Natural selection changes allele frequencies

Researchers had known that marine sticklebacks which colonized freshwater, an event that had happened naturally multiple times, will gradually lose their bony armor plating. This was presumed to be due to an increase in fitness sans armor when living in a freshwater environment, but armor is been maintained in the marine environment due to a fitness advantage. The armor is controlled mainly by a single locus, Eda. The allele of Eda which causes decreased armor is ancient and is segregating at low frequency in the natural marine populations.

Barrett, Rogers, and Schluter trapped marine armored sticklebacks and transplanted them into freshwater ponds. They observed the expected loss of armor in the now-freshwater fish. They tracked allele frequencies at the Eda locus as well as phenotypes of the fish over time to determine if the loss of armor was the result of positive selection. The combination of natural founding populations, natural environments, and phenotype & allelic correlations makes this experiment a particularly simple and elegant example of modern evolutionary research.

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