Encyclopedia of Evolution.pdf - Online Reading Center

population genetics
If allele A is present in 0 percent of the gametes (eggs and sperm) of
a population, and allele a in 0 percent of the gametes, the resulting
frequencies of the organisms will be percent AA, percent Aa, and
percent aa, under conditions of the Hardy-Weinberg equilibrium.
took blood samples from 20 birds representing six of the 13
species of Darwin’s finches on the Galápagos Islands.
They measured the number of alleles in these 20 birds for a
single locus, for the MHC II B class 1 gene. In 20 birds, there
can be at most 40 alleles. These birds had 21 alleles. Since
the rate at which this gene mutates has been estimated, the
researchers concluded that the common ancestor of all these
birds lived at least 10 million years ago. But the islands are
not that old. The original immigrant finches may have lived
on one of the islands that has eroded away and migrated to
their current locations. Furthermore, in order for the finches
on the Galápagos Islands to have this much genetic variability,
the original population would have had to contain at
least 40 genetically distinct individual birds.
Genetic Changes in Populations
In order to determine whether or not natural selection is
occurring in a population, biologists need to determine what
the frequency of each allele (for a given locus) would be in the
absence of natural selection, that is, if the population was at
equilibrium rather than being influenced by natural selection.
Consider a simple example in a species of animal, such
as a kind of fish, that has external fertilization and in which
a certain locus has just two alleles, A and a. Forty percent
of the sperm and eggs released into the water by the fish in
this population carry allele A, while 60 percent of the sperm
and eggs carry allele a. The frequency of allele A is p = 0.4,
and the frequency of allele a is q = 0.6. Because there are
only two alleles, p + q = 1. If the eggs and the sperm come
together randomly, you would expect the following results
(see figure).
1. Since 40% of the eggs, and 40% of the sperm, carry allele
A, there is a 40% × 40% = p 2 = 16% chance that the
zygote will have the homozygous AA genotype.
2. Since 60% of the eggs, and 60% of the sperm, carry allele
a, there is a 60% × 60% = q 2 = 36% chance that the
zygote will have the homozygous aa genotype.
3. There is a 40% × 60% = pq = 24% chance that a sperm
carrying allele a will meet an egg carrying allele A; there is
also a 24% chance that the opposite will happen, a sperm
carrying allele A will meet an egg carrying allele a. There
is therefore a 24% + 24% = 2pq = 48% chance that the
zygote will have the homozygous Aa genotype.
4. All of the zygotes will have one of the three genotypes AA,
Aa, or aa. Therefore p 2 + 2pq + q 2 = 16% + 48% + 36%
= 100%.
In a population that is at equilibrium, one would expect these
results simply due to genetic recombination: 16% AA, 48%
Aa, and 36% aa. This principle was first explained by G. H.
Hardy, a mathematician, and W. Weinberg, a biologist, and
is therefore called the Hardy-Weinberg equilibrium. If the
population differs from this pattern, the departure from equilibrium
could be caused by a number of possibilities:
1. The population may not be very large. Random departures
from equilibrium can occur in small populations (such as
genetic drift; see founder effect).
2. The mixture of gametes may not be occurring randomly
among individuals in the population. This could be caused
by nonrandom mating patterns, in which individuals within
the population prefer to mate with one genotype more
than with another.
3. Mutations have occurred.
4. Migration (of one genotype more than of another) has
occurred into or out of the population.
5. Natural selection has occurred.
In this last possibility, one of the alleles may be less frequent
than its equilibrium value because natural selection
has “selected against” individuals that carry this allele. The
Hardy-Weinberg equilibrium is the background against which
natural selection, defined as changes in allele frequencies in a
population, might be detected.
If natural selection operates against an allele, the allele
will become less common in the population. Strong selection
against a dominant allele (an allele whose presence always
shows up in the phenotype) may quickly eliminate that allele.
(This rarely occurs, because an allele that produces an inferior
phenotypic trait would probably never have become dominant
in the first place.) Strong selection against a recessive
allele (such as a lethal mutation), however, may never eliminate
the allele from the population. This is because the recessive
allele can hide in the heterozygotes; the heterozygotes are
therefore carriers of this allele. A lethal recessive will decline
in frequency according to this formula:
q n = q 0/1+nq 0,
in which q is the frequency of the lethal recessive allele, q 0
is the initial frequency of the allele, and q n is the frequency
after n generations. Starting with a frequency of 25 percent,