Friday, January 11, 2013

Notes on Simpson's Tempo and Mode in Evolution

Chapter II: Determinants of Evolution (p. 93-96)

  • Variability: 
    • High variability in groups is usually a result of other factors
    • Can lead to rapid differentiation on low taxonomic levels but cannot be responsible for creating new high taxonomic levels
    • Cannot be responsible for moderate-high rates of evolution
    • Most lineages show a constant variability over evolutionary time and a rapid deviation to increase variability
    • Maximum rates of evolution usually have low rather than high variability
  • Rate of mutation: mutation necessary for evolution to occur
    • Mutation rate NOT same as rate of evolution
  • Character of mutation
    • Single mutations with large, discrete phenotypic effects usually unimportant in evolution
    • Saltation (large step change) could arise from practically impossible genetic scenarios
    • Mutations recognizable in sequence usually have no/little phenotypic effect
    • Many small mutations consistent with high rates of evolution--small fluctuations in developmental fields
  • Length of generation
    • Temporal rate of evolution should vary inversely with generation time
    • May influence unusually high rates of evolution
  • Population Size
    • Large populations: evolution is extremely slow under selection and evolution is proportional to selection intensity--tend to be at genetic equilibrium even though more variable which is not good for rapid evolution
    • Small populations: more susceptible to drift--maximum rates of evolution seen in small populations but it is nonadaptive and most likely lead to extinction or rare adaptive reorientation
  • Selection: has direction and intensity--crucial factor for evolution but may be ineffective at times
    • Direction can either be centripetal (concentrate population to single modal type), centrifugal (diverge population), or linear (shift modal type to one position or another). 
Chapter III: Micro-evolution, macro-evolution, and mega-evolution (p. 97-124)
  • Investigates the question of saltations and their likelihood
  • Discusses many discontinuities in fossil record
  • Mega-evolution normally evolves among small populations that become pre-adaptive and evolve continuously to different ecological conditions
    • Large population fragments and new mutations randomly fix (rarely preadaptive)
Chapter IV: Low rate and high rate lines (p. 147-148)
  • Bradytely: slower than standard
    • Not dependent on mutation rate
    • Usually result from rapid evolution--not necessarily primitive
    • Characters are predominantly adaptive
  • Horotely: standard rate of evolution for an organism
  • Trachytely: faster than standard--either become extinct or have massive adaptive 
  • More recent rapidly evolving groups more vulnerable to extinction
  • Less specialized bradytely survive longer than more specialized
Chapter V: Inertia, Trend, and Momentum (p. 177-179)
  • Orthogenesis (rectilinear evolution): tendency for phyla to continue to evolve in same direction for considerable periods of time [only descriptive statement]
    • Typical of large populations evolving at moderate rates
    • Not simple, linear, unbranched evolution--can have many changes in rates throughout time 
    • Most linearity due to heredity
    • Direction of mutation doesn't really have anything to do with direction of evolution
  • Response to selection is not instantaneous, and inertia (lage in following a shifting optimum), is an important element in evolution
Chapter VI: Organism and Environment (p. 181-196)
  • Adaptive zone: organism's environment and everything involved in the situation in which the organism is an element
    • Can evolve!
Chapter VII: Modes of Evolution (p. 216-217)
  • Speciation: local differentiation of two or more groups within a more widespread population
    • Low taxonomic level
    • Local adaptation and random segregation
  • Phyletic evolution: sustained, directional shift of the average characters of populations
    • Post adaptation-- little random change
  • Quantum evolution: relatively rapid shift of biotic population in disequilibrium to an equilibrium distinctly unlike an ancestral condition
    • High taxonomic level
    • Preadaptation (usually preceded by inadaptive change)
Yay, I finished a book :)

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