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Fig. 24-5

Fig. 24-5. Speciation can occur in two ways: Allopatric Speciation Sympatric Speciation. (a) Allopatric speciation. (b) Sympatric speciation. Allopatric (“Other Country”) Speciation.

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Fig. 24-5

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  1. Fig. 24-5 Speciation can occur in two ways: Allopatric Speciation Sympatric Speciation (a) Allopatric speciation (b) Sympatric speciation

  2. Allopatric (“Other Country”) Speciation • In allopatric speciation, gene flow is interrupted or reduced when a population is divided into geographically isolated subpopulations

  3. The Process of Allopatric Speciation • The definition of barrier depends on the ability of a population to disperse • Separate populations may evolve independently through mutation, natural selection, and genetic drift

  4. Fig. 24-6 A. harrisi A. leucurus Allopatric Speciation of antelope squirrels on opposite rims of the Grand Canyon.

  5. Sympatric (“Same Country”) Speciation • In sympatric speciation, speciation takes place in geographically overlapping populations

  6. Polyploidy • Polyploidy is the presence of extra sets of chromosomes due to accidents during cell division • Polyploidy is much more common in plants than in animals 2n 2n = 6 4n = 12 4n Failure of cell division after chromosome duplication gives rise to tetraploid tissue. Gametes produced are diploid.. Offspring with tetraploid karyotypes may be viable and fertile.

  7. Allopolyploid • An allopolyploid is a species with multiple sets of chromosomes derived from different species Species B 2n = 4 Unreduced gamete with 4 chromosomes Unreduced gamete with 7 chromosomes Hybrid with 7 chromosomes Meiotic error Viable fertile hybrid (allopolyploid) 2n = 10 Normal gamete n = 3 Normal gamete n = 3 Species A 2n = 6

  8. Habitat Differentiation • Sympatric speciation can also result from the appearance of new ecological niches • For example, the North American maggot fly can live on native hawthorn trees as well as more recently introduced apple trees

  9. Sexual Selection EXPERIMENT Monochromatic orange light • Sexual selection can drive sympatric speciation • Sexual selection for mates of different colors has likely contributed to the speciation in cichlid fish in Lake Victoria Normal light P. pundamilia P. nyererei

  10. Allopatric and Sympatric Speciation: A Review • In allopatric speciation, geographic isolation restricts gene flow between populations • Reproductive isolation may then arise by natural selection, genetic drift, or sexual selection in the isolated populations • Even if contact is restored between populations, interbreeding is prevented • In sympatric speciation, a reproductive barrier isolates a subset of a population without geographic separation from the parent species • Sympatric speciation can result from polyploidy, natural selection, or sexual selection

  11. Adaptive Radiations • Adaptive radiation is the evolution of diversely adapted species from a common ancestor upon introduction to new environmental opportunities

  12. Worldwide Adaptive Radiations • Mammals underwent an adaptive radiation after the extinction of terrestrial dinosaurs • The disappearance of dinosaurs (except birds) allowed for the expansion of mammals in diversity and size • Other notable radiations include photosynthetic prokaryotes, large predators in the Cambrian, land plants, insects, and tetrapods

  13. Regional Adaptive Radiations • Adaptive radiations can occur when organisms colonize new environments with little competition • The Hawaiian Islands are one of the world’s great showcases of adaptive radiation

  14. Fig. 25-18 Close North American relative, the tarweed Carlquistia muirii 1.3 million years MOLOKAI KAUAI 5.1 million years Dubautia laxa MAUI OAHU 3.7 million years Argyroxiphiumsandwicense LANAI HAWAII 0.4 million years Dubautia waialealae Plants all descended from an ancestral tarweed that arrived on the islands about 5 million years ago Dubautia scabra Dubautia linearis

  15. Patterns Of Evolution • The fossil record includes examples of species that appear suddenly, persist essentially unchanged for some time, and then apparently disappear • Niles Eldredge and Stephen Jay Gould coined the term punctuated equilibrium to describe periods of apparent stasis punctuated by sudden change • The punctuated equilibrium model contrasts with a model of gradual change in a species’ existence - known as gradualism. Darwin supported the idea of slow gradual change. • Transition species found in the fossil record are a key component to gradualism.

  16. Fig. 24-17 (a) Punctuated pattern Time (b) Gradual pattern

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