Explain the logical steps of natural selection and distinguish it from genetic drift, gene flow, and mutation as mechanisms of evolutionary change.
Natural selection follows four linked facts. First, individuals within a population vary in their heritable traits. Second, populations tend to produce more offspring than the environment can support with available resources. Third, individuals whose traits happen to suit the current environment better tend to survive and reproduce more successfully than others -- differential reproductive success, which is what "selection" actually means here, not a conscious choice made by the organism or the environment. Fourth, over many generations, the heritable traits that improve survival and reproduction become more common in the population. Precision matters: organisms do not "try" to adapt on purpose within their own lifetime; a trait helpful in one environment can be useless or harmful in a different one, so there is no single universal "better," only "better-suited to a particular environment at a particular time"; and selection acts on populations across generations, never on one individual during its own lifetime.
Natural selection is not the only mechanism that changes a population's allele frequencies. Genetic drift is a random, chance-driven change, with its strongest effects in small populations; the founder effect (a small group starts a new population, carrying only a limited sample of the original population's alleles) and the bottleneck effect (a population's size crashes sharply, and survivors' alleles may not represent the original population's full variation by chance alone) are two specific patterns of drift. Gene flow is the movement of alleles between populations through migration and interbreeding. And mutation, from Unit 8, remains the ultimate original source of any new variation for either natural selection or drift to act upon. Evolution can happen through several distinct mechanisms, of which natural selection is only one, though it is the one that specifically produces adaptation.
Mastery looks like: They can state the four linked facts behind natural selection in order and correctly distinguish natural selection from genetic drift, gene flow, and mutation.
Common stumbles: Describing natural selection as an organism actively trying to adapt within its own lifetime; treating any allele frequency change as automatically "natural selection" rather than checking whether it was chance-driven drift instead.