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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Fossils & Evolution—Chapter 3 Cline Fossils & Evolution—Chapter 3

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

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

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

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

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

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