Lecture 7: Genetic Drift

evolution

a change in allele frequencies in a population over time

evolution occurs when…

natural selection, genetic drift, or gene flow alter the allele frequencies of a population over time

random process are processes that can have ___

different outcomes

genetic drift is a …

random process

genetic drift

random change in allele frequencies from one generation to the next due to “random sampling”

we can observe drift over time by…

tracking the frequency (proportion) of one allele

genetic drift is ____ at some level in real populations

always acting

genetic drift represents the constant ___ of evolution

“background noise”

what causes random sampling in real populations

any process that has the effect of randomly adding or subtracting fitness irrespective of genotype

meiosis lottery

one large source of random sampling

oogenesis

which allele becomes the ovum is effectively two coin flips: random sampling of the four gametes

random environmental events are another large source of random sampling

• natural disasters can kill individuals randomly

• resources and mates can be randomly encountered or lost

drift is the sum of…

all random events that add or subtract from the fitness of individuals

because drift is always happening…

• it is the main driver of allele frequency change (evolution) at the genetic level

• it is the “null hypothesis” when testing for other evolutionary processes (ex. selection)

buri drift experiment

• shows how drift causes the loss of genetic diversity

• most populations in buri’s experiment fixed (100% frequency) one of the two alleles

the effects of drift are ____ in small populations

more extreme

drift causes more variable and extreme changes in allele frequency in ____ populations

smaller

alleles are fixed more rapidly in small populations, resulting in a ___

loss of variation

smaller populations evolve differences ___

faster

“census size” or N_c

count of all the individuals in a population

census size is not the same as the size of the actual gene pool

• every individual may not contribute to the gene pool, equally or at all

  • ex. animals where a single male monopolizes multiple females (so some males don’t breed)

• population size may also be fluctuating through time

effective population size: N_e

the number of breeding individuals in an idealized population that would show the same amount of genetic drift as seen in the population being studied

N_e indicates what?

the strength of drift

reductions in effective population size can cause drift to become ___

stronger

the bottleneck effect magnifies the effect of __

genetic drift

founder effects occur when …

some individuals become isolated from a larger population

genetic variation in a population can be quantified by __

heterozygosity

we expect populations that have experienced more drift to have ___ heterozygosity

lower

5 key points about genetic drift

• it is unbiased: the frequency of any allele is just as likely to go up as to go down (unlike selection)

• it is stronger in smaller populations: smaller samples = more drift

• it causes genetic variability to become different: can give populations the appearance of specialization to a site

• drift causes alleles to fix (reach 100%): even in the absence of all selection

genetic drift is ___ occurring, even when selection is acting

always

mutations with ______ can easily overcome drift

large fitness effects (s)

mutations with ______ can not overcome drift

small fitness effects (s)

in small populations, drift can cause deleterious alleles to __

fix

when drift is weaker (large N_e), ____ effect mutations can overcome it

smaller

the ability of selection to overcome drift depends on …

their relative “strengths”

strength of drift

1/N_e

strength of selection

s

if 1/N_e > s for an allele …

drift will overwhelm selection

in small populations, natural selection needs to be ____ to overcome drift

very strong

in large populations, ____ alleles can overcome drift

weakly selected

drift is random, but will eventually cause the fixation or loss of…

ALL alleles

fixation

an allele reaches 100%

fixation probabilities are based on what?

current allele frequencies

the probability that an allele will eventually fix is what?

its current frequency (p) : P_fix = p

a higher starting frequency (ex. 0.9) means…

it is more likely to fix

a lower starting frequency (ex. 0.01 or a new mutation) means…

it is less likely to fix (will be lost)

why are most new mutations lost from genetic drift

because they start out with low frequencies

new mutations occur in a ___ copy in the population

single

diploids have two copies of their genomes, so the initial frequency of a mutation is:

1/2N

P_fix for a new mutation will fix is:

P_fix = 1/(2N)

why do cheetahs have such low genetic diversity?

they have experienced multiple bottleneck events