Evolution: Macroevolution & origin of species

1. Evolution:Macroevolution & origin of species Chapter 17

2. Macroevolution • large-scale change at or above species level • includes development of new species • fundamentally linked to microevolution • continued microevolution often leads to development of new sp. • What is a Species? • many different definitions • Ernst Mayr’s “Biological Species Concept” • group of interbreeding natural populations • produce fertile offspring • reproductively isolated from other such groups • problems? • subspecies • geographical variety of the same species

3. Speciation • formation of a new species • key factors are geography and environmental conditions • ultimate goal is reproductive isolation • allopatric speciation • large, continuous pop.  geographical change  pop. split into two groups geographic isolation env. conditions different for each group  two groups evolve separately due to nat. sel., etc. reprod. isol. • many processes can lead to geographic isolation • natural environmental change • human causes • individuals leaving on their own • dispersal • founder effect A schematic view of allopatric speciation

4. Ensatinaeschscholtzipicta 1 Members of a northern ancestral population migrated southward. Ensatinaeschscholtzi oregonensis Subspecies are separated by California’s Central Valley. Some interbreeding between populations does occur. 2 Central Valley Barrier Ensatinaeschscholtziplatensis Ensatinaeschscholtzi xanthoptica Ensatinaeschscholtzi croceater Ensatinaeschscholtzi eschscholtzii Ensatinaeschscholtzi klauberi Fig. 17.8 Allopatric speciation in salamanders Evolution has occurred, and in the south, subspecies do not interbreed even though they live in the same environment. 3

5. Fig. 17.9 Allopatric speciation among sockeye salmon. In Lake Washington, salmon that matured (a) at Pleasure Point Beach do not reproduce with those that matured in (b) Cedar River. The females from Cedar River are noticeably larger and the males are more slender than those from Pleasure Point Beach, and these shapes help them reproduce in the river.

6. sympatric speciation • occurs in pops. occupying the same area • no geographic isolation, but reproductive isolation still occurs • most common in plants • hybridization, polyploidy, etc. A schematic view of sympatric speciation

7. Fig. 17.12. An example of sympatric speciation – polyploidy in plants. Reproduction between two species of Clarkia results in a sterile hybrid. Doubling of the chromosome number results in a fertile third Clarkia species that can reproduce only with itself.

8. Reproductive Isolating Mechanisms • means by which species maintain their integrity • prevent two species from interbreeding to produce fertile offspring • prezygotic barriers • occur before a zygote is formed • habitat isolation • behavioral isolation • temporal isolation • mechanical isolation • gamete isolation • postzygotic barriers • take place only if all prezygotic barriers are bypassed • zygote mortality • F2 fitness • hybrid sterility Fig. 17.5 Temporal isolation

9. Fig. 17.4 Reproductive isolating mechanisms