Chapter 20: Speciation and Macroevolution

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What is a Species?

• According to the biological species concept, a species consists of one or more populations whose members interbreed in nature to produce fertile offspring and do not interbreed with members of different species.
• One problem with the biological species concept is that it applies only to sexually reproducing organisms.
  • Also, although individuals assigned to different species do not normally interbreed, they may occasionally successfully interbreed.

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Reproductive Isolation

• Reproductive isolating mechanisms restrict gene flow between species.
• Prezygotic barriers are reproductive isolating mechanisms that prevent fertilization from taking place.
  • Temporal isolation occurs when two species reproduce at different times of the day, season, or year.
  • In habitat isolation two closely related species live and breed in different habitats in the same geographic area.
  • In behavioral isolation distinctive courtship behaviors prevent mating between species.
  • Mechanical isolation is due to incompatible structural differences in the reproductive organs of similar species.
  • In gametic isolation, gametes from different species are incompatible because of molecular and chemical differences.
• Postzygotic barriers are reproductive isolating mechanisms that prevent gene flow after fertilization has taken place.
• Hybrid inviability is the death of interspecific embryos during development.
  • Hybrid sterility prevents interspecific hybrids that survive to adulthood from reproducing successfully.
  • Hybrid breakdown prevents the offspring of hybrids that survive to adulthood and successfully reproduce from reproducing beyond one or a few generations.

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Speciation

• Speciation is the evolution of a new species from an ancestral population.
  • Allopatric speciation (e.g., in Death Valley pupfishes) occurs when one population becomes geographically isolated from the rest of the species and subsequently diverges.
  • Speciation is more likely to occur if the original isolated population is small because genetic drift is more significant in small populations than in large populations.
• Sympatric speciation does not require geographic isolation
• In plants sympatric speciation can result when a polyploid individual (one with more than two sets of chromosomes) is an allopolyploid hybrid derived from two species.
  • One example of sympatric speciation by allopolyploidy is the hemp nettle.
• Sympatric speciation occurs in animals, such as fruit maggot flies and cichlids, but how often it occurs and under what conditions remain to be determined.

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The Rate of Evolutionary Change

• According to the punctuated equilibrium model, evolution of species proceeds in spurts.
  • Short periods of active speciation are interspersed with long periods of stasis.
  • According to the phyletic gradualism model, populations slowly diverge from one another by the accumulation of adaptive characteristics within a population.

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Macroevolution

• Macroevolution concerns large-scale phenotypic changes in populations that typically warrant the placement of the populations in taxonomic groups at the species level and higher, that is, new species, genera, families, orders, classes, and even phyla, kingdoms, and domains.
• Evolutionary novelties may be due to changes during development, the orderly sequence of events that occurs as an organism grows and matures.
  • Slight genetic changes in regulatory genes could cause major structural changes in an organism.
• Evolutionary novelties may originate from preadaptations, structures that originally fulfilled one role but changed in a way that was adaptive for a different role.
  • Feathers are an example of a preadaptation.
• Allometric growth, varied rates of growth for different parts of the body, results in overall changes in the shape of an organism.
  • Examples include the expanded tail region of the ocean sunfish and the enlarged claw of the male fiddler crab.
• Paedomorphosis, the retention of juvenile characteristics in the adult, occurs because of changes in the timing of development.
  • Adult axolotl salamanders, with external gills and tail fins, are an example of paedomorphosis.
• Adaptive radiation is the process of diversification of an ancestral species into many new species.
  • Adaptive zones are new ecological opportunities that were not exploited by an ancestral organism.
  • When many adaptive zones are empty, colonizing species may rapidly diversify and exploit them.
  • Hawaiian honeycreepers and silverswords both underwent adaptive radiation after their ancestors colonized the Hawaiian Islands.
• Extinction is the death of a species.
  • When species become extinct, the adaptive zones that they occupied become vacant, allowing other species to evolve and fill those zones.
  • Background extinction is the continuous, low-level extinction of species.
  • Mass extinction is the extinction of numerous species and higher taxonomic groups in both terrestrial and marine environments.

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