Genetic drift is the change in frequency of existing alleles that occurs because random sampling alters the ratio of variants. This process happens purely by chance without any organism being better or worse than another.
Sewall Wright coined the term genetic drift in 1929 to describe directed processes like natural selection before refining the concept. Random drift by means of sampling error later came to be known as the Sewall-Wright effect.
The Wright-Fisher model assumes non-overlapping generations where each copy of a gene is drawn independently from the previous pool. The Moran model allows for overlapping generations where one individual reproduces while another dies during each time step, causing genetic drift to run twice as fast.
Numbers plummeted from about 100 million birds in 1900 to roughly 50 birds by the 1990s due to hunting and habitat destruction. This decline resulted from environmental events contracting the group to a significantly smaller size over a short period.
Motoo Kimura rekindled the debate with his neutral theory of molecular evolution in 1968. He claimed most genetic changes are caused by genetic drift acting on neutral mutations rather than natural selection alone.