Fossils & Evolution—Chapter 31 Ch. 3—Key concepts A biological species is defined as a group of...

Fossils & Evolution—Chapter 3 1

Ch. 3—Key concepts• A biological species is defined as a group of

potentially interbreeding populations that are reproductively isolated from other such groups under natural conditions.

• It is impossible for paleontologists to utilize the biological species concept, because of the inability to observe reproductive behavior in fossils.

• In practice, paleontologists recognize fossil species as discrete groups of individuals that are separated from other such groups by morphologic gaps.


Ch. 3—Key terms• Species

– Biological species concept

– Morphological species concept

• Sympatric / Allopatric populations• Allopatric speciation• Cline / chronocline / chronospecies• Anagenesis (phyletic gradualism)• Punctuated equilibrium


Biological species concept

• A species is a group of potentially interbreeding populations that are reproductively isolated from other such groups– Most species are separated geographically into

local breeding populations– Populations are reproductively isolated only if

interbreeding would not occur if they lived in the same area


Biological species

• Most biologic species are distinct because they belong to evolving lineages that have been reproductively isolated for a long time

• Biologists encounter difficulty only when a lineage is branching at the present time!

a-d all discrete

morphology

morphology


Biologic methods of species discrimination

• How do biologists establish that two populations represent two species?– Reproductive isolation is difficult to document

• Experience shows that distinct populations cannot live sympatrically unless they belong to different species

• Therefore, if two distinct populations overlap in their geographic range, they probably represent two species

– Mainly, biologists rely on morphologic differences (occasionally with the benefit of biogeographic info)


Sympatric geographicranges: partial overlap

Allopatric geographicranges: no overlap


The “paleontologic species problem”

• It is not possible to apply the biologic species definition to fossils

• In practice, a paleontologist includes in a fossil species those specimens that he/she believes would have formed a biologic species had they lived together at the same time

Are a and f discrete?

morphology

morphology


Origin of species

• Speciation vs. anagenesis– Speciation = the splitting of a lineage resulting

in an increase in the number of species– Anagenesis = gradual evolution within a

lineage whereby one species changes into another (without an increase in the number of species)


Chronocline (produced by anagenesis)


Speciation vs. anagenesis

Time 1

Time 2

Species A Species A

Species BSpecies B

speciationevent

Arbitrary boundary“pseudoextinction”


Natural selection and anagenesis

• All populations are variable– Variation in the gene pool is expressed outwardly as

variation in the phenotypes of individuals– Sources of variation are

• Point mutations• Genetic shuffling from sexual reproduction

– As time passes, natural selection operates on phenotypic variation

• Certain kinds of individuals will have greater reproductive success; these kinds will become more common in the population after many generations


Natural selection and anagenesis

• Natural selection is a sorting of individuals– So that a population becomes progressively better

adapted to a constant environment, or– So that a population adapts to changing

environmental conditions• In a chronologic series of populations, there is a point at

which the accumulated differences between a population and the starting population are so great that the two would be reproductively isolated if they had been living together at the same time—at this point, a new species has been formed by anagenesis


Allopatric speciation

• If two or more breeding populations of a given species are geographically isolated from one another, they may undergo anagenesis independently until reproductive isolation occurs– One species becomes two (geographic speciation has

occurred)

– In nature, speciation occurs rapidly in small peripheral populations that have become isolated from the larger parent population (i.e., allopatric speciation)


Allopatric speciation

Time 1: homogeneous population

Time 3: geographic & reproductive isolation

Time 4: secondary sympatry, but with reproductive isolation

Time 2: migration & geographic variation


Clines and ring species

• Gene flow between adjacent populations of a species may be restricted by partial geographic barriers

• If so, then subtle genetic differences among populations may develop (genetic gradient)– Cline = series of populations whose gene pools differ

slightly along a genetic gradient

• Populations at the extreme edges of a species’ range may be reproductively isolated even though they are connected by a chain of interbreeding populations


Cline


Ringspecies


Rates of evolution• Most biologists believe that well-established

species evolve slowly, because:– Genotypes are so complex and highly integrated that

random changes are unlikely to be advantageous– Changes may not “take” in a large, thoroughly

interbreeding population

• Because large populations resist change, most evolution probably is concentrated in relatively rapid speciation events in peripherally isolated populations


Punctuated equilibrium

• Eldridge and Gould 1972

• Once a species is established, it undergoes no significant, directional change: gene pool reaches a state of equilibrium (stasis)

• New species arise suddenly through allopatric speciation, then themselves become static entities


Punctuated equilibrium

• “Paleontologic species problem” is alleviated somewhat if anagenesis is rare and most evolutionary change is associated with allopatric speciation events

• In reality, both anagenesis and speciation occur

S.E. = speciation event


Paleontologic species problem

• Conclusion: There is no way to overcome the PSP

• In practice, fossil species are recognized by the morphologic gaps between one another

• Where there is no clearcut gap, there is uncertainty!


Morphologic clusters

3 4 5 6 7 8 9 10 11 12 13 14 15

length

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