(Mendelian) Genetic Evolution in Population: Genetic Drift (or Random Drift) (lect 10)

(Population genetic) evolution

change in population allele frequencies

Selection

change in population allele frequencies due to beneficial allele being (non-randomly) favored for survival and reproduction

(Genetic) Drift

change in population allele frequencies due to random effects (the random sampling of allele over multiple generations)

Sampling error

finite-sized populations, NEVER produce sample element frequencies that exactly reflect population element frequencies

The larger the sampling error

The smaller the population size

Sampling error becomes more and more larger

As population sizes get smaller

Smaller populations (than larger)

Genetic drift has more of an impact on

In small populations

genetic drift fixes alleles faster

In large populations

genetic drift takes longer to fix alleles

Census population size

the population size in terms of the number of individuals

Effective population size, (N_e)

the number of individuals who actually breed

Not all individuals reproduce

natural selection and seuxal selection result in fewer individuals breeding than are present in the population

Than the Census population size

Effective population size is often far smaller

WITHIN population

Genetic drift “erodes” genetic variation as alleles get fixed

Monomorphic (instead of polymorphic)

Population eventually becomes ____ in genetic drift

N_e is smaller

Genetic variation is lost faster when

Rare alleles

tend to be lost more easily in Genetic variation

Rate of neutral substitution (under random genetic drift)

is determined by the mutation rate alone, and independent of the population size!

BETWEEN populations

Drift increases genetic variation

Chance

Different neutral alleles get fixed / lost in different populations by

Isolation

Drift is important to cause populations to diverge after

Bottleneck Effect

populations go through extreme reduction in size

Founders Effect

new populations initiated by small numbers of “founders”represent a very small non-representative sample of the source population

Both selection & genetic drift

can fix (and cause the loss) of alleles in a population

Selection is NOT random

Selection will fix beneficial alleles and remove deleterious ones

Genetic drift IS random

genetic drift is ___ with respect to the alleles beneficialness

Genetic drift can overcome selection to fix deleterious alleles

If both genetic drift and selection occur in a population, and selection is weak enough, and the population size is small enough

Random fixation or loss of alleles

Why does genetic drift reduce genetic variation?

Genetic drift is random (natural selection changes based on fitness)

How does genetic drift differ from natural selection?