Final answer:
The delta 32 allele isn't more abundant due to evolutionary forces such as genetic drift, particularly on islands where small populations can lose alleles quickly. Factors such as multiple introductions and propagule pressure, and selective pressures from new predators or use of pesticides like DDT, also play a role.
Step-by-step explanation:
The frequency of the delta 32 allele in populations is influenced by various evolutionary forces. One key reason delta 32 isn't more abundant could be due to a combination of genetic drift, where alleles can be lost from a population by chance as shown in the example of rabbits with brown and white coat color alleles. In a small population, such as those on islands, genetic drift can occur more quickly leading to a loss of alleles at a faster rate than in a larger mainland population. Furthermore, the impact of genetic drift is closely associated with the size of the population and how frequently the allele in question confers a survival advantage to individuals.
Additionally, the success of non-native versus native populations in maintaining genetic diversity, as discussed in Roman & Darling (2007), may be affected by multiple introductions and high levels of propagule pressure, which would result in a less significant bottleneck effect. Situations such as the introduction of new predators can also alter the genetic makeup of a population, as the survival of different alleles would depend on how those alleles affect an organism's ability to survive the new threat. Other events, like the use of DDT, can selectively remove individuals with certain alleles from a population, as seen in fruit fly populations.