Population Genetics III: Effective Population Size, Non-random Mating, Migration Flashcards
effective population size variable
Ne
effection population size Ne
- the size of an idealized population in which the rate of genetic drift is the same as in the actual population
- Ne is usually less than the census N
example of variation in the number of progeny
- Elephant Seals
- polygynous
- males establish narems
- a single bull male may get 77% of all the matings
- 6% of males get 88% of all matings
Ne equation
Ne = 4 Nm Nf / Nm + Nf
example of unequal numbers of males and females
if due to heavy hunting pressure, a population of deer has 100 females and 1 male
when is Ne the highest?
when Nm = Nf
variation of population size
Ne is the harmonic mean of the census population size through time
equation: variation in population size
Ne = t / 1/Ne1 + 1/Ne2 + …. 1/Net
example of variation in population size
- elephant seals
- 1890s: the population went through a severe bottleneck due to overhunting
- Ne approx 3.6
- no variation at 24 allozyme loci
when does non-random mating occur?
when individuals mate assortatively
example of non-random mating
if large females are more likely to mate with large males and vice versa
is non-random mating a violation of HW?
yes
inbreeding
the mating among genetic relatives, which is a form of nonrandom mating
in what kind of populations is inbreeding a problem?
small populations
where is inbreeding a serious concern?
conservation biology
what is the genetic problem with inbreeding?
inbreeding increases homozygosity
_______ frequencies change, _____ frequencies do not
genotype, alleles
the difference between inbreeding and mate choice
inbreeding impacts the entire genome, whereas mate choice only impacts the loci that influence mate choice
what if two distinct populations are mixed into a single sample?
you get the same signature as inbreeding: a deficit of heterozygotes and an excess of homozygotes
Wahlund Effect
a deficit of heterozygotes and an excess of homozygotes
coefficient of inbreeding
F
F
the probability that 2 alleles are Identical by Descent
Identical by Descent
- IBD
- both copies descended from the same ancestral allele in an earlier generation
another name for alleles that are IBD
autozygous
inbreeding can be quantified as the reduction in heterozygosity
F = Hexp - Hobs / Hexp = 1 - Hobs / Hexp
1 - F
- the probability that two alleles are not IBD
- these individuals are called allozygous and can be either heterozygotes or homozygotes
is F allozygous or autozygous?
autozygous alleles
what type of equation when F=0
you have the normal equation
by increasing the frequencies of homozygotes…
inbreeding increases the probability of homozygous deleterious alleles
is inbreeding a problem in captive populations?
yes
why is inbreeding prevalent in endangered populations?
endangered populations are often small, which results in inbreeding
inbreeding avoidance
- mate choice
- dispersal from natal site
- self incompatibility loci in plants
management of inbreeding avoidance
move individuals between populations to combat the decline in heterozygosity
example of migration
- water snakes
- banded snakes are most fit on the mainland. unbanded are most fit on islands where the snakes bask in the open or on rocks
- exception: islands will have only unbanded snakes
when will migration not change allele frequencies in a population?
when the equation m (pc - pi) = 0
m
frequency of new migrants/generation
pc
frequency of A allele in the mainland population
pi
frequency of A allele in the islands population
will the migration change the allele frequencies when m = 0 and when the frequency of the A allele is the same in both populations?
no
impacts of gene flow
- homogenizes populations genetically
- decreases genetic variation between populations
- increases genetic variance within populations
the effect of migration is ______ genetic drift
opposite
in what direction does gene flow and selection work?
opposite directions
population structure
the presence of allele frequency differences among populations
population structure is due to several evolutionary processes, including:
- natural selection
- genetic drift
- founder effects
- gene flow
- mutation
the most important consequence of population structure
a reduction in the average heterozygosity relative to that expected if all the subpopulations were a single, randomly-mating population
what does population structure lead to?
loss of heterozygosity
Hs
observed average heterozygosity
Ht
expected heterozygosity
little Fst
0.00-0.05
moderate Fst
0.05-0.15
great Fst
0.15-0.25
very great Fst
0.25-1
genetic drift
increases divergence among populations over time
consequence of genetic drift
increase in Fst over time among populations
does migration make population similar or different from each other?
more similar
what will gene flow do in the absence of any other evolutionary processes?
completely homogenize subpopulations
what does genetic drift increase?
divergence among populations over time
island model
subpopulations of equal Ne with equal amounts of gene flow (m) between them
