Final answer:
Genetic drift acts on genotypes by chance, whereas natural selection acts on phenotypes based on their adaptive value. Genetic drift can reduce genetic variation, while natural selection can lead to adaptations in populations.
Step-by-step explanation:
Genetic Drift and Natural Selection
Genetic drift acts on the genotypes of a population by chance, not by selection due to advantageous or disadvantageous traits. For example, when a natural disaster occurs, it may kill individuals randomly, not favoring specific genotypes; hence genetic drift can affect the genetic makeup of survivors irrespective of their phenotypes.
On the contrary, natural selection primarily acts on phenotypes, as it favors individuals with traits that provide a survival or reproductive advantage in a given environment. These traits are the outward expressions of an organism's genotype, and if they are heritable, the supporting alleles increase in frequency within the population.
Both mechanisms play a significant role in evolution, with natural selection driving the adaptation of populations to their environments, while genetic drift can lead to random fluctuations in allele frequencies, especially in small, isolated populations. The effects of genetic drift might include reduced genetic variation and, potentially, random changes that are not necessarily adaptive, also known as non-adaptive evolution.