2 Gene Flow and Population Subdivision

Gene Flow and Population Subdivision

Definition of Gene Flow

• Gene Flow:

  • Refers to the transfer of genetic material (alleles) between populations of the same species.

  • If left unchecked by other influences, gene flow promotes genetic homogenization among populations of a species.

Population Subdivision

• Population Subdivision:

  • Describes the extent to which gene flow is reduced between separate populations of a species.

  • This reduction can be measured quantitatively.

Quantifying Population Subdivision

• Methods for Quantification:

  • There are various equations used to quantify the variation in allele frequencies across populations.

Key Equations

• FST (Fixation Index):

  • A crucial measure for quantifying population subdivision.

  • The equation is given by:
    $FST = \frac{Vq}{q(1-q)}$

  • Where:

    • $q$ = mean/average of allele frequencies

    • $Vq$ = variance in allele frequencies from the mean

• Alternative forms of FST:

  • $FST$ is also represented as:
    $FST = \frac{1}{4Nm + 1}$

  • Where:

    • $Nm$ = number of migrants between populations.

  • Another formulation based upon heterozygosity is:
    $FST = \frac{(HT - HS)}{HT}$

  • Where:

    • $HT$ = expected heterozygosity if all subpopulations were pooled and mated randomly (calculated as $2pq$ for the total population)

    • $HS$ = expected heterozygosity for the subpopulation (calculated as $2pq$ for the specific subpopulation).

Interpretation of FST Values

• Values of FST and Implications:

  • $FST$ values range between 0 to 1:

  • If $FST = 0$:

    • Indicates complete panmixia—total random mating between subpopulations.

    • Suggests there is no population subdivision, as heterozygosity is equal across subpopulations (i.e., $HS = HT$).

  • If $FST = 1$:

    • Indicates populations are FIXED for alternative alleles at the analyzed gene locus.

    • Implies that no heterozygotes are present ($HS = 0$), and the populations are completely diverged.

Impacts of Gene Flow on Speciation

• Gene Flow's Role in Speciation:

  • Only one individual is required to maintain gene flow between populations to prevent divergence.

  • Without gene flow, populations begin to accumulate mutations independently which leads to divergence.

  • This process can lead to the formation of new species (speciation) even in the absence of natural selection.

• Mechanisms of Speciation:

  • Speciation has been described as a process influenced solely by random and mechanical processes in the absence of selective pressures.

• Consequences of No Gene Flow:

  • If populations do not experience gene flow, they may diverge significantly, potentially leading to the emergence of distinct species over time.