The One-Island Model of Migration

 The one-island model of migration maintains that a far larger population inhabiting a spacious area will significantly influence the allelic distribution of a nearby smaller population that resides in an area both minuscule in comparison and diverse in it's environment. This model relies on the migration of some organisms from the larger population to the smaller populations place of inhabitance. This type of migration will have a far greater impact on the small population than it would if some organisms from the small population migrated to the area where the larger population resides. While this description may seem confusing at first, I will utilize an example of water snakes in Lake Erie to better convey how this type of migration occurs.

In Lake Erie there are two unique populations of water snakes possessing different phenotypes based on their environment and ability to camouflage. The first population of water snakes reside on the mainland and they often exhibit a striped phenotype that allows them to blend in with the leaf litter of forest streams. The second and far smaller population of water snakes reside on a small collection of islands in Lake Erie that are in the reachable vicinity of the mainland population. These water snakes have a reduced striped phenotype with some even displaying a solid grey color with no stripes at all. These phenotypes are advantageous for the island population of water snakes as they often reside around large and bland colored rocks.  The phenotype distribution of a sample of water snakes inhabiting the mainland and 4 nearby islands is given to analyze how the one-island model of migration is applicable.

The first graph shows a sample of 63 water snakes from the mainland of Ontario and all 63 organisms exhibit the same striped phenotype. However, the second graph shows the distribution of phenotypes in the peninsular mainland and this graph is different as only 80% of the water snakes exhibit the striped phenotype, while the 3 other phenotypes compile the remaining 20%. This distribution so far is consistent with the one-island model because as the geographical distance between the differing population is narrowed, the distribution of the phenotypes becomes more influenced, but still not too significantly. While this slight increase is attributed to the distance between populations, it is also attributed to the larger population maintaining less of an influx in its phenotypic distribution due to its sheer number of inhabitants. However, when the phenotypic distributions are graphed for the island nearest to the mainland (Kelleys island) a far more unique distribution is clearly evident. On this island the most plentiful phenotype is a version of the less striped phenotype, but it still only compiles around 45% of the total sample. The striped and grey phenotypes make up about 10% of the total sample each, with the remaining water snakes exhibiting a different less striped phenotype. The next island sampled was the bass complex islands which are further from the mainland than the Kelleys island. This sampled population exhibited diversity as well in their phenotypic distribution, but the stripe-less grey phenotype and less stripped phenotype were more plentiful with each accounting for approximately 40% of the total phenotypes sampled. Finally, the most remote islands in this example sampled were the Middle and Pelee Islands in which the grey stripe-less phenotype was the most common phenotype in the sample making up roughly 65% of the total. The striped phenotype was the most rare phenotype on this island while the two semi-striped phenotypes made up about 18% and 14%. 

By comparing these sampled populations of each geographic location, there are a few key aspects that become evident. First, it is shown that the further away a smaller population is from the large population, the less influence the large population's emigrants will have on the small population. To cement this claim it is pertinent to analyze the less striped phenotype prevalence for each of the 5 populations. This less-striped phenotype most likely arises from the breeding of either striped and grey snakes, or the breeding of less striped phenotype with basically any of the other water snake phenotypes. As a given island's actual distance increases from the mainland area, the less prevalent the phenotypes of the striped and semi-striped snakes is. Finally, referring to the samples of the Ontario mainland and peninsular mainland water snakes, it is evident that this example reflects a one-island model of migration. Furthermore, this is true because the large populations inhabiting the two mainland areas exhibit a minuscule number of water snakes exhibiting all other phenotypes aside from the striped phenotype. These distributions all support the one-island model of migration. 

My hypothesis on the impact the one-island model of migration has on a given population's evolutionary track is that it will most likely hinder the smaller populations, while the larger populations will be relatively undisturbed. I presume this based off this example because it is clear that the island population of water snakes would be better suited to their environment if the grey phenotype was the most common as it allows for the best camouflage. However, due to the striped phenotype water snakes migration, the population of these water snakes will stray away from a grey advantageous phenotype over multiple generations, and the less advantageous phenotypes will start to become increasingly prevalent in the small populations. Despite this however, I am sure there are examples in which a one-island model of migration will actually improve the evolutionary path of a smaller population if advantageous phenotypes emigrate to said populations from a large population.