Evolution Biol 4802

Lecture 13, Chapter 9

 

Topics for today

    1. Detecting evolutionary change at the genetic level
    2. Effect of evolutionary processes on genetic variation

 

In the absence of evolutionary processes, genotype frequencies will remain constant over generations

1.      Prove conceptually to ourselves that this is true

2.      Explore underlying assumptions

3.      Statistically test whether this is true for any given population

In the absence of evolutionary processes, gene frequencies will remain constant over generations

Assumptions?

•         Everyone has an equal chance of reproducing

•         Everyone has an equal chance of surviving

•         Amount of reproduction of any given genotype is proportional to its frequency

•         Number of alleles (forms of the gene) doesn’t change

Why would any of these be false?

1.      Nonrandom mating

2.      Mutation

3.      Gene flow

4.      Genetic drift (Chapter 10)

5.      Natural selection (Chapter 11)

How can we test for these violations our assumptions?

1.      If observed and expected genotypic frequencies do match, the population is in Hardy-Weinberg equilibrium

2.      Evolutionary changes are not occurring

3.      If observed and expected genotypic frequencies do not match, evolutionary change of some kind is happening

4.      How bad is the deviation? Is it statistically significant?

5.      Chi-square test of significance

Fig. 9.6 (old 9.3)

Genotype frequencies that are in HW equilibrium

Heterozygotes at highest frequency when p = q = 0.5

Fig. 9.8 (old 9.5)

Most genes have more than 2 alleles

1.      HW equations can be derived for any number of alleles

2.      p2 + 2pq + q2 + r2 +2rp + 2rq + s2 + 2sp +2sq + 2sr + t2 + 2pt + 2qt + 2rt + 2st

Fig. 9.10 (9.13)

Do new mutations cause deviation from HW?

  • Only for a single generation
  • One generation of random mating restores HW equilibrium
  • Small effects of a single mutation probably not detected

HW is a tool to generate hypotheses

Assumptions:

1.      Nonrandom mating

2.      Mutation

3.      Gene flow

4.      Genetic drift

5.      Natural selection

An example

Calculate

•         Observed genotypic numbers

•         Observed genotypic frequencies

•         Allele frequencies

•         Expected genotype frequencies if in HW

•         Expected genotypic numbers

•         Do they match?

•         Major difference?

 

How does inbreeding effect genetic diversity?

 

Heterozygous class reduced by ½ with each generation of inbreeding

 

How do you measure the extent of inbreeding?

F-statistics compare the observed heterozygosity to what is expected based on HW

                  F = (Ho – H)

                              Ho

 

F ranges from -1 to 1

  0 = randome mating - no inbreeding              as expected under HW

  1 = full inbreeding                                        everyone is homozygous

 <0 = negative assortative mating               more heterozygotes than expected

Box 9B (new and old)

What effect does inbreeding have?

-Exposes recessive alleles because they are more often in the homozygous state

-Italian population 1903-1973

-Should you kiss your more distant cousins?

-New study of data from 1800-1965 in Iceland suggests it might not be so bad

Helgason et al. 2008 on web

How can you relieve inbreeding depression?

-Small isolated population of snakes was declining

-Added 20 males from a different population in 1992

-Removed in 1996

-Observed population recovery

Fig. 9.14 (old 9.12)

Can you avoid inbreeding depression?

1.      Don’t mate with your relatives

2.      Keep natural population size large

3.      Inbreeding avoided in captive breeding programs

a.      Golden lion tamarin is an endangered monkey

b.      Complex pedigree designs

c.      140 zoos, 500 individuals

Fig. 9.13 (old 9.11)