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Evolution
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- 2. Speciation • Formation ofa new species • Species: – a population that can breed freely and produce fertile offspring • Speciation often occurs when part of the population is isolated from another part – Selective pressures of the environment in one area may be different from pressures in another area
- 3. What is aSpecies? • Definition : • Morphospecies - based on appearance • Biologic species - a population that can breed freely and produce fertile offspring • The largest unit of population in which gene flow is possible • Limitations: – doesn’t work for asexual organisms – extinct life forms – populations that are geographically isolated - sometimes call subspecies • No clear answer; idea is arbitrary
- 4. Patterns of Speciation •Fossil record shows 2 patterns: • Anagenesis (phyletic evolution) – the transformation of an unbranched lineage of organisms, sometimes creating an organism different enough to be a new species • Cladogenesis – branching evolution; budding of one or more new species from a parent species that continues to exist.
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- 6. Causes of Speciation •Speciation often occurs when part of the population is isolated from another part • Geographic Isolation – most common – a physical barrier develops (changing course of a river; separation of an island) – Selective pressures in one area are different from pressures in another area • Reproductive Isolation – another form of isolation
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- 8. Geographic Isolation • Biogeographyof Speciation • Classified according to geographic relationship between new and old species • Sympatric – population becomes reproductively isolated in the midst of the parent population – ranges of new and old species overlap. • Allopatric – species are physically separated – more likely in small populations • Adaptive radiation is allopatric : – emergence of numerous species from a common ancestor that spreads to several new environments.
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- 12. Reproductive Isolation • Example:organisms breed at different times • Reproductive barriers are of 2 types: • Prezygotic – before the formation of fertilized eggs – impedes mating or fertilization • Postzygotic – after
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- 14. Prezygotic Isolation • Impedesmating or fertilization • Habitat isolation – not geographically separated, but occupy different niches within an area, e.g. trees versus ground • Temporal isolation – breed at different times • Behavioral isolation – don’t produce appropriate courtship signals • Mechanical isolation – anatomically incompatible • Gametic isolation – mating occurs but gametes rarely fuse to form zygotes
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- 16. Postzygotic Barriers • Hybridinviability – offspring don’t make it • Hybrid sterility – e.g. mules • Hybrid breakdown – F2 are sterile or weak
- 17. Introgression • Alleles passa reproductive barrier when a fertile hybrid mates with a parent species • Increases variation • Rare – 2 species remain distinct
- 18. Post Speciation Evolution •Divergent Evolution – Process by which related organisms become less alike – occurs after speciation – at first 2 new species are very similar, but over time become more & more different. • Adaptive radiation is a special type of divergent evolution – Many new species from a single parent species
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- 20. Timing of Evolution •Most scientists accept natural selection as the process of evolution • The timing is controversial • Gradualism – the traditional view – a slow, steady accumulation of changes, leads to new species • Punctuated Equilibrium – long periods of inactivity followed by big jumps • Fossil record provides evidence that the pace of evolution varies – The same evidence is used to support different ideas – Could be some of both
- 21. Gradualism vs. PunctuatedEquilibrium