Final answer:
The situation where the heterozygote has higher fitness than either homozygote is known as overdominance or heterozygote advantage. This genetic variation results from mechanisms such as natural selection and can lead to increased genetic variance within a population.
Step-by-step explanation:
For genetic variation involving a single gene, a selection may arise when the heterozygote has a higher fitness than the corresponding homozygote, a situation called overdominance or heterozygote advantage.
In terms of genetic terminology, genetic variance refers to the diversity of alleles and genotypes in a population. Promoting genetic variance is essential in breeding programs to maintain phenotypic diversity and prevent inbreeding depression. Natural and sexual selection can influence the frequencies of different alleles in the gene pool, affecting the population's genetic structure over time. Natural selection can lead to traits becoming more common if they offer a survival or reproductive advantage. Situations in which multiple phenotypes are advantageous lead to diversifying selection, increasing genetic variance.