Mutation, Migration, Inbreeding, & Genetic Drift in natural populations

How do mutation, migration, inbreeding, and genetic drift interact with selection? Do they maintain or reduce variation?
      Can they maintain variation at a high level?
      What is their significance in population (short-term) & evolutionary (long-term) biology?

(1) Mutation / selection equilibrium

      Deleterious alleles are maintained by recurrent mutation.
      A stable equilibrium(where q = 0) is reached
            when the rate of replacement (by mutation)
            balances the rate of removal (by selection).

       µ = frequency of new mutant alleles per locus per generation
           typical µ = 10-6: 1 in 1,000,000 gametes has new mutant
            then =(µ / s)      [see derivation]

        Ex.: For a recessive lethal allele (= 1) with a mutation rate of µ = 10-6
               then û(10-6 / 1.0) = 0.001

mutational genetic load
    Lowering selection against alleles increases their frequency.
        Medical intervention has increased the frequency of heritable conditions
            in Homo (e.g., diabetes, myopia)
    Eugenics: modification of human condition by selective breeding
            'positive eugenics': encouraging people with "good genes" to breed
            'negative eugenics': discouraging people with 'bad genes'' from breeding
           e.g., immigration control, compulsory sterilization
                          [See: S. J. Gould, "The Mismeasure of Man"]

       Is eugenics effective at reducing frequency of deleterious alleles?
            What proportion of 'deleterious alleles' are found in heterozygous carriers?

       (2pq) / 2q2 = p/q  1/q    (if  q << 1)

             if s = 1 as above, ratio is 1000 / 1 : most of variation is in heterozygotes,
                                                         not subject to selection

(2) Migration / selection equilibrium

      Directional selection is balanced by influx of 'immigrant' alleles;
            a stable 'equilibrium' can be reached iff migration rate constant.

Consider an island adjacent to a mainland, with unidirectional migration to the island.
The fitness values of the AA, AB, and BB genotypes differ in the two environments,
      so that the allele frequencies differ between the mainland (qm) and the island (qi).

W0 W1 W2 q
Island 1 1-t 1-2t q 0
Mainland 0 0 1 q 1

 B has high fitness on mainland, and low fitness on island.
    [For this model only, allele A is semi-dominant to allele B,
        so we use t for the selection coefficient to avoid confusion]

      m = freq. of new migrants (with qm) as fraction of residents (with qi)
      if m << t    qi = (m / t)(qm)      [see derivation]

      Gene flow can hinder optimal adaptation of a population to local conditions.

     Ex: Water snakes (Natrix sipedon) live on islands in Lake Erie (Camin & Ehrlich 1958)
            Island Natrix mostly unbanded; on adjacent mainland, all banded.
            Banded snakes are non-cryptic on limestone islands, eaten by gulls
            Suppose   A = unbanded     B = banded   [AB are intermediate]
                   Let      qm = 1.0     ["B" allele is fixed on mainland]
                               m   = 0.05   [5% of island snakes are new migrants]
                                 t    = 0.5     so  W2 = 0  ["Banded" trait is lethal on island]
                   then    qi = (0.05/0.5)(1) = 0.05
                 and     Hexp = 2pq = (2)(0.95)(0.05)  10%
                            i.e, about 10% of snakes show intermediate banding, despite strong selection

       => Recurrent migration can maintain a disadvantageous trait at high frequency.

(3) Inbreeding / selection

      Inbreeding is the mating of (close) relatives
                         or, mating of individuals with at least one common ancestor

     F (Inbreeding Coefficient) = prob. of "identity by descent":
            Expectation that two alleles in an individual are
                exact genetic copies of an allele in the common ancestor

               or, proportion of population with two alleles identical by descent

                This is determined by the consanguinity (relatedness) of parents.

      Inbreeding reduces Hexp  by a proportion F
                (& increases the proportion of homozygotes). [see derivation]

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

      Inbreeding affects genotype proportions,
            inbreeding does not affect allele frequencies.

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

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

      Inbreeding coefficient of a population can be estimated from experimental data:

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

Ex.: Selander (1970) studied structure of Mus house mice living in chicken sheds in Texas
Variation at Est-4 locus
since p = 0.226 + (1/2)(0.400) = 0.426

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

Then F = (0.489 - 0.400) / (0.489) = 0.182
    which is intermediate between Ffull-sib      = 0.250
                                                    &  F1st-cousin = 0.125

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

      Paradoxes of inbreeding:
            Inbreeding is usually thought of as "harmful":
                inbreeding increase the probability that deleterious recessive alleles
                    will come together in homozygous combinations
                    "Harmful" alleles are reinforced
            Inbreeding depression: a loss of fitness in the short-term due to
                    difficulty in conception, increased spontaneous abortion, pre- & peri-natal deaths
                    Ex.: First-cousin marriages in Homo
                        Two-fold increase in spontaneous abortion & infant mortality
                        Every human carries 3 ~ 4 "lethal equivalents"

               Demonsration #2: Selection & inbreeding in small populations

         However, in combination with natural selection, inbreeding can be "advantageous":
           increases rate of evolution in the long-term (q 0 more quickly)
                    deleterious alleles are eliminated more quickly.
           increases phenotypic variance (homozygotes are more common).
                    advantageous alleles are also reinforced in homozygous form

(4) Genetic Drift / selection

      Genetic Drift is stochastic q [unpredictable, random]
                        (cf. deterministic q [predictable, due to selection, mutation, migration)

      Sewall Wright (1889 - 1989): "Evolution and the Genetics of Populations"

      Stochastic q  > deterministic q in small populations:
            allele frequencies drift more rapidly in 'small' than 'large' populations.

      Drift is most noticeable if 0, and/or N small (< 10)  [ 1/s]
            genetic varianceq2 = (q)(1 - q) / 2N
if q = p = 0.5, then q2 = 1 / 8N

<>       q drifts between generations (within population variance decreases) [DEMO];

             eventually, allele is lost (q = 0) or fixed (q = 1) (if qi = 0.5, 50:50 odds)

       q drifts among populations (among population variance increases);
                  eventually, half lose the allele, half fix it.

           Ex [Demonstration #3]
                      [Try: q = 0.5, W0 = W1 = W2 = 1.0, and N = 10, 50, 200, 1000;
                                repeat 10 trials each, note q at endpoint

      **=> Variation is 'fixed' or 'lost'  &  populations will diverge by chance <=**

      Evolutionary significance:
            "Gambler's Dilemma" : if you play long enough, you win or lose everything.
            All populations are finite: many are very small, somewhere or sometime.
            Evolution occurs on vast time scales: "one in a million chance" is a certainty.
            Reproductive success of individuals in variable: "The race is not to the swift ..."

       What happens in the really long run?

      Effective Population Size (Ne)
            = size of an 'ideal' population with same genetic variation (measured as H)
                  as the observed 'real' population.
            = The 'real' population behaves evolutionarily like one of size Ne :
                  e.g., the population will drift like one of size Ne
            loosely the number of breeding individuals in the population

    Consider three special cases where Ne < or << Nobs  [the 'count' of individuals]:

      (1) Unequal sex ratio

     Ne = (4)(Nm)(Nf) / (Nm  + Nf)

                 where Nm & Nf are numbers of breeding males & females, respectively.

           "harem" structures in mammals (Nm << Nf)
             Ex.: if Nm  = 1 "alpha male" and  Nf = 200
                    then     Ne = (4)(1)(200)/(1 + 200)  4
              A single male elephant seal (Mirounga) does most of the breeding
                    Elephant seals have very low genetic variation

           eusocial (colonial) insects like ant & bees (Nf << Nm)
                     Ex.: if Nf = 1 "queen" and  Nm= 1,000 drones
                                   then     Ne = (4)(1)(1,000)/(1 + 1,000)  4
                    Hives are like single small families

      (2) Unequal reproductive success
             In stable population, Noffspring/parent = 1
             "Random" reproduction follows Poisson distribution (N = 1  1)
                  (some parents have 0, most have 1, some have 2, a few have 3 or more)

X Ne = Reproductive strategy
1 1 Nobs Breeding success is random
1 0 2 x Nobs A zoo-breeding strategy
1 >1 < Nobs K-strategy, as in Homo
1 >>1 << Nobs r-strategy, as in Gadus

      (3) Population size variation over time

           Ne = harmonic mean of N  = inverse of arithmetic mean of inverses
                    [a harmonic mean is much closer to lowest value in series]
           Ne = n /  [ (1/Ni) ]  where Ni = pop size in i th generation

       Populations exist in changing environments:
                Populations are unlikely to be stable over very long periods of time
                  10-2 forest fire / 10-3 flood / 10-4 ice age

           Ex.: if typical N = 1,000,000  & every 100th generation N = 10 :
                         then  Ne = (100) / [(99)(10-6) + (1)(1/10)]  100 / 0.1 = 1,000

       Founder Effect & Bottlenecks:
                Populations are started by (very) small number of individuals  (N=2),
                    or undergo dramatic reduction in size (N < 10)).

         Ex.: Origin of Newfoundland moose (Alces):
                            2 bulls + 2 cows at Howley in 1904
                                [maximum 8 alleles / locus, likely less if single source population]
                            [1 bull + 1 cow at Gander in 1878 didn't succeed].

       Population cycles: Hudson Bay Co. trapping records (Elton 1925)
                Population densities of lynx, hare, muskrat cycle over several orders of magnitude
                Lynx cycle appears to "chase" hare cycle

The effect of drift on genetic variation in populations

          Larger populations are more variable (higher H) than smaller
                  if s = 0H reflects balance between loss of alleles by drift
                              and replacement by mutation

           H = (4Neµ) / (4Neµ + 1) [the "neutralist equation"]

   Ex.: if µ= 10-7   & Ne = 106   then   Neµ = 1   and Hexp = (0.4)/(0.4 + 1) = 0.29

            But typical Hobs 0.20  which suggests  Ne 105
             Most natural populations have a much smaller effective size than their typically observed size.

             Ex.: Gadus morhua in W. Atlantic were confined to Flemish Cap during last Ice Age 8~10 KYBP
                    mtDNA sequence variation occurs as "star phylogeny":
                         most variants are rare and related to a common surviving genotype
                    Carr et al. 1995 estimated Ne = 3x104  as compared with Nob s 109
                        Effective size is ca. 5 orders of magnitude smaller than observed

                 Marshall et al. (2004) showed that "genome-types" at Flemish Cap are distinctive

            Stochastic effects may be as or more important than deterministic processes in long-term evolution.

Text material © 2004 by Steven M. Carr