A2 Biology 9700: Stabilising, Directional and Disruptive Selection

The three scenarios below each describe a natural population that experiences a selection pressure acting on a phenotypic characteristic.

(a) In a large hospital's long-term records, babies born with a body mass close to 3.3 kg have the highest survival rate to one year old, while babies born considerably lighter or considerably heavier than this have a higher risk of dying in their first year. State and explain the type of natural selection shown by these data, and describe the effect this type of selection has, over many generations, on the range of birth mass seen in the population. [3]

(b) A population of beetles living among volcanic rocks varies in the colour of its wing cases, from very pale grey to very dark grey. After a nearby eruption coats the surrounding rock surfaces in dark ash, records kept over the next 40 generations show that the average wing case colour of the population becomes progressively darker, and pale-coloured beetles become rare. Identify and explain the type of natural selection shown, and explain how it changes the beetle population's phenotypes, and the underlying allele frequencies, over this time. [3]

(c) On a small island, a population of seed-eating birds feeds on seeds from a single plant species. Following a change in climate, this plant now produces only two seed types (small, soft seeds and large, hard seeds) and no longer produces medium-sized seeds. Over several generations, birds with either a small, narrow beak or a large, powerful beak become more common, while birds with a medium-sized beak become rare, because medium-beaked birds cannot process either seed type efficiently. Identify and explain the type of natural selection shown, and state one likely long-term outcome for this bird population if the two beak-size groups became reproductively isolated from one another. [3]

Part (a): Human birth mass. Stabilising selection

The data show that survival is highest for babies close to the population mean birth mass (3.3 kg), and lowest at both extremes (much lighter and much heavier babies). Because both extremes are selected against, while the mean phenotype is favoured, this is stabilising selection.

Selection pressure: babies far from the average birth mass are more likely to die before reproducing (very low mass is often linked to complications of prematurity/underdevelopment; very high mass is often linked to complications during birth).
Effect over generations: individuals with extreme phenotypes are consistently removed from the breeding population more than those near the mean, so the alleles associated with extreme birth mass become rarer. This narrows the range (reduces the variation) of birth mass around the existing mean. The mean itself does not shift, unlike in directional selection.

Part (b): Beetle wing case colour. Directional selection

Here, the environment itself changes (dark ash now covers the rocks), which favours one extreme of the existing colour range (dark) over the other (pale) and over the previous average. Because the mean phenotype shifts progressively in one direction over many generations, this is directional selection.

Selection pressure: pale beetles are now more conspicuous against the dark, ash-covered rock and are eaten by predators more often, so they survive and reproduce less successfully than darker beetles.
Mechanism: the alleles for darker colouration were already present in the population (as part of the original grey-to-dark-grey range of variation) before the eruption. The eruption did not create these alleles. It changed which phenotype had a survival advantage.
Effect over generations: darker beetles pass on their alleles for dark colouration more often than pale beetles do, so the frequency of “darker” alleles increases each generation, and the population’s average wing case colour becomes progressively darker (pale beetles become rare, but do not necessarily disappear completely).

Part (c): Island seed-eating birds. Disruptive selection

Here, the environment now favours both extremes of beak size (small-narrow and large-powerful) over the intermediate (medium) beak size, because the food supply has split into two very different seed types with no medium-sized seeds available. This is disruptive selection.

Selection pressure: medium-beaked birds cannot efficiently process either the small, soft seeds or the large, hard seeds, so they obtain less food, and are less likely to survive and reproduce than birds at either extreme.
Effect over generations: alleles associated with both small and large beak size increase in frequency, while alleles for medium beak size decrease. The population’s beak-size distribution becomes bimodal, two increasingly distinct groups, rather than having a single central peak.
Long-term outcome: if the small-beaked and large-beaked groups stopped interbreeding (became reproductively isolated), the two groups could accumulate further genetic differences independently and, given enough time, diverge into two separate species without ever becoming geographically separated, an example of sympatric speciation.

Final answers

(a) Stabilising selection, extremes selected against, mean favoured; the range of birth mass narrows around the unchanged mean.
(b) Directional selection, dark environment favours the dark extreme; the population mean phenotype shifts towards darker colouration as “dark” alleles increase in frequency.
(c) Disruptive selection, both extremes favoured over the intermediate; the population becomes bimodal, with possible sympatric speciation if the two groups become reproductively isolated.