Evolution & Natural Selection

Evidence for Evolution

Darwin's theory of evolution by natural selection is supported by multiple independent lines of evidence. Fossils reveal transitional forms and document change over geologic time. Comparative anatomy identifies homologous structures (shared ancestry) versus analogous structures (convergent evolution). Molecular biology shows conserved DNA and protein sequences across taxa - the more similar the sequences, the more recent the common ancestor. Biogeography explains why island species often resemble nearby mainland organisms yet diverge in response to local selective pressures, as Darwin observed in the Galapagos finches.

Vestigial structures such as the human appendix or pelvic bones in whales provide additional evidence. These reduced or non-functional features reflect ancestral traits that lost selective advantage over evolutionary time.

Natural Selection and Types of Selection

Natural selection acts on phenotypic variation within a population. Individuals with traits that improve survival and reproduction in a given environment contribute more offspring to the next generation. Over time, advantageous alleles increase in frequency. Sexual selection, a subset of natural selection, favors traits that enhance mating success even if they reduce survival (e.g., the peacock's tail).

Genetic Drift and Gene Flow

Genetic drift is the random change in allele frequencies due to chance events, and its effects are strongest in small populations. The bottleneck effect occurs when a catastrophe drastically reduces population size, randomly eliminating alleles regardless of fitness. The founder effect occurs when a small group colonizes a new area, carrying only a subset of the original population's genetic diversity.

Gene flow (migration) transfers alleles between populations. It tends to reduce genetic differences between populations and can introduce new alleles or increase the frequency of existing ones. Gene flow counteracts divergence caused by drift and natural selection in isolated populations.

Hardy-Weinberg Equilibrium

The Hardy-Weinberg principle provides a null model for population genetics. When a population is in equilibrium, allele and genotype frequencies remain constant from generation to generation. The equations are:

Five conditions must hold for equilibrium: (1) no mutations, (2) random mating, (3) no natural selection, (4) very large population size (no drift), and (5) no gene flow. Real populations rarely meet all five, so deviations indicate which evolutionary forces are acting.

On the AP exam, you may need to calculate expected genotype frequencies given allele frequencies, or determine whether observed data deviate from Hardy-Weinberg expectations using a chi-square test.

Speciation and Reproductive Isolation

Allopatric speciation occurs when a physical barrier (mountain range, river, ocean) separates a population. Isolated groups accumulate genetic differences through mutation, drift, and divergent selection until they can no longer interbreed. This is the most common mode of speciation in animals.

Sympatric speciation occurs without geographic isolation. In plants, polyploidy (genome duplication) can create instant reproductive barriers. In animals, sympatric speciation may arise from habitat differentiation or assortative mating within a continuous range.

Reproductive isolation mechanisms are classified as prezygotic (prevent mating or fertilization) or postzygotic (hybrid inviability, sterility, or breakdown). Temporal isolation, behavioral isolation, and mechanical isolation are common prezygotic barriers tested on the AP exam.

Phylogenetics and Cladograms

A cladogram is a branching diagram that represents hypothesized evolutionary relationships among taxa. Each branch point (node) represents a common ancestor, and shared derived characters (synapomorphies) define monophyletic groups (clades). An outgroup is a taxon outside the clade of interest that roots the tree.

Molecular clocks use the rate of neutral DNA mutations to estimate divergence times between lineages. They assume a roughly constant mutation rate over time and are calibrated with fossil data. Hemoglobin and cytochrome c sequences are classic examples used in molecular phylogenetics.