Slide 1
So far,
we have dealt with the behavior of genes between generations and focused on
the phenotypes of individuals. Next we'll take a broader look at genetics and
see how the genetics of populations are studied.
Population genetics is the field of biology that studies the genetic composition of populations, and the changes in genetic composition that result from the operation of various evolutionary factors
Slide 2
Population geneticists study
populations. Populations are groups of individuals of the same species
that live in the same geographic area or range.
When population geneticists study a population - they study the gene pool of the population. The gene pool is the sum of the alleles present in a population at a particular time.
Populations can vary in the amount of genetic variation (and thus alleles) that is found in the population. For example, at the Penn State - University Park campus - the population of gray squirrels is monomorphic for fur color and are all gray while the squirrels on the Kent State campus are polymorphic - there are gray and black squirrels. This means there is more genetic variation in the Kent State population with respect to coat color.
Slide 3
Population geneticists have a
tool that allows them to keep track of allele and genotype frequencies in a
population (it is similar to the Punneqq square tool the geneticists use to
analyze crosses between individuals).
This tool is known as the Hardy-Weinberg equation and it has two components. The first is the equation that sums allele frequencies (we will start by looking at a gene with two alleles).
In a gene with two alleles - we will indicate one allele as p (this is typically the dominant allele) and the other allele as q (this is typically the recessive allele). For a gene with just these two alleles - the frequency of these alleles must sum to 1. So p + q = 1.
For
a gene with two alleles there are three possible genotypes - homozygous
dominant, heterozygous, and homozygous recessive. The frequency of the
homozygous dominant individuals is p^2, the frequency of the heterozygous
individuals is 2pq, and the frequency of the homozygous recessive individuals
is q^2. These indivdiual
frequencies must sum to 1.
Slide 4
We
can look at an example using a hypothetical gene A in which the frequency of
the dominant allele is .2 and the frequency of the recessive allele is .8.
We can also visualize the Hardy-Weinberg equation by using a Punnett
Square - in this example we are using a Punnett square to visualize a
potential cross between all individuals in the population so we must include
the allele frequencies along with the alleles along the axes.
When we fill out the Punnett square we see the frequency of homozygous dominant individuals is .04, the frequency of heterozygotes is .32 (using the 2pq part of the equation). The frequency of homozygous recessive individuals is .64. The sum of .04 and .32 and .64 is 1.
In the absence of outside factors, the frequencies of gametes, genotypes, and phenotypes stay the same from one generation to the next.
Population geneticists use this tool to watch for changes in allele and genotype frequencies.
Slide
5
As you have learned earlier, some genes have more than two alleles.
So we can take a more complicated look at population genetics and the
Hardy-Weinberg equation by looking at an example of a gene with three alleles.
Typically, in a three allele situation, we use the letters p, q, and r to describe the three alleles. The genotype frequencies are derived from multiplying (p+q+r) times (p+q+r). This gives genotype frequencies of p^2 + 2pq + q^2 + r^2 + 2pr + 2qr.
Slide 6
Remember that our definition
of evolution is a change in the genetic structure of a population. This
means a change in allele and genotype frequencies. The Hardy-Weinberg
equation is a tool that allows us to monitor changes in allele and genotype
frequencies and thus observe evolution occuring in populations.
When allele and genotype frequencies change in populations - we refer to the population as moving out of Hardy-Weinberg equilibrium. It is the forces of evolution - such as natural selection - that move a population out of Hardy-Weinberg equilibrium.