Concept 17.1   Species Are Reproductively Isolated Lineages on the Tree of Life

• 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.
• Lineage species concepts, which recognize independent evolutionary lineages as species, allow biologists to consider species over evolutionary time.

Concept 17.2   Speciation Is a Natural Consequence of Population Subdivision

• Genetic divergence results from the interruption of gene flow within a population.
• The Dobzhansky–Muller model describes how reproductive isolation between two descendant lineages can develop through the accumulation of incompatible genes or chromosomal arrangements. Review Figure 17.3, Figure 17.4 and ANIMATED TUTORIAL 17.1
• Reproductive isolation increases with increasing genetic divergence between populations. Review Figure 17.5

Concept 17.3   Speciation May Occur through Geographic Isolation or in Sympatry

• 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 Figure 17.6, Figure 17.7 and ANIMATED TUTORIAL 17.2
• Sympatric speciation results when two species diverge in the absence of geographic 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 combines the chromosomes of two species (allopolyploidy). Review ANIMATED TUTORIAL 17.3

Concept 17.4   Reproductive Isolation Is Reinforced When Diverging Species Come into Contact

• 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 Figure 17.9, Figure 17.10, and Figure 17.12
• Hybrid zones may persist between species with incomplete reproductive isolation. Review Figure 17.13

See ACTIVITY 17.1 for a concept review of this chapter.