Final answer:
The statement is true; when heterozygotes show higher fitness than homozygous genotypes, this can lead to balanced polymorphism due to heterozygote advantage. This condition results in an increase of 2pq, representing the frequency of heterozygous individuals in the population.
Step-by-step explanation:
When heterozygotes show higher fitness compared to homozygous genotypes, this scenario is known as heterozygote advantage. This condition can indeed lead to balanced polymorphism, where multiple alleles are maintained in the population. This occurs because the heterozygous individuals have a competitive advantage over both homozygous genotypes, which maintains the genetic diversity within the population.
Considering the allele frequencies represented by p and q in a population, under the influence of heterozygote advantage, the value of 2pq, which represents the frequency of the heterozygous genotype in the population, would likely increase. The evolutionary reason for this is that the heterozygous individuals have an increased survival and reproductive fitness, thus the allele frequencies of both p and q are maintained, preventing either allele from becoming fixed or lost.
As for the example featuring mice with different coat colors, natural selection can become more complex when intermediate phenotypes are less fit than either extreme. In this case, it leads to a form of disruptive selection that maintains both the light and the dark-colored alleles in the population because the intermediate phenotype is less adaptive.