Biological Evolution: Unity and Diversity
Core concepts
Evolution is change in allele frequencies in a population over generations. Natural selection acts on existing heritable variation: individuals with traits that improve survival and reproduction in a given environment leave more offspring, so those alleles become more common. Mutation creates new alleles; gene flow moves alleles between populations; genetic drift changes allele frequencies by chance, especially in small populations. Reproductive isolation can lead to speciation. Evidence includes fossils, homologous structures (shared ancestry), analogous structures (similar function, different ancestry), vestigial structures, embryology, and molecular comparisons such as DNA and protein sequences. Phylogenetic trees and cladograms show common ancestry and derived traits. Populations, not individuals, evolve. Hardy–Weinberg equilibrium (p² + 2pq + q² = 1) describes allele frequencies in a non-evolving population and serves as a null model.
Traps to watch
- An individual does not evolve during its lifetime; the population's allele frequencies change.
- "Needing" a trait does not create it; variation must already exist.
- Homologous structures indicate common ancestry; analogous structures indicate similar selection pressures, not close relatedness.
- Genetic drift is random, not survival-based; it is strongest in small populations.
- A trait that is favorable in one environment may be harmful in another.
Worked example
A beetle population has both green and brown individuals. Birds eat green beetles on brown soil. Brown beetles survive and reproduce more, so the brown allele increases. This is directional selection. Rule of thumb: identify (1) heritable variation, (2) differential survival or reproduction, and (3) a change in allele frequency.