3.4.4 - Genetic diversity and adaptation

Genetic diversity

A population of clones can be wiped out by one disease; a diverse one is likely to have survivors.

A population is a group of organisms of the same species living in the same place at the same time. An allele is a version of a gene, and individuals can carry different alleles.

Count the different alleles across the whole population, not just the two one individual might carry at a locus.

Why low diversity is risky

Temperature, food supply, disease, predators and competitors can all change. A population with high genetic diversity is more likely to include some individuals whose phenotype suits the new conditions. An organism's phenotype is the set of characteristics it shows, produced by its alleles and their interaction with the environment.

Edible bananas show the extreme case. They reproduce asexually, so a plantation of one variety is a set of clones, and the export trade once relied on Gros Michel. A fungal strain that could infect one plant could infect the others, and by the 1960s Panama disease had collapsed the variety.

Low genetic diversity therefore leaves a population more vulnerable to change. High diversity does not guarantee survival, but it gives natural selection something to act on, as long as the variation is heritable. A difference caused only by the environment, such as a plant growing taller in richer soil, need not be inherited.

Natural selection in populations

A new allele starts in one individual, yet it can spread through a whole population over many generations.

From new allele to common allele

1. Individuals already carry different alleles. Random mutation, which you met in 3.4.3, can add a new allele.
2. Many mutations are harmful and others are neutral, but in certain environments a new allele may benefit its possessor.
3. Individuals with the advantageous phenotype therefore survive better or reproduce more successfully.
4. So the advantageous allele is inherited by members of the next generation.
5. This repeats, so over many generations the frequency of that allele increases in the population.

Selection acts on phenotype, but the long-term result is a change in allele frequency, and that change is evolution.

The mutation appears first, by chance; the environment then determines whether that allele is helpful, harmful or neutral. An environmental factor that affects which phenotypes survive and reproduce best is called a selection pressure.

Success and frequency

What counts is reproductive success, not merely survival. An allele that reduces damage by herbivores, and so raises seed production, can be favoured even when it does not change lifespan.

An allele's frequency is its proportion among all copies of that gene in the population. A bigger count does not prove the frequency rose, because the whole population may have grown: 10 resistant cells among 100 cells and 100 among 1000 both give a frequency of 0.10.

Directional selection

An antibiotic does not create resistance; it removes the bacteria that lack it.

Directional selection usually happens when environmental conditions change, making one end of the range more successful.

How antibiotic resistance spreads

1. A random mutation produces an allele that gives resistance to an antibiotic.
2. When an effective antibiotic is present, susceptible bacteria are killed or their reproduction is inhibited.
3. Resistant bacteria are therefore more likely to survive and reproduce under this selection pressure.
4. They reproduce rapidly by binary fission, and daughter cells inherit the resistance allele.
5. So the resistance allele becomes more common over many generations.

High resistance is the favoured extreme, so the population's mean resistance shifts towards it, and repeated use of the same antibiotic can make the population harder to treat. The antibiotic does not cause the mutation: it selects bacteria already carrying the allele.

What a full selection answer needs

The same chain explains selection in an unfamiliar organism: variation, selection pressure, the phenotype that survives, reproduction, passing on alleles, rising allele frequency. AQA's 2025 examiners noted that some broad answers missed reproduction and passing on alleles.