Final answer:
The evolution of populations by chance is known as genetic drift. The movement of alleles among populations because of migration is called gene flow. Natural selection is the process where inherited traits that enhance survival and reproduction become more common over time.
Step-by-step explanation:
The evolution of populations due to chance events is genetic drift. The transfer of alleles into and out of populations due to the movement of fertile individuals or their gametes is gene flow. Natural selection occurs when individuals with inherited traits better suited to their environment survive and produce more offspring than other individuals.
Genetic drift can significantly affect small populations where chance events can lead to large changes in allele frequencies. Gene flow changes allele frequencies as individuals migrate in or out of a population, potentially introducing new alleles or changing the proportions of existing ones. Natural selection is a process whereby individuals who have characteristics which are more suited to their environment tend to survive longer and reproduce more, passing on these adaptive traits to their offspring, leading to changes in the allele frequencies in a population over generations.