Final answer:
Heterozygote advantage, also known as genetic drift, is the situation where a heterozygote has a characteristic that makes it more likely than either homozygous individual to reproduce in a given environment.
Step-by-step explanation:
The situation you are referring to is called heterozygote advantage or genetic drift.
Natural selection favors heterozygotes that have a characteristic that makes them more likely to reproduce in a given environment than either of the homozygous individuals. This advantage can lead to an increase in the frequency of the allele associated with the heterozygote trait in the population over time.
For example, imagine a population of butterflies where a certain gene determines wing color. Homozygous individuals with two copies of the dominant allele have bright wings that attract predators, while homozygous individuals with two copies of the recessive allele have dull wings that blend in with the environment. Heterozygotes with both alleles have an advantage as their wings are a more intermediate color, providing better camouflage. Therefore, they are more likely to survive and reproduce, passing on the advantageous allele.