Non-Random Mating

What is it?

Non-random mating- the probability that two individuals in a population will mate is not the same for all possible pairs of individuals.

Why? Human Populations

Easily observable traits Cultural values Social rules Mating usually occurs between similar people with

respect to certain traits: skin color, stature, and personality.

Animal breeders essentially so the same thing Intentionally try to improve varieties or create new

ones Select mates for their animals with desired traits

Hoping to increase the frequency of those traits in future generations

Why? Cont…

Animal PopulationsSelect mates carefully, even without

interference from humans

  Why Animals Mate Non-randomly: Tale of the Peacock        video clip from PBS 2001 series Evolution        requires RealPlayer to view         (length = 4 mins, 2 secs)

Types of Non-Random Mating

Disassortative- individuals only mate with others who are phenotypically different from themselves for selective traits (opposites)

Assortative- individuals mate with others who are like themselves phenotypically for selected traits (similar) Inbreeding (extreme) – close relatives

1st, 2nd, cousins Siblings

Clicker Question

What is Inbreeding Depression? Choose the best answer.

a) A large-scale but short-term reduction of population size followed by an increase in population size.

b) Feelings of sadness due to inbreeding within a species.

c) Reduction in the mean fitness of a population due to the presence of deleterious alleles.

d) Reduced fitness in individuals or populations resulting from kin matings.

Inbreeding Depression

Is a reduced fitness in a given population as a result of breeding between related individuals.The reduced fitness is generally caused by

recessive deleterious alleles.

What are Deleterious Alleles?

Deleterious alleles refer to a version of a gene that, on average, decreases the fitness of the organism carrying it.

Deleterious alleles arise constantly through mutations, so they are always present in a population at low frequencies.

Inbreeding Depression

Over time, natural selection weeds deleterious alleles out of a population—when the dominant deleterious alleles are expressed, they lower the carrier’s fitness, and fewer copies wind up in the next generation.

Recessive deleterious alleles are “hidden” from natural selection by their dominant non-deleterious counterparts. An individual carrying a single recessive deleterious allele will be healthy and can easily pass the deleterious allele into the next generation.

When the population is large, this is generally not a problem—the population may carry many recessive deleterious alleles, but they are rarely expressed. However, when the population becomes small, close relatives end up mating with one another, and those relatives likely carry the same recessive deleterious alleles. When the relatives mate, the offspring may inherit two copies of the same recessive deleterious allele and suffer the consequences of expressing the deleterious allele.

The Greater Prairie Chicken

200 Years ago, the state of Illinois was almost entirely covered with prairie and was home to millions of greater prairie chickens.

Greater Prairie Chickens Cont…

The numbers fell to just 25,000 in 1933; 2,000 in 1962; 500 in 1972; 76 in 1990. By 1994, there were fewer than 50 greater prairie chickens.

These remaining birds belonged to two remnant populations—one in Marion County and the other in Jasper County.

The Question Is…

Why did the Jasper County prairie chicken population continue to decline from the mid-1970’s to the mid-1990’s, even though the amount of habitat available was increasing?

Ronald Westemeier and Colleagues (1998) hypothesis… Destruction of the prairie did two things.

1. Directly reduced the size of the birds’ population.

2. Isolated the few surviving birds from each other and from populations in other states.

Remember…small populations with little gene flow are precisely the setting in which genetic drift is most powerful.

Clicker Question

True or False, a reduction in fitness due to genetic drift is reminiscent of inbreeding depression.

Consequences… Genetic drift resulted in the fixation of alleles.  Some of

these were deleterious, reducing the mean fitness of the population.

As populations shrunk, more matings occurred among close relatives.  Thus inbreeding increased the frequency of homozygotes.  As deleterious recessive alleles were exposed to selection, inbreeding depression resulted.

As the population mean fitness was reduced, population size continued to shrink, exacerbating the problems that caused it to shrink in the first place.

Evidence to support this explanation (the case of the Jasper County population):

From 1963 to 1990 there was a steady decline in hatching success.  This could have been due to inbreeding depression (there are other possibilities).

If inbreeding depression/genetic drift was the culprit, genetic diversity (e.g. number of alleles per locus) should have been lower in Jasper Co. individuals than in individuals from other, larger populations. It was.

If inbreeding depression was the culprit, than introducing alleles from other populations should have increased hatching success.  It did - introducing individuals from other populations had a dramatic impact.

Understanding Non-Random Mating

Random MatingThere are nine possible mating patterns for a

trait that is controlled by two alleles (A and a):

AA  X  AA Aa  X  AA aa  X  AA

AA  X  Aa Aa  X  Aa aa  X  Aa

AA  X  aa Aa  X  aa aa  X  aa

With random mating, the expected offspring genotype frequencies are 25% homozygous dominant (AA), 50% heterozygous (Aa), and 25% homozygous recessive (aa).

This ratio does not change from generation to generation.

Assortative Mating Taken to the extreme…

There are only three possible mating patterns with respect to genotypes for traits controlled by two alleles (A and a):

AA x AA Aa x Aa aa x aa

Net effect of assortative mating: Progressive increase in

the number of homozygous genotypes (AA and aa)

Corresponding decrease in the number of heterozygotes (Aa)

This trend will continue from generation to generation.

Disassortative Mating

There are six possible mating patterns with respect to genotypes for traits controlled by two alleles (A and a):

AA x Aa Aa x aa aa x AA

AA x aa Aa x AA aa x Aa

Net effect of disassortative mating: Progressive increase in the

frequency of heterozygotes (Aa)

Corresponding decrease in the number of homozygous genotypes (AA and aa)

This trend will continue from generation to generation.

Note: Has the opposite effect as assortative mating

PopGenLab Assortative Mating

What is the effect on genotype frequencies?Allele frequencies?Heterozygosity?Population size?Bottleneck?

Disassortative MatingWhat is the effect on genotype frequencies?Allele frequencies?Heterozygosity?Population size?Bottleneck?

Evolutionary Effects/ Consequences

1) Which type of mating pattern is most likely to lead to evolution in allele frequencies? Why?

2) Which type of mating pattern promotes a wider genetic variation in populations? Why?

Assortative MatingLoss of heterozygosityLoss of variation among genesLeads to genetic drift and fixation (small pop.)

Leads to inbreeding depression

Disassortative Mating Increase in heterozygosity Increase in variation among genes