Allele Fixation in Island Populations

Reduction of Heterozygosity in Small Demes

Consider an archipelago consisting of 1000 similar islands. Each island contains an isolated population of Species X, which has a gene locus segregating a dominant (A) and recessive (a) allele that is not under selective pressure.

  • All 1000 populations start with equal freqencies of dominant (A) and recessive (a) alleles at a particular locus (i.e., p and q both = 0.5)
  • As the number of generations passed increases, the 1000 populations begin to diverge from the initial 0.5 frequencies due to SAMPLING ERROR.
  • As some populations go to q = 1.0, they will also go to p = 0.0
  • Approximately the same number are predicted to go to p = 1.0
  • Hence, the distribution of allele frequencies found in the 1000 populations spreads symmetrically.
  • When the allelic frequency in a population reaches 1.0, the allele is the only one left in the population, and it becomes fixed for that allele. The other allele is permanently lost.
  • In populations in which an allele has become either fixed or lost, the process of random genetic drift stops at that locus.
  • Without further input (mutation), the populations that have allelic frequencies of 1.0 or 0 for either allele will maintain those allelic frequencies forever.

  • Somewhere between 100 (N) and 200 (2N) generations, the distribution of allele frequencies flattens out as some of the 1000 initial populations are lost to fixation/loss of alleles. (Think of the sides of the graph as "sinks" into which these fixed/lost allele populations disappear.)
  • With the population size we chose for our example, there's a loss of about 0.5% of the total 1000 populations to fixation/loss with each generation.
  • Subsequent loss to fixation/loss becomes vanishingly small after the generations reach about 300. It would take a HUGE amount of time for the rest of the populations to be absorbed--practically infinite.

    NOTE that if the initial allelic frequency is not 0.5, the curve will be shifted to the right or left (depending on which allele is more frequent), but the processes of movement towards homozygosity will be the same.

    In this example, we started out with a very low frequency of a. The probability of its being LOST from the population is far greater than its being fixed. In this graph, most of the populations are fixed with AA--not aa.