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  1. 1. Speciation The Process of Evolution
  2. 2. Speciation <ul><li>Formation of a new species </li></ul><ul><li>Species : </li></ul><ul><ul><li>a population that can breed freely and produce fertile offspring </li></ul></ul><ul><li>Speciation often occurs when part of the population is isolated from another part </li></ul><ul><ul><li>Selective pressures of the environment in one area may be different from pressures in another area </li></ul></ul>
  3. 3. What is a Species? <ul><li>Definition : </li></ul><ul><li>Morphospecies - based on appearance </li></ul><ul><li>Biologic species - a population that can breed freely and produce fertile offspring </li></ul><ul><li>The largest unit of population in which gene flow is possible </li></ul><ul><li>Limitations: </li></ul><ul><ul><li>doesn’t work for asexual organisms </li></ul></ul><ul><ul><li>extinct life forms </li></ul></ul><ul><ul><li>populations that are geographically isolated - sometimes call subspecies </li></ul></ul><ul><li>No clear answer; idea is arbitrary </li></ul>
  4. 4. Patterns of Speciation <ul><li>Fossil record shows 2 patterns: </li></ul><ul><li>Anagenesis ( phyletic evolution) </li></ul><ul><ul><li>the transformation of an unbranched lineage of organisms, sometimes creating an organism different enough to be a new species </li></ul></ul><ul><li>Cladogenesis </li></ul><ul><ul><li>branching evolution; budding of one or more new species from a parent species that continues to exist. </li></ul></ul>
  5. 5. Anagenesis vs. Cladogenesis
  6. 6. Causes of Speciation <ul><li>Speciation often occurs when part of the population is isolated from another part </li></ul><ul><li>Geographic Isolation </li></ul><ul><ul><li>most common </li></ul></ul><ul><ul><li>a physical barrier develops (changing course of a river; separation of an island) </li></ul></ul><ul><ul><li>Selective pressures in one area are different from pressures in another area </li></ul></ul><ul><li>Reproductive Isolation </li></ul><ul><ul><li>another form of isolation </li></ul></ul>
  7. 7. Isolation
  8. 8. Geographic Isolation <ul><li>Biogeography of Speciation </li></ul><ul><li>Classified according to geographic relationship between new and old species </li></ul><ul><li>Sympatric </li></ul><ul><ul><li>population becomes reproductively isolated in the midst of the parent population </li></ul></ul><ul><ul><li>ranges of new and old species overlap. </li></ul></ul><ul><li>Allopatric </li></ul><ul><ul><li>species are physically separated </li></ul></ul><ul><ul><li>more likely in small populations </li></ul></ul><ul><li>Adaptive radiation is allopatric : </li></ul><ul><ul><li>emergence of numerous species from a common ancestor that spreads to several new environments. </li></ul></ul>
  9. 9. Allopatric vs. Sympatric
  10. 10. Allopatric Barriers
  11. 11. Geographic Isolation
  12. 12. Reproductive Isolation <ul><li>Example: organisms breed at different times </li></ul><ul><li>Reproductive barriers are of 2 types: </li></ul><ul><li>Prezygotic </li></ul><ul><ul><li>before the formation of fertilized eggs </li></ul></ul><ul><ul><li>impedes mating or fertilization </li></ul></ul><ul><li>Postzygotic </li></ul><ul><ul><li>after </li></ul></ul>
  13. 13. Reproductive Isolation
  14. 14. Prezygotic Isolation <ul><li>Impedes mating or fertilization </li></ul><ul><li>Habitat isolation </li></ul><ul><ul><li>not geographically separated, but occupy different niches within an area, e.g. trees versus ground </li></ul></ul><ul><li>Temporal isolation </li></ul><ul><ul><li>breed at different times </li></ul></ul><ul><li>Behavioral isolation </li></ul><ul><ul><li>don’t produce appropriate courtship signals </li></ul></ul><ul><li>Mechanical isolation </li></ul><ul><ul><li>anatomically incompatible </li></ul></ul><ul><li>Gametic isolation </li></ul><ul><ul><li>mating occurs but gametes rarely fuse to form zygotes </li></ul></ul>
  15. 15. Behavioral Isolation: Courtship Barrier
  16. 16. Postzygotic Barriers <ul><li>Hybrid inviability </li></ul><ul><ul><li>offspring don’t make it </li></ul></ul><ul><li>Hybrid sterility </li></ul><ul><ul><li>e.g. mules </li></ul></ul><ul><li>Hybrid breakdown </li></ul><ul><ul><li>F 2 are sterile or weak </li></ul></ul>
  17. 17. Introgression <ul><li>Alleles pass a reproductive barrier when a fertile hybrid mates with a parent species </li></ul><ul><li>Increases variation </li></ul><ul><li>Rare </li></ul><ul><ul><li>2 species remain distinct </li></ul></ul>
  18. 18. Post Speciation Evolution <ul><li>Divergent Evolution </li></ul><ul><ul><li>Process by which related organisms become less alike </li></ul></ul><ul><ul><li>occurs after speciation </li></ul></ul><ul><ul><li>at first 2 new species are very similar, but over time become more & more different. </li></ul></ul><ul><li>Adaptive radiation is a special type of divergent evolution </li></ul><ul><ul><li>Many new species from a single parent species </li></ul></ul>
  19. 19. Adaptive Radiation
  20. 20. Timing of Evolution <ul><li>Most scientists accept natural selection as the process of evolution </li></ul><ul><li>The timing is controversial </li></ul><ul><li>Gradualism </li></ul><ul><ul><li>the traditional view </li></ul></ul><ul><ul><li>a slow, steady accumulation of changes, leads to new species </li></ul></ul><ul><li>Punctuated Equilibrium </li></ul><ul><ul><li>long periods of inactivity followed by big jumps </li></ul></ul><ul><li>Fossil record provides evidence that the pace of evolution varies </li></ul><ul><ul><li>The same evidence is used to support different ideas </li></ul></ul><ul><ul><li>Could be some of both </li></ul></ul>
  21. 21. Gradualism vs. Punctuated Equilibrium