A2 Biology 9700: Heritable Variation and the Response to Selection

Two populations of the same plant species are being studied.

Population 1: plants growing in a field with patchy soil fertility. Researchers take cuttings (genetically identical clones) from several plants and grow them in both nutrient-poor and nutrient-rich patches. In every case, the clones grown in nutrient-poor soil develop small leaves, while the genetically identical clones of the same individuals grown in nutrient-rich soil develop much larger leaves. Leaf size in this population is entirely explained by local soil nutrient availability.

Population 2: plants growing in a separate field with uniform soil fertility throughout. When researchers grow offspring from many different parent plants together in identical, controlled conditions (a common-garden experiment), the offspring still show a consistent range of leaf sizes that closely resembles the leaf size of their own parents, regardless of exactly where in the field they are grown.

Both populations are now exposed to a new, sustained environmental pressure that favours plants with larger leaves. Which statement correctly predicts the long-term evolutionary response of each population to this pressure?

Step 1: Determine the cause of variation in each population

In population 1, genetically identical clones develop very different leaf sizes depending only on the soil they are grown in. Because genotype is held constant (the clones are genetically identical) while the phenotype (leaf size) still changes with the environment, this shows the variation in leaf size is entirely environmental, with no genetic contribution.

In population 2, a common-garden experiment removes environmental differences by growing all offspring in identical conditions. Because offspring still consistently resemble their own parents in leaf size even when the environment is the same for everyone, this consistent, parent-offspring-linked variation must have a genetic (heritable) basis.

Step 2: Apply the requirement for a response to selection

Natural selection can only produce lasting evolutionary change, a shift in allele frequencies carried through to future generations, if the variation it acts on is heritable. If a favoured phenotype is caused only by the environment (as in population 1), then even if larger-leaved individuals are favoured, this will not change which alleles are passed on: the offspring’s leaf size will still simply depend on the soil they happen to grow in, not on any inherited “large leaf” allele.

In population 2, because the leaf-size variation has a genetic basis, individuals with alleles associated with larger leaves will be favoured by the new pressure, survive and reproduce more successfully, and pass those alleles on. Over generations, this increases the frequency of “large leaf” alleles in population 2, producing genuine evolutionary change.

Step 3: Evaluate the options

A correctly identifies that population 1 cannot evolve (its variation is non-heritable) while population 2 can evolve (its variation is heritable), this matches the reasoning above.
B is incorrect because selection can only produce an evolutionary response where variation is heritable; acting on non-heritable variation (population 1) produces no lasting genetic change.
C is incorrect because the common-garden experiment is exactly the kind of evidence that supports a genetic basis for the variation, not against it, offspring resembling parents despite identical conditions indicates heritability.
D is incorrect because purely environmental “effects” in population 1 do not accumulate or increase across generations under selection at all, so there is no meaningful evolutionary response to compare in speed; only population 2 shows a genuine (if gradual) evolutionary response.

Final answer

Option A. Only population 2, whose leaf-size variation has a heritable genetic basis, can show an evolutionary response to the new selection pressure; population 1’s purely environmental variation cannot.