*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).
