Evolution and Diversity of Life
Evolution and Diversity of Life tests the mechanisms that change allele frequencies, the evidence for common ancestry, speciation, phylogenetic interpretation, and the broad organization of living things. Questions often combine a Hardy-Weinberg calculation with a claim about selection or ask you to read a phylogenetic tree correctly.
Mechanisms of evolution
- Mutation, gene flow, genetic drift, and natural selection change allele frequencies.
- Natural selection requires variation, heritability, and differential reproductive success.
- Directional selection shifts a phenotype; stabilizing selection favors intermediates; disruptive selection favors extremes.
- Hardy-Weinberg equilibrium requires no mutation, no migration, no selection, random mating, and a large population. Then p + q = 1 and p² + 2pq + q² = 1.
Speciation and phylogeny
- Allopatric speciation involves geographic isolation; sympatric speciation occurs without it.
- Prezygotic barriers prevent fertilization; postzygotic barriers reduce hybrid viability or fertility.
- On a phylogenetic tree, nodes represent common ancestors; branch tips are extant or sampled taxa; monophyletic groups include an ancestor and all descendants.
Diversity of life
- The three domains are Bacteria, Archaea, and Eukarya. Prokaryotes lack membrane-bound organelles.
- Endosymbiotic theory explains mitochondria and chloroplasts as descendants of engulfed prokaryotes.
- Fungi are absorptive heterotrophs; plants are photoautotrophs with alternation of generations.
Worked example
In a population at Hardy-Weinberg equilibrium, 16% of individuals show the recessive phenotype. Then q² = 0.16, so q = 0.4 and p = 0.6. The heterozygous frequency is 2pq = 2(0.6)(0.4) = 0.48, or 48%. The dominant phenotype frequency is p² + 2pq = 0.36 + 0.48 = 0.84.
Traps that cost marks
- Saying individual organisms evolve; populations evolve.
- Assuming a dominant allele will spread automatically.
- Using phenotype frequency directly as allele frequency unless the phenotype is recessive.
- Reading branch tips left to right as a ladder of progress.
- Confusing genetic drift with natural selection; drift is random with respect to fitness.