Why was the reduction of the bison population to under 1,000 individuals a bottleneck event for the species? a. it drastically reduced the gene pool of the species. b. it reduced the territorial range available to the species. c. foreign genes were introduced by crossbreeding with cattle. d. it reduced the prevalence of recessive traits.

The right answer is A.

A population bottleneck (also known as a genetic bottleneck) is, in the study of biological evolution, an event corresponding to the fact that a substantial proportion of the population of a species disappears or is prevented from to reproduce.

Population bottlenecks (or bottlenecks) thus reduce genetic diversity, and hence the ability of people to adapt to new environmental constraints such as climate change, or a decline in available resources. This genetic drift can eliminate certain alleles that could, by natural selection, allow the species to adapt.