Evolution and Biodiversity
Evolution and Biodiversity
Evolution and Biodiversity asks how allele frequencies change, how reproductive isolation produces species, and how organisms are classified and related. Expect Hardy–Weinberg calculations, natural selection scenarios, speciation mechanisms, and interpretation of phylogenetic trees, plus broad diversity across the three domains.
Core concepts
- Mechanisms: natural selection, genetic drift (bottleneck and founder effects), gene flow, mutation, and nonrandom mating change allele frequencies. Natural selection acts on phenotypes and can be directional, stabilizing, or disruptive.
- Hardy–Weinberg: p + q = 1 and p² + 2pq + q² = 1 describe a nonevolving population with no selection, no mutation, no migration, large population, and random mating. Departures suggest evolution.
- Speciation: allopatric speciation involves geographic isolation; sympatric speciation occurs without it, often through polyploidy or habitat/behavioral isolation. Prezygotic barriers prevent fertilization; postzygotic barriers reduce hybrid viability or fertility.
- Phylogeny and diversity: phylogenetic trees show common ancestry, not “progress.” Homologous structures reflect shared ancestry; analogous structures reflect convergent evolution. The three domains are Bacteria, Archaea, and Eukarya.
Worked example
A population is at Hardy–Weinberg equilibrium, and 4% of individuals show a recessive phenotype. Since q² = 0.04, q = √0.04 = 0.2. Then p = 1 − 0.2 = 0.8. The carrier frequency is 2pq = 2(0.8)(0.2) = 0.32, or 32%. Reasoning tip: begin with the recessive phenotype frequency, because it directly gives q² only when the trait is recessive and the population is at equilibrium. If the trait were dominant, the 4% would be p² + 2pq, not q².
Traps
Populations evolve, not individuals; dominant alleles do not necessarily spread; Hardy–Weinberg assumptions are idealized; and a tree’s branch length or tip position does not mean “more evolved.” Do not treat analogous structures as evidence of close relatedness.