Non-random mating

Mating in many species is often assortative

• Humans provide lots of examples

• Humans tend to marry people of similar intelligence

• Humans tend to marry people of similar physical appearance

Height

Skin color

Inbreeding

Inbreeding is a type of non-random mating in which mates are chosen on the basis of relatedness

Inbreeding is typically non-adaptive - increase in likelihood of deleterious recessive alleles being expressed

The loss of vigor, health and survivorship from inbreeding is called inbreeding depression

Generation 1

Generation 2

Generation 3

Generation 4

A1A1

HomozygoteA1A2

Heterozygote

100%

100%

100%

25%

25% 50%

50%

0 25 50Frequency of genotypes

25%

25%

100%

100%

100%

A2A2

Homozygote

75 100

Figure 22.6

Table 22.3

Non-random mating

In some species, pairing is dissassortative; mates are chosen that are different

• An example is the White-throated Sparrow, a common breeding bird throughout New England

• Two morphs of this bird are found

• Head with black and white stripes

• Head with black and tan stripes

White-throated Sparrow morphs

90% of the pairs are between tan- and white-striped individuals

Why is dissassortative mating seen in White-throated Sparrows?

The color of the head striping is correlated with parental qualities

• Tan-striped females tend to be good foragers

• White-striped males tend to be good defenders but relatively poor foragers

• Tan-striped males and white-striped females are intermediate in foraging ability

• Tan-striped males are less aggressive than white-striped males

Is non-random mating an evolutionary mechanism?

No. Allele frequencies in the population are not changed

Rather, the mix of genotypes differs from the Hardy-Weinberg prediction

Small population sizes

When populations are small, the phenomenon of genetic drift may come into play

Genetic drift is defined as any change in allele frequencies that is due to chance

Warwick Kerr and Sewall Wright experiments on genetic drift with Drosophila

Kerr and Wright experiments on genetic drift with Drosophila

The biologists concentrated on a single trait, the shape of leg bristles, controlled by a single locus

The wild type condition is normal, unbranched bristles; the mutant type is bristles that bend at the tip (forked)

The shape of the bristles does not affect the fitness of the flies so selection based on bristles did not occur

Experimental design

Used 96 cages, each of which received four female and four male Drosophila

Kerr and Wright began with p = q = 0.5

After the parental generation bred, Kerr and Wright raised the offspring in each cage

From each cage, they randomly chose four males and four females to continue the line in each cage

They repeated this process for 16 generations

Experimental results

Three types of outcomes:

• In 29 of the populations, the allele for normal bristles had disappeared. All of the flies were fixed (homozygous) for forked bristles.

• In 41 populations, the flies were fixed for normal bristles (the forked allele had disappeared)

• In the remaining 26 populations, both alleles were still present

• The results are surprising: in 73% of the experimental populations, genetic drift reduced allelic diversity to zero!

Flipping a coin as a model of genetic drift

Let’s flip a fair coin (heads and tails are equally likely) 20 times

Let heads and tails be the two alleles in our population. We begin with p = q = 0.5 (since heads and tails are equally likely)

Here are the results of our 20 coin flips:

• H-T-T-T-T-T-H-H-T-T-H-T-H-H-T-T-H-H-T-H

The coin model

H-T-T-T-T-T-H-H-T-T-H-T-H-H-T-T-H-H-T-H

We therefore got 11 tails and 9 heads

By random events, we changed the gene frequencies: f(H) = p = 0.45 and f(T) = q = 0.55

If we flipped our coin 1,000 times, p and q would be quite close to 0.50

If we stopped flipping the coin after only six tosses, our new values of p would be 0.17 and q would be 0.83