The Origin of Species

1. The Origin of Species Chapter 23

2. Outline • The Nature of Species • Pre and Postzygotic Isolating Mechanisms • Geography of Speciation • Hawaiian Drosophila • Darwin’s Finches • Lake Victoria’s Cichlid Fishes • New Zealand Buttercups • Diversity of Life Through Time • Pace of Evolution • Speciation and Extinction

3. Biological Species Concept • Ernst Mayr • “…groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups.” • Individuals that cannot produce fertile offspring are termed reproductively isolated, and thus members of different species. (sympatric species) • Application problems • difficult to apply the concept to populations that do not occur together in nature (allopatric populations) • no meaning for asexual species

4. The Nature of Species • Distinctiveness of sympatric species • Sympatric species are distinctive entities that are phenotypically different, utilize different parts of the habitat, and behave separately. Biological species concept • If sympatric species commonly exchange genes, their gene pools should become homogenized. • Two species that occur together and appear to be nearly identical are termed sibling species.

5. The Nature of Species • Geographic variation within species • Populations within a species that occur in different areas and are distinctive may be classified as subspecies or varieties.

6. Prezygotic Isolating Mechanisms • Prezygotic isolating mechanisms prevent the formation of zygotes. • Ecological isolation • Even if two species occur in the same area, they may utilize different portions of the environment and thus do not hybridize because they do not encounter each other. • lions and tigers in India

7. Tiglon

8. Prezygotic Isolating Mechanisms • Behavioral isolation • Many birds differ in courtship rituals which tends to keep species distinct. • Temporal isolation • varying breeding seasons • Mechanical isolation • reproductive structural differences • Prevention of gamete fusion

9. Postzygotic Isolating Mechanisms • Postzygotic isolating mechanisms prevent the proper functioning of zygotes after they form. • Hybridization often produces embryos that die during early development. • hybrid sterility • abnormal sex organ development • chromosome alterations

10. Problems with the Biological Species Concept • Extent of reproductive isolation • high levels of hybridization • Difficult to apply to species that do not occur together in nature • Cannot be applied to asexual reproduction • No single definition may be universally applicable.

11. Reproductive Isolation and Evolutionary Change • Most reproductive isolating mechanisms initially arise for some reason other than to prevent reproduction. • adaptations for changing environments • selection may reinforce isolating mechanisms • Initially incomplete isolating mechanisms are reinforced by natural selection until they are completely effective.

12. Reproductive Isolation and Evolutionary Change • Role of natural selection in speciation • Reinforcement is driven by natural selection favoring the perfection of reproductive isolation. • Random changes may cause reproductive isolation • Given long enough periods of time, any two isolated populations will diverge due to genetic drift.

13. Reproductive Isolation and Evolutionary Change • Adaptation and speciation • Adaptation and speciation are probably related in many cases. • As species adapt, they accumulate many differences that may lead to reproductive isolation.

14. Geography of Speciation • Speciation is a two-part process: • identical populations must diverge • reproductive isolation must evolve to maintain the differences • Allopatric speciation • Geographically separated populations appear much more likely to have evolved substantial differences leading to speciation.

15. Geography of Speciation • Sympatric speciation • In recent years, a number of cases have appeared that appear difficult to interpret in any way other than sympatric speciation.

16. Sympatric Speciation • Instantaneous speciation through polyploidy • individual is born that is reproductively isolated from other members of species • polyploidy - more than 2 sets of chromosomes • autopolyploidy - all chromosomes arise from a single species • allopolyploidy - two species hybridize • Disruptive selection

17. Rapid Evolution • Adaptive radiation - process producing a cluster of species, occupying a series of similar habitats, all evolving from a recent ancestor • requires both speciation and adaptation to different habitats • Adaptation may be driven by the need to minimize competition for available resources with other species (character displacement).

18. Darwin’s Finches • The 14 species of finches Darwin encountered comprise four groups, all derived from one similar mainland species, and radiated in the absence of competition from other birds. • ground finches • tree finches • warbler finches • vegetarian finches

19. Pace of Evolution • Punctuated equilibrium • proposed by Stephen Jay Gould and Niles Eldredge • Evolution normally proceeds in spurts, with long periods of little movement in between. • contrasted to the theory of gradual evolutionary change (gradualism) • The proposed stasis would be expected in large populations experiencing stabilizing selection over long periods of time.

20. Macroevolution

21. Speciation and Extinction Through Time • There have been 5 major mass extinctions interspersed within relatively consistent extinction patterns. • most famous - end of Cretaceous period (65 mya)- dinosaurs went extinct • Mammals quickly experienced evolutionary radiation. • Biological diversity tends to rebound after mass extinctions.

22. The Future of Evolution • Human impacts on the environment will affect the evolutionary process in many ways. • environment alteration • climate change • decreased population sizes • increased genetic drift • increased extinction