The One-Island Model and the Importance of Size

     The most important facet of the one-island model of migration, the way I understand it, is the homogenizing effect of migration on whatever selection is happening on the island(s). In any relationship between a mainland and an island, there is going to be a size difference in the population; and in this case, size does indeed matter. This difference leads to any movement between the island and the mainland as having a negligible impact, in terms of allele frequencies, on the mainland, while the island's frequencies can be radically impacted by migration. As seen in the example with the water snakes, populations can be selected upon differently by variable environments even within a relatively close spatial distribution and thus have different phenotypes. However, this variation is negatively impacted by the movement of genetic material from the mainland(s) to any of the islands. 

    As seen in the above model from the textbook, m represents migration rates between the populations of snakes (from the mainland to the islands, specifically).  With an increase in the rate of m, seen on the right, there is an increase in the speed at which the allelic frequencies between the populations is homogenized. Moreover, with the assumption that this migration is chronic and not just happening in small, impactful bursts, then there is an implication that a constant migration is going to homogenize the populations over time no matter what. I honestly don't know if I can say that this is an accurate assumption, because we see in places like Madagascar and New Zealand that there has been quite rampant speciation from the mainlands (Africa and Australia, respectively.) However, maybe this is frequently true for less extreme spatial differences between the mainland and the island(s), and that's why there is a maintenance of a single species between differently selected upon populations in more confined locales (like this water snake example). So, it appears that this adverse effect of migration on selection's work could in turn be affected by the amount of space between the populations and, thus, how easy it is for migration to occur.