1. Fossils & Evolution—Chapter 3
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.
Fossils & Evolution—Chapter 3

2. Fossils & Evolution—Chapter 3
Ch. 3—Key terms
Species
Biological species concept
Morphological species concept
Sympatric / Allopatric populations
Allopatric speciation
Cline / chronocline / chronospecies
Anagenesis (phyletic gradualism)
Punctuated equilibrium
Fossils & Evolution—Chapter 3

3. 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
Fossils & Evolution—Chapter 3

4. Fossils & Evolution—Chapter 3
Biological species
a-d all discrete
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!
morphology
morphology
Fossils & Evolution—Chapter 3

5. 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)
Fossils & Evolution—Chapter 3

6. Fossils & Evolution—Chapter 3
Sympatric geographic
ranges: partial overlap
Allopatric geographic
ranges: no overlap
Fossils & Evolution—Chapter 3

7. The “paleontologic species problem”
Are a and f discrete?
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
morphology
morphology
Fossils & Evolution—Chapter 3

8. Fossils & Evolution—Chapter 3
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)
Fossils & Evolution—Chapter 3

9. Chronocline (produced by anagenesis)
Fossils & Evolution—Chapter 3

10. Speciation vs. anagenesis
Time 2
Species B
Species B
Arbitrary boundary
“pseudoextinction”
speciation
event
Time 1
Species A
Species A
Fossils & Evolution—Chapter 3

11. 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
Fossils & Evolution—Chapter 3

12. 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
Fossils & Evolution—Chapter 3

13. 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)
Fossils & Evolution—Chapter 3

14. Allopatric speciation
Time 4: secondary sympatry,
but with reproductive isolation
Time 3: geographic & reproductive isolation
Time 2: migration & geographic variation
Time 1: homogeneous population
Fossils & Evolution—Chapter 3

15. 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
Fossils & Evolution—Chapter 3

16. Fossils & Evolution—Chapter 3
Cline
Fossils & Evolution—Chapter 3

17. Fossils & Evolution—Chapter 3
Ring species
Fossils & Evolution—Chapter 3

18. Fossils & Evolution—Chapter 3
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
Fossils & Evolution—Chapter 3

19. 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
Fossils & Evolution—Chapter 3

20. 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
Fossils & Evolution—Chapter 3

21. 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!
Fossils & Evolution—Chapter 3

22. Fossils & Evolution—Chapter 3
Morphologic clusters
Fossils & Evolution—Chapter 3