Core Concept
PILLAR 1 — MOLECULAR/CONCEPTUAL MECHANISM
Step-by-Step Analysis
The peppered moth (Biston betularia) exhibits a phenotypic polymorphism governed by allelic variation at a single locus controlling melanin biosynthesis in wing epidermal cells. The carbonaria allele produces a dominant phenotype wherein elevated transcription of genes in the melanin pathway deposits eumelanin granules densely across the wing scale cells, yielding the dark (melanic) morph. The typica allele, recessive to carbonaria, produces far less eumelanin, resulting in the light, speckled morph. Eumelanin is a nitrogenous polymer synthesized from the amino acid tyrosine via the enzyme tyrosinase, generating a pigment that absorbs broadly across the visible spectrum and thus appears black or dark brown. In the typica morph, melanin deposition is restricted to discrete scattered scales, so most wing area reflects ambient light and appears white or pale gray.
Why Other Options Are Wrong
Natural selection operates on this heritable phenotypic variation through differential predation by visual hunters, notably birds such as great tits (Parus major) and thrushes (Turdus philomelos). These predators possess tetachromatic vision and detect moth prey by matching the visual pattern of a resting moth against the background substrate. A moth whose wing reflectance profile closely matches the reflectance of lichen-covered or soot-darkened bark receives fewer predatory strikes because the bird's visual processing fails to register a discrete prey-shaped object against that background. Survival to reproductive age translates directly into higher allele transmission to the next generation. Thus, the selective agent is not temperature or drift but an ecological interaction — directional visual predation — that shifts allele frequencies in proportion to crypsis quality.
PILLAR 2 — STEP-BY-STEP LOGIC
Before the Industrial Revolution, English forests supported pale, lichen-encrusted tree trunks (e.g., oak, Quercus robur). On this substrate, the typica morph's speckled reflectance pattern blended with the lichen mosaic, reducing bird detection. The carbonaria morph stood out as a dark silhouette and was consumed disproportionately, keeping the carbonaria allele at very low frequency. Industrialization (c. 1840–1900) released massive quantities of particulate soot and sulfur dioxide from coal combustion. Soot particles settled on bark, darkening trunks, while SO₂ killed the light-colored crustose lichens (e.g., Lecanora conizaeoides), removing the pale background entirely.
With the substrate now dark, the selective dynamic inverted. On soot-blackened bark, the carbonaria morph's high eumelanin content made the moth virtually invisible, whereas the typica morph appeared as a conspicuous pale patch. Avian predators now struck typica individuals at a higher rate. Because survival governs reproductive output, carbonaria individuals transmitted their dominant allele to a greater proportion of the subsequent generation's offspring. Over successive decades, the carbonaria allele frequency climbed from under two percent to over ninety percent in heavily industrialized regions such as Manchester, constituting one of the most rapid documented instances of directional natural selection in a wild population. This is precisely why option C is correct: the differential survival advantage conferred by crypsis on darkened urban bark directly drove the observed population shift.
PILLAR 3 — DISTRACTOR ANALYSIS
Option A entices students by invoking temperature, conflating melanin's role in thermal absorption with the actual selective agent. Melanic ectotherms do absorb more solar radiation, which can be advantageous in cold microclimates; however, the peppered moth case was driven by visual predation, not thermoregulatory selection. No data link regional temperature change to moth color frequency in industrial England.
Option B mischaracterizes the event as a population bottleneck followed by recolonization. While severe population reductions can alter allele frequencies stochastically via the bottleneck effect, the moth population did not crash and was not replaced by outside colonizers. Instead, standing genetic variation at the melanin locus was filtered by selective predation within a continuous population — a fundamentally different mechanism.
Option D invokes genetic drift triggered by non-native species introduction. Genetic drift refers to random fluctuations in allele frequency unrelated to fitness, often via founder effects or small population sizes. The peppered moth shift was decidedly non-random: carbonaria moths survived at measurably higher rates on soot-darkened bark due to adaptive crypsis. Furthermore, no non-native moth species was introduced into England during this period. The observed change therefore reflects deterministic natural selection, not stochastic drift or exotic species dynamics.