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 and INTERACTIVE TUTORIAL 17.1
- Reproductive isolation increases with increasing genetic divergence between populations.
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, ANIMATED TUTORIAL 17.1
- 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). See ANIMATED TUTORIAL 17.2
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 Figures 17.8 and 17.10
- Hybrid zones may persist between species with incomplete reproductive isolation. Review Figure 17.12 and WORKING WITH DATA 17.1
See WEB ACTIVITY 17.1 for a concept review of this chapter