From the ever eloquent Today In Science History:
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Wright is best known for his concept of genetic drift, called the Sewell Wright effect - that when small populations of a species are isolated, out of pure chance the few individuals who carry certain relatively rare genes may fail to transmit them. The genes may therefore disappear and their loss may lead to the emergence of new species, although natural selection has played no part in the process.
Check out genetic drift at The Biology Project at The University of Arizona.
Learn more about Wright HERE. Image from HERE