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 Hexp  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 - Hobs ) / 2pq  = (Hexp - Hobs) / Hexp [see derivation]


F as measure of population structure
     Ex.: Selander (1970) studied structure of Mus house mice living in chicken sheds in Texas

Genotypic variation at the 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 Ffull-sib      = 0.250
                                   &  F1st-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


Text material © 2021 by Steven M. Carr