23.1 Species are the basic units of evolution.
The Nature of Species
• Any species concept must be able to account for species that occur together and for populations of the same species that are geographically separated. (p. 472)
• Although sympatric species occur together, they are usually phenotypically different and utilize different parts of the shared habitat. (p. 472)
• Ernst Mayr developed the biological species concept which defines species as: "groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups." (p. 473)
• Reproductive isolating mechanisms prevent genetic exchange between species. (p. 473)
23.2 Species maintain their genetic distinctiveness through barriers to reproduction.
Prezygotic Isolating Mechanisms
• Prezygotic isolating mechanisms prevent the formation of zygotes. (p. 474)
• Two species may not hybridize because they occupy different portions of an area and do not encounter each other. (p. 474)
• Related species often maintain distinctiveness due to behavioral differences such as courtship rituals. (p. 474)
• Sympatric species may utilize visual and sensory communication, pheromone reception, and electroreception to avoid mating with the wrong species. (pp. 474-475)
• Temporal isolation, mechanical isolation, and prevention of gamete fusion are other types of prezygotic isolating mechanisms. (p. 475)
Postzygotic Isolating Mechanisms
• Postzygotic isolating mechanisms prevent proper functioning of zygotes after they form. (p. 476)
• Hybrids that are weaker than their parents will almost certainly be eliminated in nature. (p. 476)
• Some hybrids are healthy, but infertile. (p. 476)
Problems with the Biological Species Concept
• Hybridization is more common than previously believed. (p. 477)
• Reproductive isolation may not be the only force maintaining species integrity. (p. 477)
• An alternative explanation is that species distinctions are maintained by natural selection. (p. 477)
• Due to extreme diversity, no single species definition may be adequate. (p. 477)
23.3 We have learned a great deal about how species form.
Reproductive Isolation May Evolve as a By-Product of Evolutionary Change
• Partial reproductive isolation might lead to reinforcement. Individuals in parental populations that avoid hybridization would be more successful at transmitting their genes to future generations. This would lead to continual improvement of prezygotic isolating mechanisms until the populations are completely reproductively isolated. (p. 478)
• Partial isolation might also lead to the loss of genetic distinctiveness due to gene flow. (p. 478-479)
• Genetic drift may also cause reproductive isolation, but such isolation usually evolves more rapidly in the presence of selective pressures. (p. 479)
The Geography of Speciation
• In order for speciation to occur, similar populations must diverge, and then reproductive isolation must evolve. (p. 480)
• Allopatric populations are much more likely to diverge into separate species. (p. 480)
• Polyploidy, either by autopolyploidy or allopolyploidy, leads to immediate sympatric speciation in many plants and some animals. (pp. 480-481)
• Disruptive selection may lead to sympatric speciation, but this is controversial. (p. 481)
23.4 Clusters of species reflect rapid evolution.
• Adaptive radiation occurs when groups of closely related species evolve from a common ancestor by adapting to different habitats. (p. 482)
Hawaiian Drosophila
• When the ancestors of Drosophila reached the Hawaiian Islands, they encountered many empty habitats. Rampant speciation and ecological opportunity led to intense insect diversity. (p. 482)
Darwin's Finches
• Darwin's finches are derived from a single mainland species. The 14 species found on the islands comprise four groups: ground finches, tree finches, warbler finches, and vegetarian finches. (p. 483)
Lake Victoria Cichlid Fishes
• Many of the varied species originated after the lake dried down and isolated local populations. (p. 484)
• Ecological and morphological diversity was quite high. (p. 484)
• Widespread extinction occurred after the predatory Nile perch was introduced into the lake. (p. 484)
New Zealand Alpine Buttercups
• Ranunculus speciation and diversification have been fostered by repeated cycles of glacial advance and retreat. (p. 485)
The Pace of Evolution
• Darwin advocated slow evolutionary change (gradualism). (p. 486)
• Niles Eldredge and Stephen Jay Gould argued that species went through long periods of little change (stasis) and then experienced rapid bursts of evolutionary change over short periods of time during speciation events. (p. 486)
Speciation and Extinction Through Time
• Over the long term, speciation has outpaced extinction. (p. 487)
• Five major mass extinctions have interspersed the long-term increase, and have been followed by large-scale, although slow, periods of diversification. (p. 487)
• Human activities may be leading to a sixth major extinction. (p. 487)
The Future of Evolution
• Humans are changing the patterns of natural selection, and some of these changes will be so drastic that many species will not be able to adapt quickly enough to survive. (p. 488)
• Decreased population size and increased geographic isolation could also remove the homogenizing effect of gene flow and lead to moderate to high levels of speciation in species that are able to avoid extinction. (p. 488)