Module 6.1: The One-Island Model of Migration and Water Snakes in Lake Erie

  Migration is a phenomenon which is used to describe movement of individual organisms from one place to another. It takes place when individuals are moved through random mating from one subpopulation to another, when gametes move from one place or subpopulation to another which is then followed by fertilization. Migration results in continuation of gene flow. Gene flow is referred to the movement of alleles between populations. Migration can be local or long distanced. The phenotypes A, B, C and D according to the question, are alleles from the continent which represents a large fraction of the island gene pool. The gene frequencies in the continent are not affected due to the alleles from the island. Chromosome banding refers to alternating light and dark regions along the length of a chromosome, produced after staining with a dye. Banding is controlled by a single locus. Let us assume, AA and Aa individuals are banded and aa are not banded. From Ontario and Peninsular mainland, we can see that phenotype A is not banded and D is banded. The other island populations are considered polymorphic. It is a real example of gene flow and selection: Snakes which live in islands usually bask in the sun on the limestone rocks near the water.  Snakes with no banding or unstriped snakes are not much differentiated from the limestone rocks. Therefore, predators are more likely to eat striped or banded snakes. It suggests that selection prefers unstriped snakes on the islands. As snakes without banding (aa) have higher survival than banded snakes (AA or Aa), phenotype A is seen to be lost from the island population. To summarize the evolutionary consequences of migration for populations, migration introduces novel genetic variation into populations. It sets the spatial scale for evolution, opposing local adaptation. Migration with an evolutionary impact enables gene flow.