Download Non-Random Mating

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 Populations
 Select
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

a)
b)
c)
d)
What is Inbreeding Depression? Choose
the best answer.
A large-scale but short-term reduction of population
size followed by an increase in population size.
Feelings of sadness due to inbreeding within a species.
Reduction in the mean fitness of a population due to the
presence of deleterious alleles.
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.
Directly reduced the size of the birds’
population.
2. Isolated the few surviving birds from each
other and from populations in other states.
1.

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 Mating
 There
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 Mating
 What
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 Mating
 Loss
of heterozygosity
 Loss of variation among genes
 Leads to genetic drift and fixation (small pop.)


Leads to inbreeding depression
Disassortative Mating
 Increase
in heterozygosity
 Increase in variation among genes