Mechanisms of Speciation and Macroevolution

1. Chapter 19 Speciation and Macroevolution

2. Reproductive isolating mechanisms • Prevent gene flow between species • Prezygotic barriers • Prevent mating or fertilization • Postzygotic barriers • Reproductive failure after fertilization

3. Temporal isolation • Mating at different times of year • Mating at different times of day

4. Habitat isolation • Different habitats in the same area • Gametic isolation • Incompatible egg and sperm • Molecular recognition on the surface of the cells

5. Behavioral (sexual) isolation • Required courtship behaviors The male satin bowerbird builds a bower of twigs (a dark tunnel) to attract females

6. Mechanical isolation • Incompatible genital organs Only large bees brush against the stamens of the white sage Only small bees can land on the petal of the black sage

7. Postzygotic barriers • Hybrid inviability • hybrid embryos die when genetic regulation fails during development • Hybrid sterility • problems during meiosis cause abnormal gametes

8. Postzygotic barriers • Hybrid breakdown • Hybrid cannot reproduce because of some defect

9. Speciation • A population becomes reproductively isolated • The separated gene pools diverge • Genetic exchange stops

10. Allopatric speciation • Geographically separated populations • Most common form of speciation • Genetic drift in small populations

11. Squirrel species separated by the Grand Canyon have diverged in fur color

12. Sympatric speciation • Divergence of two populations in the same geographic region • Other reproductive isolating mechanisms at work • Especially common in plants

13. Allopolyploid • Hybrid with multiple sets of chromosomes from two species • Autopolyploid • Hybrid with multiple sets of chromosomes from a single species

14. Sympatric speciation by allopolyploidy in plants

15. Sympatric speciation in animals • Population occupies a new ecological niche • No gene flow even though species live in the same area

16. Two species of maggot flies are sympatric in the northern half of one fly’s range

17. Rate of evolutionary change • Fossil record often lacks transitional forms between two species • Is the fossil record simply incomplete? • Or does it accurately reflect evolution as it really occurs?

18. Punctuated equilibrium • Long periods of stasis (~2 My) • Punctuated by periods of rapid speciation (~100,000 y) • Triggered by changes in the environment • Abrupt appearance of new species in the fossil record

19. Gradualism • Continuous evolution over long periods • The traditional view • Populations gradually accumulate adaptations • Different selective pressures in different environments

20. Gradualism

21. Punctuated Equilibrium

22. Macroevolution • Large-scale phenotypic changes in populations, classified at the species level or higher • Appearance of evolutionary novelties • Adaptive radiation • Mass extinction

23. Preadaptations • Features that fulfilled one role but later became adaptive in a different role • Feathers originally provided thermal insulations • After gradual change they served a new function in flight

24. Allometric growth • Varied rates of growth for different parts of the body • A change in development can result in a new species when the change is adaptive

25. Allometric growth

26. Paedomorphosis • Retention of juvenile characteristics in the adult • A change in the timing of development

27. Paedomorphosis in an axolotl salamander

28. Adaptive radiation • Speciation fills new ecological niches • New adaptive zones may appear when the environment changes • One species colonizes an island and diversifies into new species

29. Adaptive radiation

30. Extinction • Facilitates evolution by opening adaptive zones • Background extinction at a steady rate

31. Mass extinctions • Five or six mass extinctions of many species and higher taxonomic groups • Major climate changes • Catastrophes such as asteroid impacts

32. Mass extinction of the archosaurs