Final answer:
Option D is correct; genetic drift reduces genetic variation within a population and can lead to divergence between populations. It is a random process that does not necessarily increase individual fitness.
Step-by-step explanation:
Regarding the effects of genetic drift, option D, which states that it reduces genetic variation within a population and can lead to divergence between populations, is correct. Genetic drift refers to random fluctuations in allele frequencies in a population, especially in small populations. It can indeed lead to decreased genetic variability within a population because some alleles may become fixed while others may be lost entirely. Furthermore, when populations are separated, these random changes can cause populations to evolve differently, leading to divergence.
It is important to note that genetic drift does not necessarily lead to higher fitness of individuals in a population, as the changes in allele frequencies are random and not directed towards an advantageous state. Therefore, option A is incorrect. Additionally, genetic drift is one of several factors that can cause deviations from the Hardy-Weinberg equilibrium, potentially leading to microevolution.