Final answer:
The banded and unbanded island water snakes demonstrate the combined action of migration and natural selection leading to divergent evolution, possibly resulting in the speciation of these snakes on different islands.
Step-by-step explanation:
The banded and unbanded island water snakes are an example of the combined unidirectional effect of migration and natural selection.
Habitat isolation results from populations being separated physically, which prevents them from breeding with each other. Over time, different environmental pressures lead to divergent evolution, where each isolated population adapts to its own unique environment. This process can result in allopatric speciation, where new species emerge due to these divisions and the species-specific adaptations they acquire.
Migration can introduce new alleles to an isolated population, leading to genetic drift. For instance, if a few snakes from a mainland population with a unique genetic make-up migrate to an island, it might lead to an increase in a particular allele within that isolated population. Natural selection, on the other hand, will favor traits that are advantageous in the new environment, causing those traits to become more common within the population over time. If both processes are acting in the same direction, it can cause rapid divergence between the populations.