What does evolution explain ?Extending the evolutionary theory.

Philippe HunemanIHPST (CNRS/ Université Paris I

Sorbonne)

• EXTENDED evolutionary theory.Dawkins 1982; Turner 2001; Sterelny et al. 1996;

Odling-Smee et al. 2003; Pigliucci 2007; Gould 2002 (« expansion »)

Extending the explananda ? Extending the explanantia ?

I. THE EXPLANANDA – AND ESPECIALLY DIVERSITY

Evolutionary theory explains (Lewontin) :

Adaptation

Diversity

Transformations

Evolutionary theory explains (Lewontin) :

Adaptation-> explained (defined ?) by natural selection

DiversityAdaptive radiation, « principle of divergence »

Evolution -> several causes, the major one being natural selection

Diversity

• Variety, but not much novelty (Darwin)-> unity through diversity

• Finally, then, although in many cases it is most difficult even to conjecture by what transitions organs could have arrived at their present state; yet, considering how small the proportion of living and known forms is to the extinct and unknown, I have been astonished how rarely an organ can be named, towards which no transitional grade is known to lead. It is certainly true, that new organs appearing as if created for some special purpose rarely or never appear in any being; as indeed is shown by that old, but somewhat exaggerated, canon in natural history of "Natura non facit saltum." We meet with this admission in the writings of almost every experienced naturalist; or, as Milne Edwards has well expressed it, "Nature is prodigal in variety, but niggard in innovation." Why, on the theory of Creation, should there be so much variety and so little real novelty? Why should all the parts and organs of many independent beings, each supposed to have been separately created for its own proper place in nature, be so commonly linked together by graduated steps? Why should not Nature take a sudden leap from structure to structure? On the theory of natural selection, we can clearly understand why she should not; for natural selection acts only by taking advantage of slight successive variations; she can never take a great and sudden leap, but must advance by the short and sure, though slow steps.

Origin of species (6th ed.) ch. 6.

Similarity, through difference : The concept of homology: ‘the same organ in different animals under every variety of form and function’. (Owen 1843)

Distinguishing homology and analogy (adaptive convergence)

Diversity

Idea of a morphospace

Diversity

Idea of a morphospace

Generalised morphospace

Issue : The clustering within the morphospace

The Clustering question : Why ?

Diversity, 2

The pattern of branchingWhat is the explanans ????

5 Organisms (taxa, specimens) A, B, C, D, E

Possible comparisons -> (ab) (cde)), (abc (de)), (a (bcde)) (ab ((cd)e)) etc.

• (A (B (CD))• (A (CB) D)• (A (BC) D)

• (A (D (BC)) = (A (DBC)) + (A D (BC))

• Suppose you have (A (DB) C) also … : then no possible branching pattern

The Branching question

• The fact is that we have something like a tree !Why ??

-> Common descent (by any means ??).

Plant cladogram

Vertebrates cladogram

Partial cladograms

Dinosaurs

• Evolution = two questions about rare facts in spaces of possible spaces

• Are there equal ? Or is an answer to the Clustering question not enough to answer the Branching question ?

• Is there any process likely to answer the two questions ?

II. DARWINIAN EVOLUTIONARY THEORY EXPLAINING DIVERSITY

Darwin’s view

• Unity through diversity ?

The Conditions of existence and the Unity of type

(E.S. Russell Form and function, 1916; Cuvier vs. Geoffroy St Hilaire 1830)

Forme (Function)

Geoffroy St HilaireUnity of type

Common descent

CuvierConditions of existence

Natural selection

• It is generally acknowledged that all organic beings have been formed on two great laws Unity of Type, and the Conditions of Existence. By unity of type is meant that fundamental agreement in structure, which we see in organic beings of the same class, and which is quite independent of their habits of life. On my theory, unity of type is explained by unity of descent. The expression of conditions of existence, so often insisted on by the illustrious Cuvier, is fully embraced by the principle of natural selection. For natural selection acts by either now adapting the varying parts of each being to its organic and inorganic conditions of life; or by having adapted them during long-past periods of time: the adaptations being aided in some cases by use and disuse, being slightly affected by the direct action of the external conditions of life, and being in all cases subjected to the several laws of growth. Hence, in fact, the law of the Conditions of Existence is the higher law; as it includes, through the inheritance of former adaptations, that of Unity of Type.

• Chapter VI. Origin of species

• Darwin’s thesis : « unity of type » is subsumed under natural selection

• What is « common descent » for two traits at a same phylogenetic level

….resorts to « natural selection » when considering the first stages (plesiomorphic states) of the trait…

• Thus, we can hardly believe that the webbed feet of the upland goose or of the frigate-bird are of special use to these birds; we cannot believe that the same bones in the arm of the monkey, in the fore leg of the horse, in the wing of the bat, and in the flipper of the seal, are of special use to these animals. We may safely attribute these structures to inheritance. But to the progenitor of the upland goose and of the frigate-bird, webbed feet no doubt were as useful as they now are to the most aquatic of existing birds. So we may believe that the progenitor of the seal had not a flipper, but a foot with five toes fitted for walking or grasping; and we may further venture to believe that the several bones in the limbs of the monkey, horse, and bat, which have been inherited from a common progenitor, were formerly of more special use to that progenitor, or its progenitors, than they now are to these animals having such widely diversified habits. Therefore we may infer that these several bones might have been acquired through natural selection, subjected formerly, as now, to the several laws of inheritance, reversion, correlation of growth, &c.

• Consequences : micro and macroevolution• Darwin’s gradualism

• The scale-free diagram – zoom in (processes) and zoom out (branching pattern, homologies)

The Modern Synthesis view

Change in allelic frequencies as the core of evolution

« Population thinking » (Mayr) and population genetics: genes and (alleles) populations are the two main explanatory levels.

Irrelevance of development to evolution (what counts is the phenotype’s effect on the replication rates of the genotypes)

Natural selection is the main cause of the departures from gene frequencies equilibria

About processes: macroevolution as extrapolation from microevolution (Mayr, Simpson)

About patterns : Gradualism

III. WHAT WOULD BE AN ALTERNATIVE VIEW ?

Population thinking and typology

• Amundson’s classification (« Homology and homoplasy : a philosophical perspective » 2004): typology = homology more important than natural selection

« Typologists were united not by metaphysical or anti-evolutionary commitments, but by a belief in the importance of homology over adaptations”

• Micro vs macro evolution = is adaptation different than « novelty »? (Muller and Newman 2003) Should novelty need another explanans ?

Forme Function

Geoffroy St HilaireUnity of type

Common descent

HomologyUnity Typological thinking

CuvierConditions of existence

Natural selection

Adaptive radiationDiversityPopulation thinking

• MS : explains diversity through adaptation (radiations…)

• Explains unity by common descent (and in fine natural selection…)

• Typologist : explains unity (in diversity) by commonalities of structure (developmental)

The role of development

• MS view : natural selection explains the branching (through common descent) and (through both homologies and analogies) the clustering

• (Developmental) typologist view : Common developmental processes underpin homologies (hence the branching) and clustering

4. WHAT IS EMPIRICAL AND WHAT IS « PURELY » CONCEPTUAL IN THOSE CHALLENGES TO THE MODERN SYNTHESIS ?

Extending vs rebuilding evolutionary theory ?

Integrating :• Development (Gilbert, Opitz & Raff 1996)• Ecology (Odling Smee et al. 2003; Hubbell

2001)• Both(Eco evo devo, Gilbert 2008)

Several cleavages

• Genes / epigenetics (Gilbert; Kirschner & Gerhardt 2005) • Genes / developmental systems (DST)• Genes / Genomics (Fox Keller 2000, Stolz, etc.): what if

« genes » don’t exist any more in molecular genetics ?

• Genes / organisms (West Eberhardt 2003; Walsh 2008; Odling Smee et al. (2003))

(Gould & Lewontin 1979)

• Forms / Genes (Pigliucci 2007)• Structure/ Function (adaptation) (Amundson 2005) or

Externalism vs. Internalism (Godfrey Smith 1996)

The problem

Are all those « challenges » to the modern synthesis likely to be synthesised ?

What hangs upon empirical findings in those question ?

A common thread: Against « gene centrism »

1. In defining evolution• Evolution is not « change in allelic frequencies » (i)(a definition already controversial among MS writers : “Evolution is not a

change in gene frequencies, as is claimed so often, but the maintenance (or improvement) of adaptedness and the origin of diversity. Changes in gene frequency are a result of such evolution, not its cause.” (Mayr 1998, 2093)).

But• Change in developmental pathways (Caroll 2005) (ii)• Change in bodyplans (iii)Etc.

(No more assuming that i entails ii and iii)

A consequence :

• Macro evolution is not immediately derived from microevolution (vs. Mayr, Simpson, Dobzhanski)

• The challenges are in the same time about the patterns of evolution

(here, emphasis on the processes; see Gayon 2008)

Against gene centrism

2. In explaining evolutiona. Development is as important as genotypesb. (Evolution is not led by genes) c. Organism is a fundamental level of evolution;

About c. : Darwin vs. Neodarwinism (Ariew 2008; Pigliucci 2007)

d.Evolutionary genes don’t match genes from genomics/molecular genetics (pb. for a theory of variation and inheritance ?)

Two options:

1) explain other explananda (e. g. 1.a Form, the unity of forms ; or a synthesis of allelic evolution and form, or adaptation and form) (e.g. Amundson 2005; Pigliucci 2007;

1.b, novelty (Newman and Muller 2003; Watson 2005))

2) explain differently (some of) the same explananda• 2a. Another explanans than natural selection (Kauffmann;

walsh; odling smee et al. ?; Gould ?; Jablonka & Lamb ?;• 2b. Natural selection differently understood is an explanans

(DST; Caroll 2005; West Eberhardt ?)

• (1) calls for a synthesis of MS and something else in order to build a general evolutionary biology

• (2a) needs to extend MS• (2b) means a re-writing of MS

What may be the targets of criticism ?

The 2 controversial gene-related theses in the Modern Synthesis :

Darwinism (natural selection as a major cause of evolution, adaptation and diversity)

Mendelism (Particular inheritance)-> genes are the substrate of inheritance (p)Weissmanism (germen soma distinction)-> development does not impinge on evolution (q)

The typological developmentalist attacks q.

But : Possibly : very sophisticated developmental

phenomena (involving genes, epigenetics etc.) without making development relevant for evolution

-> the argument from genotype phenotype maps

Gspace, Pspace

Gspace, Pspace, with DspaceDspace is evolutionary irrelevant

Genotype spaceDevelopmental space

Phenotype space

Gspace, Pspace, with DspaceDspace is evolutionary relevant

CONCLUSION

- Plurality of alternatives to MS (is there one typologism?)

- Not all have the same explananda (even when it comes to diversity)

- The emphasis on development as explanation for evolution and unity-through-diversity is contingent upon empirical facts concerning P-D-G spaces mappings.