Hardy-Weinberg and speciationEdexcel A-Level Biology A: Revision notes

All the alleles of all the genes in a population make up its gene pool. The allele frequency is the proportion of a particular allele in the gene pool, given as a decimal between 0 and 1.

For a gene with two alleles, the dominant allele has frequency $p$ and the recessive allele has frequency $q$. All the alleles in the gene pool add up to 1:

$p + q = 1$

If a population is evolving, the allele frequencies change from one generation to the next. The Hardy-Weinberg equation gives a way to check for this.

If a population is in Hardy-Weinberg equilibrium, the allele frequencies stay the same from generation to generation, and the genotype frequencies can be predicted:

$p^2 + 2pq + q^2 = 1$

• $p^2$ = frequency of the homozygous dominant genotype
• $2pq$ = frequency of the heterozygous genotype
• $q^2$ = frequency of the homozygous recessive genotype

The equation holds only if:

• mating is random
• there is no selection
• there is no mutation
• there is no migration (no gene flow)
• the population is large

Cystic fibrosis is caused by a recessive allele. About 1 in 2500 babies in a population is born with it.

1. $q^2 = 1 \div 2500 = 0.0004$
2. $q = \sqrt{0.0004} = 0.02$
3. $p = 1 - 0.02 = 0.98$
4. Carriers: $2pq = 2 \times 0.98 \times 0.02 = 0.0392$

So about 3.9% of people, roughly 1 in 26, are carriers. In a population of 100 000, we expect $0.0392 \times 100\,000 = 3920$ carriers.

The equation describes a population in which nothing is changing. We can use it as a reference:

• calculate the allele frequencies (or the expected genotype frequencies) at one time
• repeat for a later generation
• if the allele frequencies have changed, the population is not in equilibrium, and one or more of the conditions is not met

A change in allele frequency over time shows that evolution is occurring. Causes include selection, mutation, migration (gene flow) and chance in small populations. Non-random mating changes the genotype frequencies, but does not change the allele frequencies by itself.

A species is a group of organisms that can interbreed to produce fertile offspring. Reproductive isolation stops populations interbreeding, so there is no gene flow between them.

• Geographical isolation: a physical barrier such as a mountain range, sea or river separates populations.
• Seasonal (temporal) isolation: populations breed at different times of year.
• Behavioural isolation: differences in courtship or mating behaviour prevent mating.

Once isolated, each population undergoes its own mutations and, in different conditions, different selection pressures. The allele frequencies change in different ways.

After a population is divided, the sequence is:

1. No gene flow between the populations.
2. Mutations arise independently, giving different alleles.
3. Different selection pressures favour different alleles.
4. Allele frequencies change in different ways; the populations accumulate different genetic information.
5. Eventually members of the two populations cannot interbreed to produce fertile offspring: they are separate species.

This is called speciation.