In a hypothetical population of butterflies, a dominant mutation in an allele that controls wing color has resulted in purple spots developing on the wings. The genetic change is heritable such that the offspring of butterflies who inherit at least one mutated allele will develop purple spots. The population of butterflies inhabits a windy field that contains many purple flowers. Individuals with purple spots are so well camouflaged on the purple flowers that they are eaten less frequently by predators. There are other fields with populations of the same species of butterfly nearby, but purple flowers are less prevalent in these nearby habitat areas. There are no borders between these fields and it is common for individuals from one population to be blown into another population by the wind. Despite the advantage the mutated allele confers to the butterflies in the field with many purple flowers, the mutated allele never occupies 100% of the gene pool and the population of butterflies always contains a mix of individuals with and without purple spots. What is the most likely explanation for why this trait does not spread entirely throughout the population