Biology Practice Test: Evolution and Natural Selection

Natural selection acts on heritable variation: some inherited traits help individuals survive and reproduce, so those traits become more common. If all individuals were identical, there would be nothing to select. Individuals cannot change their genes on demand.

Predators found light moths more easily on darkened bark, so dark moths left more offspring and the dark form became more common. Soot on wings is not inherited, and individual moths do not change their color to match the bark.

Some bacteria in the population already had genes that made them more resistant. The early doses killed the most sensitive ones, and the survivors reproduced, so the new population is mostly resistant. Resistance is a property of the bacteria, not the patient, and it does not arise because bacteria need it.

The same bone layout used for different jobs is best explained by inheritance from a shared ancestor, with each line modifying the limb over time. Structures that do the same job but evolved separately, like bird and insect wings, are called analogous.

Butterfly and bird wings both allow flight, but they are built in completely different ways and did not come from a winged common ancestor; they are analogous. The other pairs are vertebrate limbs with the same basic bones, so they are homologous.

Reduced structures that no longer serve their original purpose are vestigial. The leftover hip bones point to ancestors of whales that walked on land. Fish have no hip bones like these, so they are not analogous structures shared with fish.

In undisturbed sedimentary rock, lower layers were deposited first (the law of superposition), so fossil X is older. The layers show relative age only; they do not prove that one organism descended from the other.

One half-life leaves 1/2, two leave 1/4 and three leave 1/8. Three half-lives are 3 × 5,730 = 17,190 years. The answer 11,460 years is two half-lives, which would leave 1/4.

Differences in a protein build up over time after two lineages split, so fewer differences point to a more recent common ancestor. Species Y differs by only 2 amino acids and is the closest relative; Z, with 21 differences, is the most distant.

When a few individuals start a new population, its allele frequencies can differ from the source population purely by chance. This form of genetic drift is the founder effect. Gene flow is the movement of alleles between populations, which would make them more alike.

Once separated, the two groups collected different mutations and faced different selection, until they became reproductively isolated. That makes them separate species, formed through geographic isolation. Neither group died out, so this is not extinction.

q² = 0.16, so q = 0.4 and p = 1 − 0.4 = 0.6. Heterozygotes are 2pq = 2 × 0.6 × 0.4 = 0.48, or 48%. The 36% answer is p², the homozygous dominant share, and 84% is everyone who shows the dominant trait.

Evolutionary fitness means reproductive success: how many offspring an individual leaves that survive to reproduce. Strength or speed only matters for fitness if it leads to more offspring.

When the intermediate form is favored and both extremes are selected against, the population stays clustered around the average: stabilizing selection. Disruptive selection is the opposite, favoring both extremes over the middle.

Finches with deeper beaks get more food, survive and breed more, so their offspring shift the population's average beak depth upward (directional selection). An adult bird's beak does not grow deeper because of the drought; the change shows up across generations.