Inbeeding & Natural Selection

Inbreeding & Natural Selection

      Inbreeding: loosely, mating of relatives (individuals with at least one common ancestor)
                           In any finite population, all individuals related, more or less closely
"Inbreeding" in population genetics more precisely defined as
                               mating of related individuals at 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

F measures population structure

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

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

f(B)_obs = 0.374 + (1/2)(0.400) = 0.574
f(A)_obs = (1 - 0.574) = 0.426

f(AB)_exp= (2)(0.426)(0.574) = 0.489

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

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

Contrasting effects of inbreeding
       Inbreeding "harmful" to individuals in short-term
                increases proportion of homozygotes w/ deleterious recessive alleles
                    "Harmful" alleles "reinforced"
                Every human carries 3 ~ 4 "lethal equivalents"

        Inbreeding depression: loss of fitness in short-term due to
               difficulty in conception, increased spontaneous abortion, pre- & perinatal deaths
                    Ex.: Sib-Sib marriages have excess congenital defects
           First-cousin marriages
                     Two-fold increase in spontaneous abortion & perinatal mortality
                             Delay in time to first live birth

         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:
                            promotes Peripatric Speciation