In a hypothetical population of butterflies, a dominant mutation in an allele that controls wing color has resulted in purple sp
ots 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
The movement of alleles into a population is called gene flow. The most likely explanation for why this trait does not entirely spread in the population is 'outside alleles enter the population through gene flow'.
Gene flow is caused by the migration and further reproduction of individuals in the population, or the transport of gametes from one population to another (e.g., the transport of pollen by pollinating bees).
Gene flow is an evolutionary force capable of modifying allele frequencies in a population.
In this case, despite the evolutionary advantage that the mutated allele confers to the butterflies, the frequency of this allele can not reach 100% due to the continued introduction of alleles from other populations (i.e., due to gene flow).
