Ch 7 Migration, Drift, Non-Random mating

Migration

The movement of alleles among populations

• influences gene flow: transfer of alleles from one gene pool to the next

immigration

movement INTO a popuation

Emigration

movement OUT OF a population

Wright’s One-island model

p_i = resident

m = rate of gene flow

p = average allele frequency of A1 in source population

p= m (p-p_i)

(1-m) = proportion of gene copies from non-immigrants

p_i = frequency

p’ = allele frequency of after 1 generation of population

Example

Test Statistic F_ST

Reflects variation in allele frequencies among populations of the group

• ranges from 0 to 1

• The larger the range, the more variation in allele frequencies

• F_IS = the proportion of the variance in the subpopulation contained in an individual

What happens if the assumption of large population size is broken?

• evolution can happen by random chance. IT is not adaptive, but does lead to changes in allele frequencies

Small population with locus A w/ alleles A₁ and A₂

A₁=0.6 ; A₂=0.4

• Random mating in gene pool produces 10 zygotes

• Due to the small #, by changes, alleles will not unite in the same frequencies

Because of genetic drift, one allele can rise to fixation over time

• The smaller the population, the faster the rate of fixation

• The larger the population, the more likely It will conform to Hardy-Weinberg

Founder effect

• A small group of individuals that starts a new population in a new location

• can result in sampling error

Genetic Bottleneck

(Similar to founder effect)

• Random events cause a population to crash into a very low level

• a sudden decrease in population size because of extreme natural forces

Loss of heterozygosity

• Can decline because of drift

• Genetic drift can produce substantial changes in allele frequencies

• in terms of migration or selection, the effects of genetic drift can be lessened

What did Sewall Wright demonstrate?

that the probability of fixation for a particular allele is = to the original frequency

• if the initial frequency of an allele is 0.8, there is 80% it will drift to fixation

Haplotype

(Think of haploid (1N ) cell)

a group of genes within an organism that is inherited together from a single parent

Young’s study of plants

• used literature data

• Plotted 2 measures of overall genetic diversity against population size in 4 plant species

Genetic polymorphism

• fraction of loci that have at least 2 alleles with frequencies above 0.01

Alellic richness

avg. number of alleles per locus

F-statistics : F_IS = (H_s-H_I)/H_s

• H_I = H_o = Observed heterozygosity in a population — count # of heterozygotes

• H_s = H_E = Expected heterozygosity in a population based on HWE — H_s = 2[f(A)][f(a)]

F_ST = (H_T-H_S)/H_T

• H_s = avg. H_s among all populations

• H_T = Total expected heterozygosity among all populations

• F_ST = the level of differentiation among a set of populations

Effective population size

The size of an idealized population that would lose genetic diversity at the same rate as the actual population

• N_e= measure of a population’s genetic behavior

How does effective size differ from census size?

Any characteristic of a real population that deviates from the characteristics of an ideal population

Demographic Method A) Unequal sex-ratio — inbreeding effective size

N_e=4N_efN_em/(N_ef+N_em)

Demographic Method B) Variation in family size

N_e = (4N-2) / (V_k +2)

V_k =variance in family size

Demographic Method C) Fluctuations in population size

N_e = t/E(1/N_ei)

• N_ei= effective size in the _i^th generation

• t = number of generations

Neutral theory vs. Selectionist theory

Neutral theory

• rate of evolution = neutral mutation rate

• advantageous mutations are very rare, and most mutations are selectively neutral

Selectionist theory

• advantageous mutations are more common

• rate of substitution is determined by natural selection on advantageous mutations

Nearly neutral theory of molecular evolution

s is less than or equal to 1/ 2(N_e)

s= selection coefficient

N_e= effective population size

Neutral theory as null hypothesis

Positive selection promoting replacement substitution

• MHC proteins

• immunoglobulins

• plant S-alleles

Loci under positive selection

• recently duplicated genes that have attained new functions

• loci involved in sex determination

• species-specific interactions between sperm and egg

Hitchhiking (selective swap)

can lead to increase in frequency of neutral or even deleterious genes

Coalescence

a method that allows the calculation fo effective population size in previous generations

What happens if the assumption about non-random mating is broken?

Nonrandom mating does not cause evolution by itself because it can have great indirect effects on evolution. Mate choices cause nonrandom mating = assortative mating

• (ex. females choose males with particular phenotypes)

Positive assortative mating

individuals choose mates similar to themselves

• “omg you’re just like me!!”

• increases homozygosity and decrease in heterozygosity at all loci

Negative assortative mating

individuals choose mates different from themselves

• “you’re into rock and im into pop!! Love that~!”

• increases heterozygosity

What is the most common type of nonrandom mating?

inbreeding (dating relatives)

• self-fertilization

• increases in homozygosity at all loci regardless of what allele frequencies were

Coefficient of Inbreeding, F

Computing F in real populations

Inbreeding Depression

• exposure of deleterious alleles as homozygotes

• loss of function mutations are hidden as heterozygotes

• increases the frequency at which deleterious alleles affect phenotypes

• Can have higher mortality rates because there is a 50% likelihood of a rare disease within inbreeding that are more likely to be expressed for newer generations