Chapter 22

Summary		Questions		Media Resources
22.1 Species are the basic units of evolution.
• Species are groups of organisms that differ from one another in one or more characteristics and do not hybridize freely when they come into contact in their natural environment. Many species cannot hybridize with one another at all. • Species exhibit geographic variation, yet phenotypically distinctive populations are connected by intermediate forms.		1. Define the term sympatry. Why is sympatric speciation thought by many to be unlikely? 2. What is the biological species concept?
22.2 Species maintain their genetic distinctiveness through barriers to reproduction.
• Among the factors that separate populations and species are geographical, ecological, behavioral, temporal, and mechanical isolation, as well as factors that inhibit the fusion of gametes or the normal development of the hybrid organisms. • Some isolating mechanisms (prezygotic) prevent hybrid formation; others (postzygotic) prevent hybrids from surviving and reproducing.		3. What is the difference between prezygotic and postzygotic isolating mechanisms?· 4. What barriers exist to hybrid formation and success? Which are prezygotic and which are postzygotic isolating mechanisms? Why do some people think the term "isolating mechanism" is misleading?
22.3 We have learned a great deal about how species form.
• Reproductive isolation can arise as populations differentiate by adaptation to different environments, as well as by random genetic drift, founder effects, or population bottlenecks. • Natural selection may favor changes in the mating system when a species occupies a new habitat, so that the species becomes reproductively isolated from other species. • When two species are not completely reproductively isolated, natural selection may favor the evolution of more effective isolating mechanisms to prevent hybridization, a process termed "reinforcement."		5. How does selection relate to population divergence? 6. How many genes are involved in the speciation process? 7. When are hybrids at a disadvantage? What can be the result of this disadvantage? 8. Define the term polyploidy.		•Introduction to Speciation •Allopatric Speciation • Sympatric Speciation •Activity: Allopatric Speciation • Hybrids and Evolution in Fragile Ferns
22.4 Clusters of species reflect rapid evolution.
• Clusters of species arise when populations differentiate to fill several niches. On islands, differentiation is often rapid because of numerous open habitats. • The pace of evolution is not constant among all organisms. Some scientists believe it occurs in spurts, others argue that it proceeds gradually. • Hybridization occurs commonly among plants and even among animals. The biological species concept may not apply to all organisms.		9. What is adaptive radiation? What types of habitats encourage it? Why? 10. What is the difference between gradualism and punctuated equilibrium? 11. Why is the biological species concept no longer considered to be universally applicable?		•Key Events •Extinctions •Evolutionary Trends •Darwin's Dreampond by Goldschmidt • Evolutionary Pace

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