Inbeeding & Natural Selection

Inbreeding & Natural Selection

      Inbreeding loosely understood as mating of relatives (individuals with at least one common ancestor)
                           but in finite populations, all individuals related, more or less closely
   "Inbreeding" in population genetics more precisely defined as
                                               mating of related individuals at a higher frequency than random expectation

    F (Inbreeding Coefficient) = prob. of "Identity by Descent":
            Probability that two alleles are exact genetic copies of allele in common ancestor
            Determined by consanguinity (relatedness) of parents [HOMEWORK]

Inbreeding reduces H_exp by proportion F
(& increases proportion of homozygotes) [see derivation]

          f(AB) = 2pq (1-F)
            f(BB) = q² + Fpq
            f(AA) = p² + Fpq

      Inbreeding affects genotype proportions,
                                     not allele frequencies
           HOMEWORK

Inbreeding increases frequency of individuals
with deleterious recessive genetic diseases by factor F/q [see derivation]

Ex.: if f(B) = q = 10^-3 and F = 0.10 , then F/q = 100
=> 100-fold increase in BB births

Inbreeding coefficient of population estimated from experimental data:

F = ( 2pq - H_obs ) / 2pq = (H_exp - H_obs) / H_exp [see derivation]

F as measure of population structure
Ex.: Mus house mice live in very large chicken sheds

Genotypic variation at Esterase-4 locus

	Obs	Exp
AA	0.226	0.181
AB	0.400	0.489
BB	0.374	0.329

From p = 0.226 + (1/2)(0.400) = 0.426

& q = 0.374 + (1/2)(0.400) = 0.574 ( = 1 - 0.426 )

Then F = (0.489 - 0.400) / (0.489) = 0.182
intermediate between F_full-sib = 0.250
& F_1st-cousin = 0.125

=> Mice live in small family groups with close inbreeding
[typical for small mammals]

Countervailing effects of inbreeding
            Inbreeding "harmful" to individuals in short-term
                increases proportion of homozygotes w/ deleterious recessive alleles
                    "Harmful" alleles "reinforced"
            Inbreeding depression: loss of fitness in short-term due to
                    difficulty in conception, increased spontaneous abortion, pre- & perinatal deaths
                    Ex.: Sib-Sib & First-cousin marriages in humans
                       Two-fold increase in spontaneous abortion & infant mortality
                         Delay in time to first live birth
                       Every human carries 3 ~ 4 "lethal equivalents"

         Inbreeding "advantageous" to population in long-term, In combination with Natural Selection,
           increases rate of evolution (q 0 more quickly)
                     deleterious homozygotes exposed to selection in higher proportion
                     => deleterious alleles eliminated more quickly
           increases phenotypic variance (homozygotes more common)
                    advantageous alleles reinforced as homozygotes
karyotypic rearrangements common in small populations: source of Peripatric Speciation