Final answer:
When there is no migration in a population, allele fixation can be determined by genetic drift, positive selection, negative selection, or natural selection. These evolutionary mechanisms lead to changes in allele frequencies, and sometimes it's difficult to pinpoint which one is at play without detailed study of the population and its environment. The correct options are A B C D.
Step-by-step explanation:
In cases where populations are isolated with no migration, the mechanism that determines which allele becomes fixed is often due to genetic drift, positive selection, negative selection, or natural selection.
Positive selection occurs when an allele gives a reproductive advantage and increases in frequency. Negative selection works against alleles that are deleterious, reducing their frequency. Natural selection favors alleles that confer a survival advantage.
Hardy-Weinberg Equilibrium conditions stipulate that allele frequencies in a population will remain constant if certain criteria are met; when these conditions are violated, as in the presence of genetic drift or selection, allele frequencies will change.
The founder effect is a specific case of genetic drift where a small number of individuals establish a new population, which may have different allele frequencies than the original one, leading to a rapid change in allele frequencies due to the small genetic pool.
Overall, whenever allele frequencies in a population change, it's important to consider the complex interplay of multiple evolutionary mechanisms that could be influencing this genetic variation. The correct options are A B C D.