Adaptive Wright Landscape and Shifting-Balance Theory s vs. Fisher Theory of Mass Selection s Readings from Futuyma (1998) Evolutionary Biology
Fitness Directional selection Adaptive landscapes Interaction of selection and genetic drift Multiple loci in adaptive landscapes Shifting balance theory pg. 366-369 pg. 375-381 pg. 392 pg. 392-393 pg. 402-403 pg. 408-409 *** read twice or more
Evolution : Change in gene frequency over time.
Forces of evolution (agents that change gene frequencies): - natural selection (deterministic changes) - genetic drift (random or stochastic changes) - gene flow/migration - mutation - recombination * *some argue that recombination is a force, others believe it only reshufflesgenetic variation
Adaptive Evolution : The process of genetic change due to natural selection, whereby a population
becomes better suited to some feature(s) of its environment. Both Wright and Fisher state that natural selection is a quintessential element of adaptive evolution. They differ on the relative importance of the other forces of evolution. Wright believed genetic drift and gene flow were more important; Fisher believed mutation was more important. Some more definitions: Fitness Success of an individual (w) or a population (w) in reproducing. Directional selection An advantageous allele increases from a very low frequency (invades) and spreads throughout the population. Random genetic drift Random changes in the frequency of an allele(s) or genotype(s) within a population.
Adaptive Landscape : An adaptive landscape is a surface in multidimensional space (analogous to
a mountain range) that represents the mean fitness of a population (not the fitness of a genotype). An individual is represented as a point on the surface (mountain) and a population is represented as a cloud of points. Adaptive landscape is probably the most common metaphor in evolutionary genetic[s] Futuyma (1998) Evolutionary Biology pg. 403 Evolution may be envisioned as the movement of a population of points (individuals) on the w surface (adaptive landscape). The points move up-slope until it arrives at the peak (mountain top).
Shifting-Balance Theory : Adaptation is shifting-balance between evolutionary forces and can be
grouped in three phases: Random genetic drift allows subpopulations (demes) explore the adaptive Phase landscape II: Demic Selection Selection moves demes to the top of the adaptive peaks Phase III: Interdemic selection Demes compete with each other and most successful demes spread throughout landscape Phase I: Exploration
Wright-Fisher Controversy
Wright shifting balance theory is often contrasted to Fisher (1930) mass selection theory where most s s adaptive evolution results from Darwinian selection on large populations. Wright Origin of adaptive novelty in a constantly changing environment Combination of local natural selection, random genetic drift, migration, and interdemic selection Small, subdivided populations Epistasis and pleiotropy; contextdependence of allelic effects Inevitable by-product of local adaptation in epistatic systems Fisher Refinement of existing adaptation in a stable or slowly changing environment Mutation and natural selection
Central problem of evolutionary theory Major processes of evolutionary change Ecological context of evolution Genetic basis of evolutionary change Process of speciation
Large, panmictic populations Additive genetic effects; contextindependence of allelic effects Disruptive or locally divergent selection
(from Wade & Goodnight 1998)
More References
Hartl & Clark (1997) Principles of Population Genetics adaptive topography & random genetic drift Interdeme selection & shifting balance theory pg. 236 pg. ...