3.4.4 Genetic diversity and adaptation

Populations carry variation in their gene pools. When the environment selects against some variants and in favour of others, allele frequencies change over generations. This is natural selection — and the features it produces are what we call adaptations.

Genetic diversity is the total number of different alleles in a population's gene pool.

Genetic diversity is the total number of different alleles present across all individuals in a population. The complete set of alleles in a population is called the gene pool. High genetic diversity carries a survival consequence: when the environment changes, a population with many allele variants is more likely to contain individuals whose phenotype suits the new conditions. Low diversity — through bottleneck or inbreeding — leaves a population vulnerable.

Write allele whenever the variant is the mark point. Don't write gene. A "resistance gene" is rejected; a "resistance allele" is credited. The gene exists in every individual; the allele is what varies.

The distinction matters in every selection answer that follows. What changes in the population is the frequency of a specific allele — not a count of resistant individuals, not a vague reference to "the gene." Holding the allele vocabulary cleanly here is what makes the four-stage chain in the next section pay off.

Natural selection changes allele frequencies through a four-stage chain.

Natural selection is the process by which a population's genetic composition changes over generations in response to environmental pressures. Three conditions are necessary: heritable variation exists within the population; the variation produces phenotypic differences; and those differences affect survival and reproduction differentially. When all three hold, allele frequencies change.

1. A mutation in some individuals produces a new allele. The mutation arises spontaneously; it pre-exists any selection pressure.
2. A selection pressure in the environment kills, or otherwise prevents the reproduction of, individuals that lack the advantageous allele.
3. The individuals carrying the allele survive and reproduce, passing the allele to their offspring.
4. The allele frequency of the advantageous variant increases in the population over successive generations.

Write allele frequency for the endpoint. Don't write number of resistant individuals or the gene. A frequency is a proportion across the population; a count is not. Stopping at "more individuals have the trait" misses the chain's final mark.

Write survive and reproduce, not survive alone. Transmission is the mark point, not survival. An individual that survives but does not breed contributes nothing to allele frequency.

Pitfall — The Lamarckian trap

The mutation produces the allele; the selection pressure does not.

Antibiotics do not produce resistance alleles. Pathogens do not produce host-resistance alleles. Pesticides do not produce pesticide-resistance alleles. The selection pressure acts on variation that already exists in the population, and that variation was generated by mutation independently of the pressure.

Any sentence beginning "the antibiotic caused," "the pathogen produced," or "exposure created" inverts the mechanism. AQA treats this as the single highest-value rejection in the topic. Open every selection answer with the existing-variation framing — the resistance allele was already there before the antibiotic arrived.

Artificial selection — selective breeding — follows the same four-stage logic with one substitution: the selection pressure is human choice, not the environment. A breeder selects which individuals reproduce on the basis of a desired trait, and the alleles producing that trait increase in frequency over generations. The mechanism is identical; only the source of the pressure differs.

Write resistant in bacterial contexts. Don't write immune. Bacteria do not have immune systems. The credited descriptor for a bacterium that survives antibiotic exposure is "resistant" or "antibiotic-resistant."

AQA examines two named types of natural selection.

AQA 7402 examines two types of natural selection: directional and stabilising. Identifying which type the question is about before writing a sentence is critical. Directional selection answers walk one extreme rising and the mean shifting; stabilising selection answers walk both extremes culled and the mean unchanged. Misidentifying the type maps the answer onto the wrong template and forfeits multiple marks.

Directional selection drives the antibiotic-resistance walk-through. Before treatment, most bacteria in the population are sensitive; rare mutations have produced a resistance allele in some individuals. When the antibiotic is applied, sensitive bacteria die and resistant bacteria survive and reproduce. The resistance allele frequency rises each generation until the population is dominated by resistant individuals — the mean has shifted.

Stabilising selection drives the human birth weight walk-through. Very low birth weights carry survival risks; very high birth weights carry delivery risks. Babies at intermediate weights (~3.0–3.5 kg) survive most often, so the alleles producing intermediate phenotypes are maintained and the alleles producing extremes decline in frequency. The mean stays put; the range of variation narrows over generations.

For selective breeding, the selection pressure is human choice. Don't write "the environment changed" — the environment may be steady; what changed is which individuals the breeder allowed to reproduce.

Adaptations come in three categories: anatomical, behavioural, and physiological.

An adaptation is any heritable feature of an organism that increases its probability of surviving and reproducing in its environment. Adaptations are the product of natural selection acting over many generations. AQA classifies adaptations into three categories.

Adaptations are interpreted in the context of the niche an organism occupies. A niche is the role a species plays in its ecosystem, defined by what it does and the conditions it requires. Two species cannot occupy the same niche indefinitely; the better-adapted species outcompetes the other. Over evolutionary time, niche differentiation reduces competitive overlap and allows coexistence. AQA examines niche only as context, not as a vocabulary anchor.

Statistical tests interpret whether observed differences are due to chance.

AQA pairs natural selection content with statistical interpretation. The null hypothesis states that any difference between observed values is due to chance. The P value is the probability of obtaining the observed difference if the null hypothesis is true. The significance threshold is P = 0.05; below this, the difference is judged unlikely to be due to chance alone.

1. The probability that the difference is due to chance is less than 0.05.
2. Reject the null hypothesis.
3. The named independent variable does affect the named dependent outcome — the biological conclusion.

Write difference is due to chance, not results are due to chance. The P value quantifies the difference between groups, not the data itself.

Write reject the null hypothesis. Don't write disprove, prove wrong, or incorrect. Statistical tests reject or fail to reject at a probability threshold; they do not prove.

Test choice matters. A correlation coefficient is not a comparison-of-means tool; a Student's t-test compares the means of two datasets, not correlations; Spearman's rank handles rank-ordered monotonic data. AQA mark schemes reject the wrong test choice. For Evaluate questions in selection-and-statistics contexts, the full mark requires a quoted value from the data plus a specifically named design limitation.

For Evaluate questions, name the specific design limitation. "More research is needed" and "other factors may be involved" earn zero — those phrases identify no specific flaw. Credited limitations are study-design specifics: lab-not-in-people, single concentration tested, no statistical test reported.

Key terms

• allele
• allele frequency
• frequency
• mutation
• selection pressure
• resistance
• resistant
• antibiotic
• stabilising selection
• null hypothesis