Evolution and Diversity
Evolution and Diversity tests how populations change over time and how the resulting variety of life is organised and named. Questions usually give you a scenario or a set of data and ask you to explain a change in allele frequency, justify an evidence type, or use a classification key.
Natural selection
- Variation within a population arises from mutation and meiosis, and much of it is heritable.
- Selection pressures - predation, disease, climate, competition - mean some phenotypes survive and reproduce more successfully.
- Over generations the frequency of favourable alleles rises and the population becomes adapted. Individuals do not evolve and do not 'try' to adapt.
- Directional, stabilising and disruptive selection act on different parts of a phenotype range.
- Antibiotic resistance is evolution observed in real time.
Evidence for evolution
- Fossil record: sequence in rock strata, transitional forms.
- Homologous structures share an origin but differ in function; analogous structures share a function but differ in origin.
- Comparative biochemistry (DNA sequences, cytochrome c), embryology and biogeography.
Speciation
- Allopatric: geographic isolation separates gene pools; mutation and different selection act independently; reproductive isolation follows, giving a new species.
- Sympatric speciation can follow polyploidy in plants.
- A species is a group that can interbreed to produce fertile offspring.
Classification
- Binomial nomenclature: Genus species, italicised or underlined, with the genus capitalised.
- Hierarchy: domain, kingdom, phylum, class, order, family, genus, species.
- Three domains (Bacteria, Archaea, Eukarya) compared with the five-kingdom model.
- Dichotomous keys use paired statements, one chosen at each step.
Worked example
Before spraying, 2% of a mosquito population carries a resistance allele; after spraying, 60% of survivors are resistant. Full-mark reasoning: a mutation produced the resistance allele before the insecticide was used; the insecticide is the selection pressure; susceptible mosquitoes die while resistant ones survive and reproduce; the allele frequency in the population therefore rises. The insecticide did not create the mutation - it revealed its advantage.
Traps
- Lamarckian phrasing such as 'the mosquitoes developed resistance in order to survive'.
- Confusing homologous with analogous structures.
- Assuming a dominant allele automatically becomes more common.
- Claiming a new species has formed without stating reproductive isolation.