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 :)