Forces of evolution: migration and drift

How can unfavourable alleles become common in a population?

PKU: hypothesis 3

A simple life cycle

GAMETES ZYGOTES

JUVENILESADULTS

survival

migration

mutation

drift

Genetic drift: sampling error

juvenilesadults

f(A1) = p = 0.6 f(A2) = q = 0.4

A2A2

0

0.05

0.1

0.15

0.2

0.25

0.3

0 1 2 3 4 5 6 7 8 9 10

Number of A

Prob

abilit

y

p = 0.6

Effects of genetic drift

• Population size:

• Genetic diversity:

• Population differentiation

Comparison:

population of 50: 25 A1 alleles (p = 0.5)

population of 500: 250 A1 alleles (p = 0.5)

Fixation of alleles by genetic drift

p = 0.5

p = 0.7

p = 0.9

Fixation probability

• New mutation: q = 1 / 2N (why?)

• Probability of fixing allele:

New neutral:

Effect of selection?

Extreme case of drift: founder effects

• Pingelap:

Drift and heterozygosity

• Heterozygosity:

Heterozygosity decline

H' = (1 - 1/2N) H Ht = (1 - 1/2N)t H0

0

0.1

0.2

0.3

0.4

0.5

0.6

0 2 4 6 8 10

Generations

He

tero

zyg

os

ity N = 100

N = 10

N = 1000

Assumes no mutation.

Experimental data: decline in heterozygosity in Buri’s fruit flies

expected

actual

Figure 6.17

Low-heterozygous populations and effective population size (Ne)

I.

Example:N = 20, 20, 50, 200, 2000, 20000.

Why is the effective population size lower than the census population size?

II. Variation in reproductive success:

Effective size depends on reproducing males and females

Example:

N = 200, half are male, only one mates. All females mate.

Why is the effective population size lower than the census population size?

2nd Problem for small populations: inbreeding

Inbreeding and Hardy-Weinberg

Genotype AA AA’A’A’

Adult number 250 500250

HW prediction 250 500250

Actual offspring

Outcome of inbreeding: inbreeding depression

Inbreeding depression in humans

Figure 6.28

Why do self-fertilizing taxa exist?

Migration

Alleles: f(A) = 0.5= p f(A’) = 0.5 = qGenotype AA AA’ A’A’frequency p2 2pq q2

Initial 250 500 250

migrants

Adults

Migration and selection:

• Imagine one species in two different environments

 

• Effect of different selection pressures?

 

• Effect of migration?

Selection and migration: water snakes

Trait: banding

Mainland: banded snakes (dominant)

Islands: unbanded snakes (recessive)

Banded snakes are selected against on islands (s = -0.16)

Migration maintains banding on islands

Frequency of banding on islands

F&H, chapter 7

D= banded

A = non-banded

Migration and population differentiation

• Fst: measure of isolationm = proportion of population from migration

• Fst = 1 / (4Nem + 1) at equilibrium

More migration, lower Fst

Assumption: alleles studied are neutral!

• FST – measure of genetic population differentiation

A few migrants homogenize things nicely

Migration in a population of 100

0

0.2

0.4

0.6

0.8

1

-6 -5 -4 -3 -2 -1 0 1 2

Log # of migrants / generation

Fs

t

Selection and drift: new mutations

PopG

Selection and drift: new mutations

PopG

Summary

• Migration: – changes allele frequencies in a population– can increase genetic diversity– decreases differences between populations– can work against selection to maintain unfavorable

alleles• Drift

– decreases genetic diversity– more important in small populations– can lead to loss of beneficial mutations or fixation of

deleterious mutations– increases differences between populations

Readings and questionsSacks, Oliver. 1997. Island of the colorblind. Knopf: New York.

(Oliver Sacks is an amazing neurologist and author. His books include Awakening, The man who mistook his wife for his hat, and A leg to stand on. In this book he travels to Pingalep.)

Freeman and Herron chapter 7 (chapter 6, 3rd edn).

Questions1. Genetic diseases are often common in isolated human populations,

such as porphyria (a blood disease, potentially the cause of the madness of King George) among South Africans of Dutch descent. Suggest two reasons why genetic diseases might be common in isolated populations.

2. Consider the roles of migration, selection, drift (including founder effects), and inbreeding in a newly founded island population compared to the source mainland population. For each factor, consider whether it is likely to a more important factor in the mainland or the island. How might these factors explain that remote islands are more likely to have endemic species (i.e. species found only there) compared to islands close the mainland?

3. Founding new domesticated lines (such as new dog breeds) usually requires a great deal of inbreeding. Is it possible to generate new dog breeds without the new breed suffering from inbreeding depression - if so, how?

Readings and questions, continuedQuestions

4. Conservation organizations now devote tremendous resources to preserving corridors that link patches of habitat. There has been debate about the effectiveness of these measures. Describe how you might test the effectiveness of corridors by observations of Fst and inbreeding depression.

5. You are working on the genetics of a rare tropical parrot and discover that the population has very low heterozygosity. Describe three different scenarios that could account for the lack of genetic diversity in this population, and explain how you might be able to distinguish among these.

6. The Ivory-billed wood pecker was thought to be extinct for most of the 20th century, but reports in 2005 suggest that it may still survive in a patch of forest in Arkansas in the United States. It is similar to the pilleated woodpecker, which is relatively common throughout much of the United States and Canada. How would you expect these two species to compare in terms of heterozygosity? In terms of inbreeding? In terms of hatching success?