In a population of insects, a single gene with two alleles (A and a) controls coloration. Insects with genotype AA or Aa are green, while insects with genotype aa are brown. An environmental shift introduces a new predator that easily spots green insects, but cannot see brown insects. Which statement best explains how natural selection will act on this population?

Core Concept

PILLAR 1 — MOLECULAR/CONCEPTUAL MECHANISM

Step-by-Step Analysis

The gene controlling coloration in this insect population encodes an enzyme—most likely a pigment synthase—within the melanin or carotenoid biosynthesis pathway. Allele A produces a functional enzyme that catalyzes conversion of a precursor molecule into a green pigment, depositing it within cuticular epithelial cells. Allele a carries a loss-of-function mutation, perhaps a frameshift or missense substitution, that renders the enzyme catalytically inactive; insects homozygous for a cannot synthesize the green pigment and instead accumulate the brown precursor compound in their exoskeleton. Heterozygotes (Aa) produce sufficient functional enzyme from the single A allele to generate the green phenotype, demonstrating complete dominance at the phenotypic level.

Why Other Options Are Wrong

Natural selection operates through differential reproductive success driven by variation in heritable traits. The newly introduced predator possesses a visual system tuned to the wavelength reflected by the green pigment (approximately 510–550 nanometers). Against the environmental background—likely soil or bark absorbing broadly across the spectrum—the green insects present a high-contrast signal, triggering the predator's motion-detection and prey-capture neural circuits. Brown insects, reflecting longer wavelengths that blend with the substrate, evade detection. Survival translates directly into reproductive opportunity: brown individuals pass more copies of allele a to the next generation's gene pool. Over successive generations, the frequency of allele a increases as allele A declines—classic directional selection.

PILLAR 2 — STEP-BY-STEP LOGIC

The correct answer states that natural selection will increase the frequency of allele a (and thus brown coloration) over time because brown insects experience lower predation and consequently higher relative fitness. This follows directly from the mechanism described above. Initially, the population may be at Hardy-Weinberg equilibrium with some distribution of genotypes AA, Aa, and aa. Once the predator arrives, the selective landscape changes: green phenotypes (AA and Aa) suffer disproportionate mortality before they can reproduce. Brown phenotypes (aa) survive at a higher rate, contributing a larger proportion of gametes to the next generation. Because allele a is recessive, it can "hide" in heterozygotes during intermediate generations—selection against A is more efficient than selection directly for a, yet the net trajectory is an increase in a's frequency. The population does not shift instantaneously; rather, each generation shows a incremental rise in the proportion of aa individuals, assuming the predator remains a consistent selective pressure. This is directional selection, not stabilizing or disruptive, because one phenotypic extreme (brown) is favored over the other (green).

PILLAR 3 — DISTRACTOR ANALYSIS

Option B likely claims that allele A will be completely eliminated within one generation. This reflects a misunderstanding of how recessive alleles persist in heterozygotes; even strong predation cannot remove allele A instantly because Aa individuals still carry it. Option C may assert that the mutation rate will spontaneously generate more brown alleles in response to the predator. This is teleological reasoning—mutations arise randomly with respect to fitness needs, and selection cannot induce specific adaptive mutations. Option D probably states that genetic drift, rather than natural selection, is the primary driver of allele frequency change here. While drift operates in all finite populations, the scenario describes a consistent, directional advantage for brown insects against a visual predator, which is the hallmark of natural selection, not stochastic sampling error. Option E may suggest that the population will evolve a completely new gene to produce brown coloration. This misrepresents evolutionary mechanisms; selection acts on existing heritable variation (alleles A and a already present in the gene pool), not on de novo gene creation, which would require far longer timescales and different molecular processes such as gene duplication or horizontal transfer. Each distractor exploits a common misconception about the tempo, mode, or mechanism of natural selection.