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    Evolution3 Evolution3 Presentation Transcript

    • Speciation The Process of Evolution
    • Speciation
      • Formation of a 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
    • What is a Species?
      • 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
    • 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.
    • Anagenesis vs. Cladogenesis
    • 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
    • Isolation
    • Geographic Isolation
      • Biogeography of 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.
    • Allopatric vs. Sympatric
    • Allopatric Barriers
    • Geographic Isolation
    • 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
    • Reproductive Isolation
    • Prezygotic Isolation
      • Impedes mating 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
    • Behavioral Isolation: Courtship Barrier
    • Postzygotic Barriers
      • Hybrid inviability
        • offspring don’t make it
      • Hybrid sterility
        • e.g. mules
      • Hybrid breakdown
        • F 2 are sterile or weak
    • Introgression
      • Alleles pass a reproductive barrier when a fertile hybrid mates with a parent species
      • Increases variation
      • Rare
        • 2 species remain distinct
    • 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
    • Adaptive Radiation
    • 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
    • Gradualism vs. Punctuated Equilibrium