evolution exam 4

gene flow

transfer of genetic material between pops

migration, pollen, horizontal gene transfer

gene flow/migration allele frequencies

1. calculate initial (pre-migration) genotype/allele frequencies

2. calculate post migration genotype/allele frequencies based on # of migrants and their genotype frequencies

continent-island model

assume one way movement from a large population to a smaller population on an island

continent-island migration equation

frequency of A1 on island = p

frequency of A1 on continent = p_m

proportion of island pop who are migrants from the continent (migration rate) = m

frequency of A1 on the island after migration = p^*

p^*= mp_m + (1-m)p

change on island from one generation to the next

Δp = p^* - p = mp_m + (1-m)p-p

Δp = m(p_m-p)

equilibrium

allele frequency on the island will be the same as it is on the continent

p = p_m

clines

continuous change in allele frequency in space 

hardy-weinberg principle

allele/genotype frequencies in a population will remain the same from one gen to gen in the absence of other evolutionary influences

gene flow homogenizing populations

1. gene flow includes exchange of alleles

2. equalization of frequencies across different populations

3. counteracting divergence like genetic drift and natural selection

F_ST , allele frequencies and variance

F_ST = 0 means no genetic differentiation between populations (interbreeding and share genetic material)

F_ST = 1 means complete genetic differentiation (pops are completely isolated, share no genetic diversity) 

dispersal reasons

ephemeral (TW) habitat

get away from relatives (competition or inbreeding)

no dispersal

not possible

no suitable habitat

energetic costs

random factors in evolution

1. mass extinction

2. mutation

3. genetic drift

mass extinction

results from random occurrences (asteroid and volcanoes)

mutation

random process that gives rise to new alleles each generation at constant rate

adds allelic diversity; affects alleles frequencies at a given locus

genetic drift

random changes in allele frequencies or genotypes within a population due to sampling from a limited population

reduces allelic diversity; affects allele frequencies at all loci in the genome 

why does a genetic drift happen?

population are not infinite, sampling error randomly changes allele frequencies

does genetic drift work better in small or larger pops?

it is faster, more efficient small pops

with more time, it can be drastic in large pops

can genetic drift fix/lose alleles

in the absence of other evolutionary forces, drift causes the eventual fixation or loss of allele

the probability that an allele will eventually go to fixation is the same as the frequency of that allele in the current pop

allele fixation probability equation

N = pop size

2N = number of alleles at locus A 

x = initial number of copies of A1

what is the probability any 1 copy of these alleles will go to fixation? = 1/2N

what is the probability that allele A1 will go to fixation? = x(1/2N) = x/2N

equations for proportion of frequencies after generations

H_g+1 = H_g (1-(1/2N))

H_g+1 = expected heterozygosity in next gen

H_g = observed heterozygosity in current gen

H_t = H₀ (1-(1/2N))^t

effective population size (N_e)

smaller than census size

fewer individuals contribute to next generation’s genes than could