Evolutionary Forces
Home Back


Here is a list of the evolutionary forces that Visual PopGen uses in calculating the graphs.  You can click on the name of any of the evolutionary force to jump to it's definition.

Evolutionary Force

Genetic Parameter


Expected Range

Default Value


Genetic Drift


Effective population size

1 - 1,000,000





Inbreeding coefficient.

0 – 0.5

(1/(2N)) or 0*




Relative fitness of the three possible genotypes.

0 – 1




0 – 1




0 – 1



Gene Flow


Proportion of new migrants per generation

0 – 1




Proportion of allele 2 in migrants' gene pool

0 – 1





Forward mutation rate. (A1->A2)

0 – 1




Back-mutation rate.


0 – 1





Initial proportion of A2 in the population's gene pool

0 – 1





Number of generations involved in calculations.

10 - 1000



*F=1/2N if Inbreeding is activated for simulation; F=0 if it is not.

Definition of evolutionary forces and genetic parameters:


Genetic Drift: Parameter (N) –Effective breeding population size.

When N is small, only a small number of alleles (2N) comprise the gene pool passed from one generation to the next. Given these circumstances, allele proportions may be affected by chance (random) sampling errors. Changes could be positive or negative (flip of a coin). Genetic drift may lead to fixation of an allele by accidentally failing to pass on alternative alleles during one generation.


Inbreeding: Parameter (F) – Inbreeding coefficient.

Breeding among relatives causes some individuals who would otherwise have been heterozygotes to be homozygous because the two alleles in their genotype are "identical by descent." The inbreeding coefficient, F, is the proportion by which the expected heterozygote genotype proportion is reduced due to inbreeding. Half of that proportion is added to each of the two homozygous genotype proportions. The value of F increases by a factor of (1+F) each generation. Inbreeding will not change allele proportions (p & q) unless combined with selection--the only evolutionary force that acts directly on genotypes.


Selection:  Parameter (w11) – Relative fitness of one homozygous genotype (A1A1)

                Parameter (w12) – Relative fitness of the heterozygous genotype (A1A2).

                Parameter (w22) – Relative fitness of one homozygous genotype (A2A2).

The most fit genotype must be assigned a relative fitness of 1, the others (those being "selected against") are assigned relative values (0 <= w <= 1).

Directional selection involves selection against one of the homozygotes but not the other. This type of selection tends to favor one allele. This may result in fixation of the favored allele due to complete elimination of the alternative allele.

Stabilizing selection involves selection against both homozygotes. This type of selection results in an equilibrium (q stops changing). This situation results in a "stable polymorphism," where both alleles remain present in the gene pool.

Note: Selection tends to have a stronger effect on allele proportions when both alleles are relatively frequent. If nearly all alleles are of one kind, there is little genetic variation among individuals, and few experience their selective disadvantage; thus changes in q become smaller per generation.

Gene flow: Parameter (m) – the proportion of new migrants in a breeding                population each generation.

             Parameter (qm) – the proportion of second allele (A2) among new migrants.

The effect of migrants entering the breeding population with their own characteristic allele proportions may be very great if a high proportion of the population (m) are recent migrants each generation, and those migrants have allele proportions (qm) that are quite different from those in the rest of the population. But if the proportion of migrants is low, or if allele proportions are similar in residents and migrants, the effect of gene flow may be more similar to that of mutations (an occasional source of alternate allele).


Mutation: Parameter (u) – Forward mutation rate (A1 --> A2).

               Parameter (v) – Backward mutation rate (A2 --> A1).

Mutation is not considered an important force in changing allele proportions. The importance of mutations is as a source of alternative alleles, some of which might become abundant in a population due to genetic drift or to selection.

Mutation & Selection. To illustrate the interaction of mutation (a source of alternative alleles) and selection (a potentially strong force changing allele proportions), start with q0 = 0 so that A2 must be introduced by mutation.

Purifying selection. Start with only one allele (A1) in the gene pool (set the initial value of q to 0); then assign realistic mutation rates that cause introduction of the second allele (A2); and implement selection against the mutant by assigning w22 the lowest value, and w11 = 1.

Progressive selection. Start with only one allele (A1) in the gene pool as above; use the same realistic mutation rates; and implement selection that favors the mutant allele (w22 = 1; w11 < 1).


This web page built by Kristopher Aaron Umbarger