evolutionary theories and especially for testing any nomolog-

ical theories because historical theories cover singular events;

hence they cannot be used to test other historical events or to

test nomological theories which cover general events.

In addition to the general historical theory of evolution,

endless special theories exist which deal with the evolutionary

history and classification of all groups of organisms at all

hierarchal levels from subspecies to kingdoms. Hence the

theory of whether the Rodentia and the Lagomorpha are

closely related to one another within the Eutheria forming the

taxon Glires, or the theory of whether pinnipeds (seals and

other aquatic carnivores) had a single or a double origin from

the terrestrial carnivores, or whether birds descended from an

early group within the archosaurian reptiles (Martin 2004) or

from a later and more specialized group within the theropod

dinosaurs (Sereno 2004) or perhaps that some of the latedinosaurs, the maniraptoran radiation, are actually not

dinosaurs but flightless descendants of an early bird such as

Archaeopteryx (Paul 2001, 2002; Martin 2004) are all special

historical evolutionary theories.

Special historical evolutionary theories deal with singular

events and each must be tested independently against objective

empirical observations, which generally include an argument

chain of well corroborated nomological theories and the

observations supporting them. To be scientific, historical

evolutionary theories must also be tested against relevant,

well corroborated nomological evolutionary theories. Any

historical evolutionary theory must also be tested using other

well-tested, non-evolutionary nomological theories. A histor-

ical evolutionary theory on the origin of avian flight must also

100 Bock



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be tested with nomological aerodynamic theories as well as

those concerning functional properties of vertebrate muscle-

bone systems, metabolism, respiration, etc. If, finally, a well

corroborated and convincing historical theory has been

reached to explain the evolution of avian flight, this historical

theory works for birds only and cannot be applied to explain

the evolution of flight in bats or in pterosaurs, which are

different singular events and may well have very different

historical explanations (Bock and Bu hler 1995).

Functional explanations

Although this essay deals with diverse forms of evolution-

ary explanations, a brief word should be said about

functional explanations. Two different, but interrelated sys-

tems of explanations exist in biology, which are (1) the

dichotomy of N-D E versus H-DEs and (2) the dichotomy of

Functional Explanations versus Evolutionary Explanations.

The latter system stems from the useful division of biology

into the major areas of functional and evolutionary biology

as noted by Mayr in his Growth of Biological Thought (1982).

He did not use the terms Functional and EvolutionaryExplanations, but his division of biology into these two

major areas can be taken as the basis to consider this

dichotomy of explanations. It should be noted that the use of

functional expla

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