Biological Science: Origin and Evolution of Life; Classification of Organisms
This domain tests the evidence for common descent, the mechanisms that change allele frequencies, the leading hypotheses for the origin of life, and the taxonomic framework biologists use to organize diversity. Questions are usually conceptual: identify the mechanism at work, or classify an organism correctly.
Mechanisms of evolution
- Natural selection requires heritable variation and differential reproductive success. Fitness means surviving offspring, not strength or size.
- Genetic drift is random change in allele frequency; it dominates small populations through bottlenecks and founder effects.
- Gene flow (migration) and mutation also alter frequencies. Mutation is the ultimate source of new alleles but is slow.
- Hardy–Weinberg (large population, random mating, no mutation, migration, or selection) is the null model; departure from it signals evolution.
Evidence and origin of life
Homologous structures share ancestry but may differ in function; analogous structures share function but not ancestry. Molecular comparisons (ribosomal RNA, cytochrome c) build phylogenies. The Miller–Urey experiment (1953) sparked CH4, NH3, H2 and H2O to produce amino acids, showing abiotic synthesis is possible. The RNA world hypothesis rests on ribozymes - RNA that both stores information and catalyzes reactions. Endosymbiotic theory explains mitochondria and chloroplasts: double membranes, circular DNA, 70S ribosomes.
Classification
The hierarchy is Domain, Kingdom, Phylum, Class, Order, Family, Genus, species. Woese's three domains are Bacteria, Archaea, Eukarya; Archaea differ in membrane lipids and rRNA. Viruses sit outside the tree of life because they are not cellular and cannot replicate independently. Binomial names are italicized with the genus capitalized.
Worked example
A hurricane cuts a lizard population from 200 to 4 survivors, all of which happen to carry allele A, pushing its frequency from 0.35 to 1.00. Because survival was random with respect to genotype, this is a bottleneck - genetic drift - not natural selection, gene flow, or mutation. The test: selection requires fitness differences tied to the allele; drift requires only chance.
Traps
Equating "dominant" with "most common"; saying individuals evolve rather than populations; explaining adaptation by need or use (Lamarckian); labeling analogous structures homologous; and assigning viruses to a domain.