Allele frequency, speciation and conservationEdexcel International A Level Biology: Revision notes

The gene pool is all the alleles of all the genes in a population. The allele frequency is the proportion of all the copies of a gene in the population that are one particular allele. For a gene with two alleles, let p be the frequency of the dominant allele and q the frequency of the recessive allele.

• p + q = 1
• p² + 2pq + q² = 1, where p² is the frequency of homozygous dominant individuals, 2pq of heterozygotes and q² of homozygous recessive individuals.

The Hardy-Weinberg principle states that allele frequencies stay the same from generation to generation if: the population is large; mating is random; there is no mutation; there is no migration (no gene flow); and there is no selection, so all genotypes are equally likely to survive and reproduce.

Worked example: 9% of the plants in a population have white flowers (recessive). q² = 0.09, so q = 0.3 and p = 1 − 0.3 = 0.7. The frequency of heterozygotes is 2pq = 2 × 0.7 × 0.3 = 0.42. In a population of 500 plants, 0.42 × 500 = 210 are heterozygous.

To detect whether allele frequency has changed, calculate the frequency of an allele in successive generations, or compare the genotype frequencies that are observed with those that the Hardy-Weinberg equation predicts from the allele frequency.

If the observed and expected values differ, at least one of the conditions is not met, so evolution is taking place. Allele frequency changes through:

• mutation, which produces new alleles
• migration (gene flow), as individuals bring in or take out alleles
• natural selection, since some genotypes survive and reproduce better
• genetic drift, in which chance alone changes frequencies, most strongly in small populations
• non-random mating, which changes the genotype frequencies, for example by inbreeding

Example: the Hardy-Weinberg equation predicts that if q = 0.3, then q² = 0.09. If the next generation has 16% white-flowered plants (q² = 0.16, q = 0.4), the allele frequency has changed from 0.3 to 0.4.

A mutation is a random change in the base sequence of DNA. It can produce a new allele. Most mutations are harmful or have no effect, but a few give an advantage in a particular environment. Mutations occur at random: they do not occur because the organism needs them.

Natural selection acts on the variation that mutation (and sexual reproduction) produces:

1. Individuals vary because of different alleles.
2. A selection pressure (such as a disease, predator, poison or antibiotic) acts on the population.
3. Individuals with an advantageous allele are more likely to survive and reproduce.
4. They pass the allele to their offspring, so its frequency increases from generation to generation.

Examples: antibiotic resistance in bacteria, and warfarin resistance in rats. The advantage depends on the environment, so an allele that is helpful under one selection pressure can be harmful when it is removed.

A species is a group of organisms that can interbreed to produce fertile offspring. Speciation is the formation of new species from an existing species.

Speciation needs reproductive isolation, so that there is no gene flow between populations. In allopatric speciation a geographical barrier (such as a river, canyon, mountain or sea) separates the populations. Then:

1. Each population is exposed to different selection pressures and different mutations.
2. The allele frequencies in each population change independently.
3. The populations become so different that they can no longer interbreed to produce fertile offspring, even if they come back together, so they are separate species.

In sympatric speciation, new species form in the same area. Reproductive isolation can result from differences in the timing of breeding (seasonal), in courtship behaviour, in the structure of the reproductive organs, or from a change in chromosome number, so that hybrids are infertile.

Zoos contribute to conservation by: captive breeding of endangered animals, managed with records of relatedness to keep genetic diversity high and avoid inbreeding; reintroduction to the wild when the cause of decline has been dealt with; research into the biology and breeding of species; and education of the public, which raises awareness and funds.

Evaluation: zoos can protect species that are close to extinction, but captive populations are small (small gene pool), animals can lose natural behaviours, and the cost is high. Reintroduction fails if the original threat remains. Zoos cannot keep large numbers of species.

Seed banks store seeds at low temperature and low humidity so that they stay dormant and viable. They take up little space, are cheap, hold seeds from many individuals (conserving genetic diversity), protect species from habitat loss and allow plants to be reintroduced and researched. Limitations: some seeds cannot survive drying or freezing, viability falls with time so the seeds must be tested and renewed, and stored seeds do not evolve in response to changes in the environment.

• Hardy-Weinberg: p + q = 1; p² + 2pq + q² = 1; start from q² (recessive phenotype).
• Conditions: large population, random mating, no mutation, migration or selection.
• A change in allele frequency shows that a condition is not being met.
• Mutation produces new alleles at random; selection increases the frequency of advantageous alleles.
• New species arise when isolated populations diverge until they cannot interbreed to produce fertile offspring.
• Evaluate zoos and seed banks: benefits (protection, diversity, research, education) and limits (small gene pool, cost, habitat still needed).