Chapter Summary

• Speciation is the process by which one species splits into two or more daughter species, which thereafter evolve as distinct lineages.

• The morphological species concept distinguishes species on the basis of physical similarities; it often underestimates or overestimates the actual number of reproductively isolated species.

• The biological species concept distinguishes species on the basis of reproductive isolation.

• The lineage species concept, which recognizes independent evolutionary lineages as species, allows biologists to consider species over evolutionary time.

• Genetic divergence results from the interruption of gene flow within a population.

• The Dobzhansky–Muller model describes how reproductive isolation between two physically isolated populations can develop through the accumulation of incompatible genes or chromosomal arrangements. Review Figures 22.3, 22.4, Activity 22.1

• Reproductive isolation increases with increasing genetic divergence between populations. Review Figure 22.5

• Allopatric speciation, which results when populations are separated by a physical barrier, is the dominant mode of speciation. This type of speciation may follow founder events, in which some members of a population cross a barrier and found a new, isolated population. Review Focus: Key Figure 22.6, Figures 22.7, 22.8, Animations 22.1, 22.2

• Sympatric speciation results when the genomes of two groups diverge in the absence of physical isolation. It can result from disruptive selection in two or more distinct microhabitats.

• Sympatric speciation can occur within two generations via polyploidy, an increase in the number of chromosomes sets. Polyploidy may arise from chromosome duplications within a species (autopolyploidy) or from hybridization that results in combining the chromosomes of two species (allopolyploidy). Review Figure 22.9

• Prezygotic isolating mechanisms prevent hybridization; postzygotic isolating mechanisms reduce the fitness of hybrids.

• Postzygotic isolating mechanisms lead to reinforcement of prezygotic isolating mechanisms by natural selection. Review Figures 22.11, 22.12, Investigating Life: Flower Color Reinforces a Reproductive Barrier in Phlox

• Hybrid zones may form and persist if reproductive isolation between species is incomplete. Review Figure 22.14

• Dietary specialization, pollinator specialization, sexual selection, and dispersal ability all influence speciation rates. Review Figures 22.15, 22.16

• Evolutionary radiation refers to the rapid proliferation of descendant species from a single ancestor species. This often occurs when a species colonizes a new area with unoccupied ecological niches. If evolutionary radiation results in an array of species that live in a variety of environments and differ in the characteristics they use to exploit those environments, it is referred to as adaptive radiation.