Year 10 answers
Concepts and definitions
- Variation; selection pressure; differential survival and reproduction; heritability.
- Adaptation — an inherited trait that improves survival or reproduction in a given environment. Fitness — the number of viable offspring an individual contributes. Selection pressure — an environmental factor (predator, climate, disease, food supply) that favours some variants over others.
- Homologous: same underlying structure, different function (human arm and whale flipper share bones). Analogous: same function, different origin (butterfly wing and bat wing).
- A structure that no longer serves its original function (e.g. human appendix, wisdom teeth, tailbone).
- A group whose members can interbreed in nature to produce fertile offspring.
- A physical barrier splits a population; the separated groups diverge until they can no longer interbreed.
Evidence
- Fossil record, biogeography, comparative anatomy (homologous structures), molecular (DNA/protein) similarity. (Also embryology, direct observation.)
- They show intermediate features between ancestral and modern groups, matching the predictions of common descent.
- About .
- The continents were isolated for tens of millions of years; mammals evolved along separate lines on each land mass.
- False. Mutations arise randomly, independent of the environment. Selection then acts on the variation already present.
Case studies
- Mutations produce rare resistant cells; the antibiotic kills susceptible cells; resistant cells survive, reproduce rapidly (every min), and pass resistance to daughter cells; within weeks resistant strains dominate.
- When soot darkened the trees, birds preyed on conspicuous light moths; dark moths survived and reproduced more, raising the frequency of the dark allele. After clean-air laws pale bark returned, reversing the advantage.
- Selection pressure changes with rainfall; the trait that maximises fitness changes each year, so the mean beak size tracks the current food supply.
- Prey speed is a selection pressure; faster cheetahs caught more prey and raised more cubs; the fastest heritable traits (long legs, flexible spine, large heart and lungs) accumulated over many generations.
- Mutation produces rare resistant individuals; pesticide kills susceptible ones; survivors breed; resistance spreads. Strategies: rotate between pesticides with different modes of action; leave “refuge” areas untreated so susceptible genes persist in the population.
Analysis and argument
- Consistent with selection, but a one-off survival event could be chance. Confidence requires heritability (do offspring of survivors also tend to be larger?) and repeatability across generations or populations.
- Yes, effectively. Behavioural isolation (mate recognition) is a reproductive barrier; gene flow has stopped. They are on their way to being recognised as distinct species even if still anatomically similar.
- Vestigial limbs and embryonic hind-limb buds are expected if whales descended from four-legged ancestors. Genetic programs for limbs persist but are no longer expressed fully.
- Direct: transitional forms (light-sensitive patches in flatworms, cup eyes in snails, pinhole eyes in nautiluses, full lens eyes in vertebrates) show a gradient of working intermediate eyes. Logic: each small improvement in light detection confers survival advantage, so intermediate eyes are favoured — not disadvantageous.
Challenge
- Grow a large bacterial culture; plate onto agar with and without low-dose antibiotic. Selection pressure: the antibiotic. Heritable variation: resistance alleles from spontaneous mutation. Measure fitness by counting colonies; track allele frequency by repeated subculturing.
- Evolution is not directional. The horse fossil record is a branching bush with many extinct side lineages — not a straight march to Equus. Modern horses are one surviving branch; most lineages went extinct.
- Fraction expressing dark phenotype (1%). If only survives, the new allele frequency is (the dark allele fixes at ) since only homozygous dark individuals remain.
- Urban birds changing songs to cut through city noise; tuskless elephants becoming more common under ivory poaching pressure; peppered moth; antibiotic-resistant bacteria; herbicide-resistant weeds; salmon reaching sexual maturity earlier under heavy fishing of large fish. (Any two.)