IB Biology Topic D4.1.10: Allele Frequencies of Geographically Isolated Populations Notes

Isolated Populations Drift Apart Until They May Become New Species

1. When a species splits into populations that cannot interbreed, gene flow between them stops.
2. Selection, mutation, and genetic drift then act independently in each population, so their allele frequencies move apart.
3. Given enough time, the populations can diverge so far that they can no longer interbreed, which is allopatric speciation.

What Cuts Off Gene Flow

1. Geographic barriers
   1. New rivers, rising mountains, changing sea levels, or expanding deserts separate populations.
2. Founder effect
   1. A small group is cut off, such as insects blown to an island, and starts with only a subset of the original gene pool.

Three Forces Move Isolated Populations Apart

1. Different selection pressures
   1. Environments differ on each side, so selection favours different alleles.
2. New mutations
   1. A beneficial mutation arising on one side stays there, so each population builds its own set.
3. Genetic drift
   1. In small populations, allele frequencies swing by chance, so the two drift in different directions.

Human Example: The ADH1B Alcohol-Metabolism Gene

1. The ADH1B gene codes for an enzyme that breaks down alcohol, and its alleles vary sharply by region.
2. ADH1B*1
   1. Linked to slower alcohol metabolism.
3. ADH1B*2
   1. Linked to faster metabolism; common in East Asia but rare in Europe.

Faster metabolism may have been favoured where early rice farming raised alcohol consumption.

Finches Show Isolation Producing New Species

1. A single finch species reached the Galapagos and spread across islands with different food sources.
2. Each island population evolved a beak suited to its food, and over time 13 distinct species formed that no longer interbreed.