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Darwin’s successor

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The 25th anniversary of the death of the geneticist Sewall Green Wright (1889–1988) falls on 3 March 2013. After graduating with a biology degree from the University of Illinois in 1912, he moved on to Harvard University, concentrating his research on mammalian genetics, in particular those of the guinea pig, before returning to Chicago to research and teach genetics.

He produced groundbreaking work, applying statistical analysis to genetics and introducing the question of causation to genetic variables, rather than treating them as chance events, illustrating the importance of the effects of combinations of genes rather than their individual effect.

He was the 20th century’s leading researcher on evolution, and formulated theories showing how the frequencies of alleles and genotypes could change in response to evolutionary pressures such as natural selection, mutation and migration.

Wright had a particular interest in inbreeding (his parents were first cousins) and random genetic drift in small populations, which seemed to defy the laws of natural selection. Genetic drift is caused by random chance events in small isolated populations, which produce genotypes that are less suited to their environments than those of their ancestors. He postulated that in a small gene pool desirable alleles may disappear by chance due to the random nature of inheritance. In large or migrating populations the sheer size of the gene pool means that favourable alleles will never disappear by chance and natural selection will prevail.

An example of genetic drift occurred in an Amish population in Pennsylvania. Two of the founder members were carriers for the rare Ellis-van Creveld syndrome. After several generations of isolation and inbreeding, most of the population were either carriers or sufferers of the syndrome.

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