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Vertical Integration of Evolutionary Mechanisms: Pelvic Reduction in the Threespine Stickleback

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Published on May 12, 2011

Natural selection is the primary cause of adaptation and the evolution of biodiversity. Darwin thought that natural selection acts on minor differences and causes slow evolution in such small steps that it is invisible in the present. Although rapid evolution of pesticide resistance by agricultural pests and of antibiotic resistance by bacteria is recognized, it may be due to unnatural selection imposed by humans. Is Darwin's view on evolutionary tempos correct, or is rapid evolution of pests and bacteria typical? Two evolutionary time series from threespine stickleback fish provide insights into evolutionary tempos within vertebrate lineages in response to natural environmental change. Samples from a fossil stickleback (Gasterosteus doryssus) lineage at 250-year intervals revealed dramatic evolution of pelvic reduction. Although average pelvic structure evolved gradually over a about 4,000 years, it resulted from changes in the relative frequencies of two extreme, alternative phenotypes. Similarly, annual samples from a modern stickleback (G. aculeatus) population since 1990 in an Alaskan lake that was colonized by marine stickleback in the 1980s revealed extensive contemporary evolution. Like pelvic structure in the fossil lineage, the average number of armor plates evolved gradually, but gradual change of average phenotypes resulted from change in the relative frequencies of two extreme, alternative phenotypes. Other traits in both the fossil and modern lineages evolved by a gradual shift in the distributions of phenotypes with minor differences, as envisioned by Darwin and his successors, but they all evolved rapidly. Analysis of evolutionary time series in modern and fossil lineages with fine temporal resolution are important both to determine how fast evolution typically occurs in natural populations and to integrate evolutionary tempos with mechanisms that are observable only in contemporary populations. It is already evident that evolution can be rapid and involve natural selection on major differences, but more work is needed to infer typical rates of evolution and how important major differences are.

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