L14 Gene flow

first 11 are leftover from L13

synonymous vs nonsynonymous dn/ds

one strategy to determine if a locus is evolving by drift or by

dn = # of nonsynonymous change (difference)/ nonsynonymous site

ds = # of synonymous change (difference)/ synonymous site

dn/ds < 1

fewer non than syn mutations = purifying selection

dn/ds = 1

equal non and syn mutations = neutral evolution

dn/ds > 1

more non than syn mutation = positive selection

dn/ds is a conservative

it often has a value less than 1 even when positive selection has occurred

this is because the signal from positive selection can sometimes be swamped by historic purifying selection

Synonymous mutations

DNA changes that don't alter the amino acid sequence of a protein

Nonsynonymous mutations

change the protein sequences and are frequently subjected to natural selection

The McDonald-Kreitman test (MK test)

compares the ds/pi to dn/pi

d = number of difference between species

pi = heterozygosity (polymorphism)

if the ratio for non is higher

positive selection

if the ratio for the syn is higher

purifying selection

ratio is equal

neutral

gene flow

mixing of alleles in different populations

can introduce new alleles into one population from another population

gene flow is random

with respect to fitness

gene flow can increase

genetic variation within a population

gene flow tend to

decrease genetic variation between populations

• homogenize allele frequencies

dispersal

the movement of individuals and/or gametes

passive dispersal

• dispersal of pollen or seeds by wind

• dispersal of planktonic larvae by currents

active dispersal

• humans dispersing from Africa

• herd migration across plains

benefits of dispersal

• find better habitat (new/plentiful resources)

• better habitat for offspring

• less competition

• find new mates (reduce inbreeding)

cost of dispersal

• new habitat can be dangerous

• new habitat can be unpredictable

• energetic trade-off (moving long distances is energetically costly)

migration does NOT contribute to gene flow if it

doesn’t lead to mixing of alleles between populations

migrants move to a new population and reproduce in their new home

gene flow

migrants move to a new population and return to where they were born to reproduce (no mixing of alleles)

NOT gene flow

how is gene flow measured

to determine how to measure gene flow, we need to determine the nature of the environment

DeltaP = m(pm - p)

migration rate (m)

the fraction of individuals in a population that arrives from another population in each generation

p’ = p + dP

spatially continuous

there are no distinct populations so migration rate cannot be used

gene flow is measured by migration variance

migration variance O_m²

variance in the distribution of where individuals were born vs where they reproduce

square root of migration variance O_m

equal to the average distance between birthplaces of parent and offspring

genetic divergence between populations - measured by Fst

which measures the genetic variance found across populations that results from genetic differences between them