Download Non-random mating

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
Figure 22.6
A1A1
Homozygote
A1A2
Heterozygote
A2A2
Homozygote
Generation 1
100%
25%
50%
25%
100%
Generation 2
100%
25%
50%
100%
25%
Generation 3
100%
100%
Generation 4
0
25
50
Frequency of genotypes
75
100
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 tanand 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 HardyWeinberg 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