1. 1 The Origin of Species Chapter 23

2. 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. 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. 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. 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. 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. 7 Tiglon

8. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 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. 20 Macroevolution

21. 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. 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