What the heck is Hardy-Weinberg equilibrium?
As Darwin discovered, populations change from generation to generation to adapt to the environment their living in. What we see changing are physical characteristics or traits, however what's actually changing from generation to generation are the genes or alleles (versions of genes) responsible for making those trait we see.
Thus, when a population evolves, the genetic material present in that population is changing.
A common way scientists measure whether or not a population is evolving is by seeing if that population meets Hardy-Weinberg equilibrium.
If a population meets Hardy-Weinberg equilibrium, then it is considered to NOT be in the process of evolution. (However, know evolution takes years to occur, thus although a population may seem to not be evolving and thus meet Hardy-Weinberg equilibrium, it could actually be in the process - just extremely slowly)
If Hardy-Weinberg equilibrium is met, it means there are no mechanisms of evolution occurring - No natural selection, No genetic drift, No gene flow, No mutations, and only random mating. These are also the 5 conditions that must be met in order for this equilibrium to happen in a population.
The way scientists try to find out if a population meets Hardy-Weinberg equilibrium is be using the Hardy-Weinberg equations below. Note that usually ONE gene at a time is looked at to see if it meets this equilibrium, thus there's only 2 alleles (or two versions of that gene, p and q) that are observed.
p² + 2pq + q² = 1
Where p²stands for the AA (homozygous dominant) genotype
Where 2pq stands for the Aa (heterozygous) genotype
Where q² stands for the aa (homozygous recessive) genotype
Also...
p + q = 1
Used to find the frequency for alleles
What these equations describe are the frequency of the genotypes for a particular gene, as well as the frequency of alleles for that particular gene in a population. Because frequency is usually written in percent or decimal, and there are only two alleles present for the gene in question (p and q) then they must add up to be 1 (reasoning for the 2nd equation).
Here's an example problem to show you how you might use this equation:
What are the allele frequencies for a particular gene in a population if the frequency of homozygous recessive individuals in that population is 25%?
Using the Hardy-Weinberg Equation, p² + 2pq + q² = 1, we know the frequency of homozygous recessive genotypes are q²
Thus, q² = .25 --> square root of both sides --> q = .5
If you know q, you can now find the other allele, p by using p+q=1
Thus, p+.5=1 --> subtracting .5 from 1 --> p=.5
Therefore the two allele frequencys are .5 or 50%!
