Evolution und Biodiversität / Evolution and biodiversity
Evolution items test mechanism, evidence and phylogeny together. You must distinguish selection from drift, homology from analogy, and be able to compute allele frequencies, because a correct Hardy-Weinberg calculation is a frequent discriminator in the written paper.
Evolutionary mechanisms
- Sources of variation: mutation and recombination (meiosis, fertilisation); mutation is random with respect to need.
- Selection acts on phenotypes, changing allele frequencies; stabilising, directional and disruptive selection produce different distribution shifts.
- Genetic drift is random change, strongest in small populations (bottleneck, founder effect); gene flow through migration; only populations, never individuals, evolve.
Hardy-Weinberg
- p + q = 1 and p² + 2pq + q² = 1, assuming random mating, no selection, no mutation, no migration and a large population. Use q² for the frequency of affected homozygotes and 2pq for carriers.
Speciation and phylogeny
- Allopatric speciation via geographic isolation, then reproductive isolation (prezygotic or postzygotic); sympatric speciation can follow polyploidy, especially in plants.
- Homologous structures derive from a common ancestor; analogous structures result from convergent evolution and must not be used as relatedness evidence. Cladistics groups by shared derived characters; molecular clocks use sequence differences; ERV insertions and DNA sequence identity are strong evidence.
Human evolution
- Bipedalism appears early (position of the foramen magnum, pelvic shape, leg proportions); brain enlargement and tool use follow later; reduced jaw and brow. Evidence includes Australopithecus and early Homo fossils, plus admixture of modern humans with Neanderthals outside Africa.
Worked example
An autosomal recessive disorder affects 1 in 10 000 newborns in a population assumed to be in Hardy-Weinberg equilibrium. Reasoning: q² = 1/10 000 = 0.0001, so q = 0.01 and p = 0.99. Carrier frequency = 2pq = 2 × 0.99 × 0.01 ≈ 0.0198, that is about 2%, roughly 200 times the frequency of affected homozygotes. This explains why the allele persists largely hidden in heterozygotes.
Traps that cost marks
Writing that individuals 'adapt' in order to survive; confusing selection with drift; assuming 'more complex' means 'more highly evolved'; using analogous organs as proof of kinship; forgetting the Hardy-Weinberg assumptions before applying the equations.