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Speciation

Speciation. The formation of new species. The Biological Species Concept. In evolutionary terms a species is a group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring; they do not breed successfully with other populations.

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Speciation

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  1. Speciation The formation of new species.

  2. The Biological Species Concept • In evolutionary terms a species is a group of populations whose members have the potential to interbreed in nature and produce viable, fertile offspring; they do not breed successfully with other populations

  3. Reproductive Isolation • Reproductive isolation is the existence of biological factors (barriers) that impede two species from producing viable, fertile offspring • Reproductive isolation can be classified by whether factors act before or after fertilization

  4. Prezygotic barriers block fertilization from occurring by: • Impeding different species from attempting to mate – courtship rituals, timing, and/or habitat • Preventing the successful completion of mating - Morphological differences can prevent successful mating • Hindering fertilization if mating is successful - Sperm of one species may not be able to fertilize eggs of another species

  5. Postzygotic barriers prevent the hybrid zygote from developing into a viable, fertile adult: • Reduced hybrid viability - Genes of the different parent species may interact and impair the hybrid’s development • Reduced hybrid fertility - Even if hybrids are vigorous, they may be sterile • Hybrid breakdown - Some first-generation hybrids are fertile, but when they mate with another species or with either parent species, offspring of the next generation are feeble or sterile

  6. Fig. 24-4a Prezygotic barriers Mechanical Isolation Habitat Isolation Temporal Isolation Behavioral Isolation Individuals of different species Mating attempt (e) (f) (a) (c) (d) (b)

  7. Fig. 24-4i Prezygotic barriers Postzygotic barriers Reduced Hybrid Viability Reduced Hybrid Fertility Hybrid Breakdown Gametic Isolation Viable, fertile offspring Fertilization (h) (g) (l) (i) (j) (k)

  8. Speciation • Reproductive barriers may lead to speciation. • Speciation, the origin of new species, is at the focal point of evolutionary theory

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

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

  11. 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

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

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

  14. 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.

  15. 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

  16. 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

  17. 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

  18. 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.

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

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