Final answer:
Genetic drift causes allele frequency changes due to random chance, often in small populations, and can lead to the loss of genetic variation through events like the bottleneck and founder effects. Natural selection, in contrast, favors alleles that confer survival and reproductive advantages, leading to increased population fitness and adaptation to the environment.
Step-by-step explanation:
Genetic drift and natural selection are both mechanisms of evolution that affect a population's allele frequencies, but they operate differently. Genetic drift refers to changes in allele frequencies due to random chance events, which are more pronounced in small populations. For example, if an individual with unique alleles dies before reproducing purely by chance, those alleles may be lost, resulting in a change in the population's genetic makeup. This can happen through the bottleneck effect, where a disaster reduces a population to a small number randomly, or the founder effect, where a new population is started by a small number of individuals.
On the other hand, natural selection involves alleles being favored or disfavored based on the reproductive success they confer to their bearers. Alleles that lead to traits enhancing survival and reproduction tend to increase in frequency over time, while those that are harmful tend to decrease. Unlike genetic drift, natural selection is not random and tends to increase the fitness of the population over time.
While genetic drift can lead to the loss of genetic variation and sometimes the fixation of deleterious alleles, natural selection often results in populations becoming better adapted to their environments. Both mechanisms can operate simultaneously and are important in shaping the genetic structure of populations, but their outcomes can be quite different.