6.1.2 Populations and evolution: the meaning of a gene pool and allele frequency; the use of the Hardy-Weinberg principle to calculate allele and genotype frequencies; the factors that change allele frequencies (natural selection, genetic drift, the founder effect and migration); and the process of speciation (allopatric and sympatric).

What this dot point is asking

OCR wants you to define a gene pool and allele frequency, use the Hardy-Weinberg principle to calculate allele and genotype frequencies, explain the factors that change allele frequencies (natural selection, genetic drift, the founder effect and migration), and explain allopatric and sympatric speciation.

The answer

Gene pools and allele frequency

A gene pool is all the alleles of all the genes in a population at a given time. Allele frequency is the proportion of a particular allele in that gene pool. Evolution is a change in allele frequency over time.

The Hardy-Weinberg principle

For a gene with two alleles of frequency $p$ (dominant) and $q$ (recessive):

where $p^2$ is the frequency of the homozygous dominant genotype, $2pq$ the heterozygotes, and $q^2$ the homozygous recessive. Because the recessive phenotype shows only in $q^2$, you usually start there: take the recessive phenotype frequency as $q^2$, find $q$, then $p = 1 - q$, then calculate the genotype frequencies.

The principle assumes no mutation, no selection, no migration, random mating and a large population. If observed frequencies differ from the predicted ones, one of these conditions is not met (often selection is acting).

Factors that change allele frequencies

• Natural selection: advantageous alleles increase in frequency (directional, stabilising or disruptive selection).
• Genetic drift: random changes in allele frequency, especially in small populations, where chance can cause an allele to become more or less common regardless of its advantage.
• The founder effect: when a small group founds a new population, it carries only a subset of the original gene pool, so allele frequencies can differ markedly (a form of genetic drift).
• Migration (gene flow): individuals moving into or out of a population add or remove alleles, changing frequencies.

Speciation

A species is a group that can interbreed to produce fertile offspring. New species form when gene pools become reproductively isolated and diverge:

• Allopatric speciation: populations are geographically separated (a barrier such as a river or mountain). Different selection pressures, mutations and drift cause their gene pools to diverge until they can no longer interbreed.
• Sympatric speciation: isolation arises without geographical separation, within the same area, for example by polyploidy (common in plants), or behavioural, temporal or ecological isolation, again leading to reproductive isolation and divergence.

Examples in context

Example 1. The founder effect on islands. When a few individuals colonise an island, they carry only part of the original gene pool, so the island population may have unusual allele frequencies and even genetic disorders at higher rates, an example of drift via the founder effect.

Example 2. Polyploidy in crops. Many crop plants (such as wheat) arose by polyploidy, a form of sympatric speciation in which a doubling of chromosomes immediately produces a population reproductively isolated from its parents.

Try this

Q1. State the two Hardy-Weinberg equations. [2 marks]

• Cue. $p + q = 1$ and $p^2 + 2pq + q^2 = 1$.

Q2. Explain why genetic drift has a greater effect in a small population. [2 marks]

• Cue. In a small population, chance events (which individuals survive and reproduce) have a proportionally larger effect on allele frequencies, so alleles can be lost or fixed by chance regardless of advantage.

Q3. Name the type of speciation that occurs without geographical isolation. [1 mark]

• Cue. Sympatric speciation.