Selection and Genetic Variation
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Selection and Genetic Variation. 1) selection against recessive alleles If alleles are recessive lethal, then selection can only act on them when they are homozygous consider Dawson’s flour beetles: started with population of all heterozygotes, + / l

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Selection and Genetic Variation

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Selection and genetic variation

Selection and Genetic Variation

1) selection against

recessive alleles

If alleles are recessive lethal, then

selection can only act on them

when they are homozygous

consider Dawson’s flour beetles:

started with population of all

heterozygotes, + / l

l/ lis lethal, but + / lis same

as wildtype +/+


Selection and genetic variation

Selection and Genetic Variation

1) selection against

recessive alleles

Although selection initially

removed the lallele from

population at a rapid rate,

with each generation the

frequency of l declined

more slowly


Selection and genetic variation

Selection and Genetic Variation

2) selection against homozygotes

This population was started with 100% heterozygotes for a

viable allele V, and an allele L that is lethal when homozygous

although selection rapidly

caused the V allele to

increase in frequency,

the L allele never

disappeared

in fact, the frequency

of L stabilized at 0.21


Selection and genetic variation

Selection and Genetic Variation

2) selection against homozygotes

1/5th of the population carried the lethal allele at equilibrium

(the point where the population ceased to evolve)

Why?


Selection and genetic variation

Selection and Genetic Variation

3) selection against heterozygotes

  consider the case of flies with compound chromosomes

normal pair of

homologous

chromosomes

compound chromosomes: arms swapped

- one ends up with bothleft halves

- other ends up with bothright halves

when these flies make sperm/eggs, meiosis gets screwed up...

they make 4 kinds of gametes


Selection and genetic variation

Selection and Genetic Variation

3) selection against heterozygotes

- Flies can be homozygous for C (compound) or N (normal) allele

- two N/N flies can reproduce; all zygotes are viable (fitness =1)

- two C/C flies can reproduce; 1/4th of zygotes viable (fitness = 0.25)

- C/N flies don’t exist; they never develop (fitness = 0)

C and N flies can’t

make viable zygotes

together


Selection and genetic variation

Selection and Genetic Variation

3) selection against heterozygotes

one or the other allele quickly becomes fixed in a mixed population


Selection and genetic variation

Selection and Genetic Variation

3) selection against heterozygotes

one or the other allele quickly becomes fixed in a mixed population

- why? if there are few N/N flies, the odds of 2 mating are low

- most N/N flies will not produce viable offspring

- the allele will vanish

- if there are many N/N flies, they quickly out-breed C/C flies,

due to their 4-fold advantage in producing viable offspring

this is underdominance:


Selection and genetic variation

Models of heterozygote superiority and inferiority

- in overdominance (heterozygote fitness > homozygote fitness),

population fitness is maximized at its stable internal equilibrium,

the point to which the population naturally returns


Selection and genetic variation

Models of heterozygote superiority and inferiority

- in underdominance (homozygote fitness > heterozygote fitness),

the population fitness is minimized at the unstable internal

equilibrium, the point from which the population naturally diverges


Selection and genetic variation

Frequency-dependent selection

Attack other fish by sneaking up,

rushing them, biting off a mouthful

of scales

- Those with mouths that curve to

the right attack the left side of

victims, and vice-versa

- Handedness of mouth is

determined by a single locus

with 2 alleles (simplest case!)

- Right-handedness is dominant

scale-eating fish of

Lake Tanganyika


Selection and genetic variation

Frequency-dependent selection

- victims come to expect attacks from the direction that the

majority of the scale-eaters attack from, at that particular time

- when right-handed fish are more common, victims pay less

attention to their right side (where few attacks come from);

this gives left-handed fish the edge

- as left-handers get more food, they survive and reproduce better

- then, when left-handed offspring are the majority, the situation

reverses


Selection and genetic variation

Frequency-dependent selection

proportion of

left-handers

- squares = proportion of successful breeding adults


Selection and genetic variation

Frequency-dependent selection

proportion of

left-handers


Selection and genetic variation

Frequency-dependent selection

The equilibrium point should be 50/50 of each phenotype…

…so what are the expected allele & genotype frequencies?

Alleles:RL

Allele frequencies0.30.7

Possible genotypes:RRRLLL

Hardy-Weinberg predicts:R2+2RL+L2

Genotype frequencies:0.090.420.49


Selection and genetic variation

Frequency-dependent selection 2

Another case: pea aphid Acyrthosiphon pisum occurs in

greenand redcolor morphs

- what maintains polymorphism,

the occurrence of both phenotypes

in the population?

Differential vulnerability to predation versus parasitism,

depending on color

- green aphids are more parasitized by wasps that lay

their eggs inside aphids

- red aphids get eaten more by ladybugs (they’re more obvious

sitting there on green plants)


Selection and genetic variation

Mutation as an evolutionary force

Mutation is ultimately responsible for creating new alleles and genes, but..

- can mutation also represent an evolutionary force, by

changing allele frequencies?

- can mutation affect the predictions of Hardy-Weinberg

equilibrium?


Selection and genetic variation

Mutation as an evolutionary force

Consider a population where allele frequencies are:

Aa (a recessive, loss-of-function allele)

0.90.1

In the ordinary Hardy-Weinberg state, adult genotypes will be:

AAAaaa

0.810.180.01


Selection and genetic variation

Mutation as an evolutionary force

Now assume A mutates to a at a rate of 1 per 10,000 genes each generation

due to mutation, the allelic makeup of gametes will be:

Aa

0.9 – (0.9)(0.0001)0.1 + (0.9)(0.0001)

= 0.899991= 0.10009


Selection and genetic variation

Mutation as an evolutionary force

When gametes randomly fuse to form zygotes, the genotype

frequencies will be:

AAAaaa

0.809980.180160.01002

Hardly any change; mutation had little effect over one generation

Over thousands of generations, mutation can affect allele frequencies 


Selection and genetic variation

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


Selection and genetic variation

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


Selection and genetic variation

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:


Selection and genetic variation

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

- those arriving on an island with a small population can

make a huge difference to allele frequencies on the island


Selection and genetic variation

Migration

Example: banded vs unbanded water snakes


Selection and genetic variation

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


Selection and genetic variation

distribution of banded vs

unbanded snakes


Selection and genetic variation

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


Selection and genetic variation

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


Selection and genetic variation

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)


Selection and genetic variation

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


Selection and genetic variation

Random fixation of alleles

pop. size = 4

Given enough time, any allele will

eventually become fixed or

disappear if genetic drift is

the only mechanism at work

- 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

40

400


Selection and genetic variation

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


Selection and genetic variation

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

Question to ponder:

What forces prevent drift from fixing alleles in natural

populations?


Selection and genetic variation

Random fixation and loss of heterozygosity

Frequency of heterozygotes decreases over time, as alleles drift

towards fixation or extinction

- all else being equal (no selection, etc), the frequency of

heterozygotes should fall in every generation

- given by the relationship

Hg+1 = Hg 1 - 1 where N is population size

2N

You are trying to maintain a group of 50 endangered llamas

despite your efforts to arrange random matings...


Selection and genetic variation

Random fixation and loss of heterozygosity

Heterozygosity decreases in every

generation, but more slowly in large

populations

- the faster an allele disappears due to

drift, the more quickly you lose

heterozygosity

What can reverse this effect?


Selection and genetic variation

Random fixation and loss of heterozygosity

tested experimentally by Buri with fruit flies

- started 107 replicate populations each

with 8 boy + 8 girl flies

- all flies were initially heterozygotes for a

brown eye color allele (bw/bw-75)

- each generation, out of all offspring,

16 were chosen to start the

next generation

- monitored for 19 generations


Selection and genetic variation

Random fixation and loss of heterozygosity

Expected result:

- no selective advantage, so bw-75 allele

should drift to fixation 50% of the time

and be lost 50% of the time

- heterozygosity should decrease over time

Results after 19 generations:

- in 30 populations, bw-75 allele was lost

- in 28, it was fixed


Selection and genetic variation

Random fixation and loss of heterozygosity

Heterozygosity could also be directly scored by eye color

- decreased every generation,

as predicted by theory

- actually decreased faster

than expected, as though

N = 9 flies (not 16)


Selection and genetic variation

Founder Effect

When a population is founded by a few initial colonizers, their

genetic make-up will largely determine the allele frequencies as

the young population grows

A small group of founders will not carry all the alleles present in the

larger population they came from (reduced genetic diversity)

- if founders carry rare alleles, these alleles will be over-

represented in the new population relative to the original

large population

example: Pennsylvania Amish carry allele for a rare form of dwarfism, at a frequency of 7%

- only present at 0.1% in most populations

- one of original 200 founders had the recessive allele - consequence: way more Amish dwarves than you’d expect


Selection and genetic variation

Genetic drift and Elephant tusks

98% of 174 female elephants in the Addo National Park lack tusks

- population was reduced to 11

individuals by hunting, until

protected in 1931

- at that point, 50% of females

lacked tusks

- near loss of female tusks is

likely a result of genetic drift,

following population bottleneck

proportion of

females w/ tusks

population size


Selection and genetic variation

Genetic drift and Elephant tusks

98% of 174 female elephants in the Addo National Park lack tusks

- Alternative hypothesis: ivory

hunters imposed strong

selection against tusks

- if tusklessness were a recessive

trait, what would you expect to

happen to the frequency of

tusklessness since the

population was protected?

Why?

proportion of

females w/ tusks

population size


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