Transcript mutation, migration, genetic drift - Cal State LA

Mutation as an evolutionary force
Alleles may be kept in a population through a balance between
mutation (creating deleterious alleles) and
selection (removing them)
- in mutation-selection balance, the frequency with which
new alleles are created by mutation equals the rate at which
they are eliminated by selection
When the frequency of a harmful allele (say, cystic fibrosis) is
higher in a population than you’d expect from the mutation rate
of that gene, then you have reason to suspect some other force
(i.e., selection) may be keeping that allele around
Mutation as an evolutionary force
Why does the F508 allele, which causes cystic fibrosis, occur at
a high frequency (0.02) in populations of European descent?
- selection against homozygotes is strong
- mutation rate is too low to explain high allele frequency
- protects against infection by typhoid fever bacterium?
Mutation as an evolutionary force
More importantly, mutation promotes evolutionary change by
genetic innovation
- once a rare beneficial allele is created by mutation, it can rapidly
become fixed in the population through selective sweeps
bacteria evolved in a
series of jumps:
new mutations that
resulted in larger cell
size appeared,
rapidly spread through
the population
Migration
Migration is the movement of alleles between populations
Migration can rapidly change allele frequencies,
especially for small populations
- individuals leaving a continent make little difference to the
allele frequencies on that continent
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Migration
Example: banded vs unbanded water snakes
Migration
Example: banded vs unbanded water snakes
- one gene w/ 2 alleles determines banded, unbanded or
intermediate morph
- natural selection favors banded snakes on mainland,
where they are cryptic (hidden from predators)
- selection favors unbanded snakes on islands, where
bands stand out when snakes sun themselves on rocks
to warm up
distribution of banded vs
unbanded snakes
proportion of unbanded snakes
increases as you move further out
into middle of lake
there’s always some banded snakes
present on every island, though
Migration
Why doesn’t selection fix the unbanded allele on islands?
(drive it to a frequency of 100%)
- migrants from mainland continually introduce banded allele
into island population
- about 13 snakes per year move to islands, which have ~1300
snakes (roughly 1% migration per year)
Migration acts as a homogenizing force:
- equalizes allele frequencies among populations; makes them
more similar than they would otherwise be

What is a population?
Turns out to be a difficult thing to describe... may be defined by:
What is a population?
Turns out to be a difficult thing to describe... may be defined by:
- geography
- potential for dispersal (movement)
- genetic similarity (equal allele frequencies)
What is a population?
Turns out to be a difficult thing to describe... may be defined by:
- geography
- potential for dispersal (movement)
- birds can fly over a barrier other animals and plants can’t
1 population
2 populations
What is a population?
Turns out to be a difficult thing to describe... may be defined by:
- geography
- potential for dispersal (movement)
- genetic similarity (equal allele frequencies)
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Genetic Drift
A sampling process (flipping a coin, drawing beans from a bag)
may produce results different from theoretical expectations
- flip a coin four times, and you may get 4 heads
When the actual results differ from theory, this is sampling error
Sampling error depends largely on the number of samples drawn
- flip a coin 40 times, and you are very unlikely to get 40 heads
- will probably get ~20 heads, give or take a few

Genetic Drift
Sampling error in production of
offspring in a population is genetic drift
Initial frequencies are always heavily
skewed during random sampling
- ie, drawing alleles one at a time
from a big “batch” (= gene pool)
Genetic Drift
Sampling error is very sensitive to population size
- as population increases, effects of genetic drift diminish
- odds of getting the
expected allele
frequencies when
you make 10
zygotes by drawing
alleles at random
Random fixation of alleles
Given enough time, any allele will
eventually become fixed or
disappear if genetic drift is
the only mechanism at work
pop. size = 4
40
- when one allele is fixed, all others
have a frequency of zero
- the odds that any given allele will
be the one that goes to fixation is
the initial frequency of that allele
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Genetic Drift
(1) Every population follows a unique evolutionary trajectory,
because sampling error affects allele frequencies at random
- if selection were at work, different populations would evolve
along similar trajectories
(2) drift works faster and stronger in small populations
- allele frequencies change more dramatically if population
size is small
(3) even in large populations, drift can cause substantial evolution
over long times
- geographic isolation results in differentiated populations
eventually, different species