Natural Selection in Oysters

Oysters provide a clear, recent example of natural selection caused by disease, acting as a selection pressure.In 1957 a parasitic disease called MSX, caused by the single-celled parasite Haplosporidium nelsoni, appeared among eastern oysters (Crassostrea virginica) in Delaware Bay on the east coast of the USA. It killed the large majority of the oysters in the bay within a few years.The oyster population was not identical. Because of existing genetic variation, a small number of oysters had alleles that gave them some resistance to the parasite. The events that followed match the steps of natural selection:1. The MSX parasite acted as a strong selection pressure.2. Oysters with resistance alleles were more likely to survive the infection.3. The survivors spawned and passed on their resistance alleles to the next generation.4. Over the following decades the frequency of resistance alleles increased, and the wild population of the bay became much more resistant to MSX than oysters from areas that had never been exposed.Oyster breeders have used the same idea in artificial selection: by breeding only from surviving oysters, they produced disease-resistant strains much faster than nature did.This example shows several key ideas. The resistance alleles were already present before the disease arrived; the parasite did not create them. The population lost most of its individuals, so genetic diversity fell, but the survivors were better adapted to an environment containing the parasite.