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Fig. 24.3

Time of Day Time of Year. Flowers Snails. Bullfrog x Leopard Frog. Courtship Sounds/Songs. Plants Broadcast Spawners. Horse (2n=64) x Donkey (2n=62)  Mule (2n=63). Fig. 24.3. Reproductive Isolation Limitations of Biological Species Concept

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Fig. 24.3

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  1. Time of Day Time of Year Flowers Snails Bullfrog x Leopard Frog Courtship Sounds/Songs Plants Broadcast Spawners Horse (2n=64) x Donkey (2n=62)  Mule (2n=63) Fig. 24.3

  2. Reproductive Isolation • Limitations of Biological Species Concept • Mayr’s definition emphasizes reproductive isolation; may not work in all situations • Ex: Classifying fossil organisms • Ex: Species that reproduce asexually [prokaryotes, some protists, fungi, plants (e.g. bananas), animals (e.g. fishes, lizards)] • Ex: Multiple species are inter-fertile but remain distinct (e.g. orchids)

  3. Speciation • Occurs when a population becomes reproductively isolated from rest of species • May be allopatric or sympatric • Allopatric Speciation • Population becomes geographically separated • Over time, mutation, genetic drift, natural selection  genetic divergence • Thought to be responsible for development of most new animal species • How do populations become isolated?

  4. Fig. 24.5

  5. Speciation • Allopatric Speciation • Geographical barriers • Land bridges form, separating aquatic populations (e.g. Isthmus of Panama) • Land masses separate or split off from continents (e.g. South America & Africa) • Mountain ranges form • Water levels in water bodies become lower, creating multiple smaller pools • Rivers change course (Ex: oxbow lakes) • Glaciation occurs • Islands form and are colonized (e.g. Galàpagos, Hawaii, Madagascar) • Note: Geographic barriers for some species aren’t barriers for others • Ex: Birds and many insects can fly between isolated patches of habitat • Ex: Some fishes can swim long distances • Ex: Airborne pollen and drifting gametes in the ocean can be transported long distances

  6. Speciation • Allopatric Speciation • Conditions Favoring Allopatric Speciation • Typically occurs at edges of parent population’s range • Splinter population (peripheral isolate) may be good candidate for speciation because: • Gene pool different from parent population • Likely to represent extreme of genotypic range • Speciation more likely if founder population small • Ex: Harris’ and white-tailed antelope squirrels on rims of Grand Canyon • Genetic drift within peripheral isolate • Can lead to rapid divergence from parent population • Natural selection • Diversifying or directional selection under conditions at extremes tolerated by parent population Fig. 24.6

  7. Fig. 24.10

  8. Speciation • Sympatric Speciation • Population becomes reproductively isolated without geographic separation • May be common in plants; importance in animals less clear • Plants • Autopolyploidy • Results from error in mitosis

  9. Speciation • Sympatric Speciation • Plants • Autopolyploidy • Results from error in mitosis • Allopolyploidy • Error in meiosis + hybridization

  10. Fig. 24.11

  11. Speciation • Sympatric Speciation • Plants • Allopolyploidy • Allopolyploids typically can’t produce fertile offspring with either parent (incompatible chromosome numbers) • If population of allopolyploids becomes established, typically one of three outcomes: • New species unable to compete successfully; goes extinct • New species competes successfully; coexists with parent species • New species competes very successfully; causes extinction of one or both parent species

  12. Speciation • Sympatric Speciation • Plants • Allopolyploidy • May be very common in plants • Up to 80% of flowering plant species are polyploid • May account for 25-50% of plant species • Mechanism for very rapid speciation (single generation) • May account for rapid radiation of plants in fossil record and high diversity of flowering plants (>290,000 species)

  13. Speciation • Sympatric Speciation • Animals • Mechanisms of sympatric speciation less well understood than in plants • Polyploidy usually lethal • Habitat differentiation • Ex:North American apple maggot fly (article) • Mutation  short-term isolation reinforced by non-random mating (sexual selection) • Ex: African cichlids

  14. Fig. 24.12

  15. Speciation • Allopatric vs. Sympatric Speciation • Animals (usu. allopatric) • Isolating mechanisms? • Plants (usu. sympatric) • Isolating mechanisms?

  16. Time of Day Time of Year Flowers Snails Bullfrog x Leopard Frog Courtship Sounds/Songs Plants Broadcast Spawners Horse (2n=64) x Donkey (2n=62)  Mule (2n=63) Fig. 24.3

  17. Speciation • Adaptive Radiation • Evolution of many diversely adapted species from common ancestor • Island chains offer unutilized habitat and open ecological niches • Ex: Colonization of Hawaii by honeycreepers • Ex: Silversword alliance in Hawaii

  18. Fig. 25.20

  19. Speciation • Adaptive Radiation • Occurs when niche space is available • Ex: Radiation of mammals after K/T extinction • Radiation events often are associated with the appearance of novel features • Why? • Ex: Shells & skeletons first appeared at beginning of Paleozoic (may have facilitated radiation)

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