Species exist as one or more populations. A population is a group of individuals of the same species who live in a specific area. It is at this population level that evolution is possible.
Due to the fact that the individuals are all from the same species they’re able to interbreed. They share groups of genes that are the same and which together are called the gene pool. A gene pool consists of all the alleles and so all the phenotypes possible within a population. The number of times an allele for a specific trait occurs in regards to the total number of alleles for that trait is known as the gene frequency. When a number of gene frequencies change over a period of time then evolution can occur.
gene frequency = the number of a specific type of allele
the total number of alleles in the gene pool
A English mathematician, G. H. Hardy, and a German physician, W. R. Weinberg, both worked out independently how random mating in successive generations effects allele frequency in a population.
The principal demonstrates a hypothetical situation in which there’s no change in the frequency of the alleles. As no change in the gene pool occurs there can be no evolution. Using this hypothetical basis of stability it’s possible to measure real change.
The equation they worked out was:
p2 + 2pq + q2 = 1
Where p is the frequency of the dominant allele (A) and q is the frequency of the recessive allele (a).
All the alleles must added up to 100% so it works out that p + q = 1. To cover all possible combinations within a population would then be equal to (p + q)2 or p2 + 2pq + q2 = 1.
The frequencies of the two alleles, A and a, will stay constant if:
- the population is big so that random sampling errors are kept to a minimum
- mating is always random as opposed to any preferences between males and females
- no mutations occur
- no migration occurs which causes genes to be exchanged between populations
- there’s no natural selection