What defines the amount of variation in the genetic material within all members of a population?

Genetic diversity is a term that refers to the variety of genes and alleles present within a population. It encompasses the variations in genetic makeup among individuals of the same species and is crucial for the adaptability and resilience of the population. Higher genetic diversity generally means that there is a wider range of traits available for selection in changing environments, leading to better survival and reproduction rates.

In contrast, genetic drift pertains to random changes in allele frequencies due to chance events, particularly in smaller populations, which can affect genetic variation but does not define the amount of variation itself. Natural selection is a process that acts on variations within a population, favoring traits that enhance survival and reproduction but does not directly measure the genetic variation present. Mutation rate describes how frequently new mutations occur in DNA, contributing to genetic diversity, but again, it is not a measure of the overall variation within the population.

Overall, genetic diversity is fundamental to understanding the genetic variation in a population and is key to the long-term viability of species, making it the best answer to the question asked.