Evolution and Natural Selection
What to know
Evolution is change in the genetic composition of populations over time. Understand natural selection as the mechanism: individuals vary, some variants are heritable and improve survival or reproduction, and those variants increase in frequency across generations. Distinguish this from other mechanisms - mutation (source of new variation), gene flow (migration), genetic drift (random change, strongest in small populations), and nonrandom mating. Recognize that selection acts on phenotypes, but evolution is measured as change in allele frequencies in a population.
Know the evidence: fossil record, comparative anatomy (homologous vs. analogous structures), embryology, molecular similarities (DNA and protein sequences), and observed change such as antibiotic resistance. Understand fitness as reproductive success, not strength. Distinguish directional, stabilizing, and disruptive selection. Recognize speciation: geographic isolation (allopatric) or reproductive isolation (sympatric) leading to new species that can no longer interbreed. Hardy-Weinberg equilibrium describes a non-evolving population and is a useful baseline for identifying when evolution is occurring.
Traps
- Individual vs. population. Individuals do not evolve; populations do.
- Purposeful language. Organisms do not "try" to adapt; variation arises randomly and selection sorts it.
- Homologous vs. analogous. Homologous structures share ancestry; analogous structures share function but not ancestry (convergent evolution).
- Fitness. Fitness means reproductive output, not physical strength or longevity.
Rule of thumb
Ask: is there heritable variation, and does it affect reproductive success? If yes, selection can act.
Worked example
In a bacterial population exposed to an antibiotic, a few cells already carry a resistance mutation. The antibiotic kills susceptible cells, and resistant cells reproduce, so resistance becomes common. The mutation existed before exposure; the drug selected it.