Forces of evolution: migration and drift

1
Forces of evolution migration and drift

• How can unfavourable alleles become common in a
  population?
• PKU hypothesis 3

2
A simple life cycle
drift
ZYGOTES
GAMETES
survival
mutation
JUVENILES
ADULTS
migration
3
Genetic drift sampling error
f(A2) q 0.4
f(A1) p 0.6
A2
A2
adults
juveniles
4
p 0.6
5
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)

6
Fixation of alleles by genetic drift
p 0.5 p 0.7 p 0.9
7
Fixation probability

• New mutation q 1 / 2N (why?)
• Probability of fixing allele
• New neutral
• Effect of selection?

8
Extreme case of drift founder effects

• Pingelap

9
Drift and heterozygosity

• Heterozygosity

10
Heterozygosity decline

• H' (1 - 1/2N) H Ht (1 - 1/2N)t H0

N 1000
N 100
N 10
Assumes no mutation.
11
Experimental data decline in heterozygosity in
Buris fruit flies
expected
actual
Figure 6.17
12
Low-heterozygous populations and effective
population size (Ne)
13
Why is the effective population size lower than
the census population size?

• I.
• Example
• N 20, 20, 50, 200, 2000, 20000.

14
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.

15
2nd Problem for small populations inbreeding
16
Inbreeding and Hardy-Weinberg

• Genotype AA AA AA
• Adult number 250 500 250
• HW prediction 250 500 250
• Actual offspring

17
Outcome of inbreeding inbreeding depression
18
Inbreeding depression in humans
Figure 6.28
19
Why do self-fertilizing taxa exist?
20
Migration

• Alleles f(A) 0.5 p f(A) 0.5 q
• Genotype AA AA AA
• frequency p2 2pq q2
• Initial 250 500 250
• migrants
• Adults

21
Migration and selection

• Imagine one species in two different environments
•  
• Effect of different selection pressures?
•  
• Effect of migration?

22
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
23
Frequency of banding on islands
D banded A non-banded
FH, chapter 7
24
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!

25
A few migrants homogenize things nicely

• FST measure of genetic population
  differentiation

26
Selection and drift new mutations
PopG
27
Selection and drift new mutations
PopG
28
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

29
Readings and questions

• Sacks, 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).
• Questions
• 1. 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?

30
Readings and questions, continued

• Questions
• 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?