Biology - Homeostasis, Ecology, and Evolution
Homeostasis, ecology, and evolution questions test systems thinking: how a change is detected, corrected, or selected for. Expect negative feedback, osmoregulation and thermoregulation, neuron and hormone action, energy flow and trophic levels, population growth, and natural selection.
Core rules
- Negative feedback reverses a deviation; positive feedback amplifies it (e.g., oxytocin in childbirth).
- Blood glucose: insulin lowers glucose; glucagon raises it. Body temperature and water balance use similar feedback loops.
- Energy flows one way through ecosystems; about 10% passes between trophic levels, so pyramids of energy are steep.
- Population growth is limited by carrying capacity; density-dependent factors intensify with crowding.
- Natural selection acts on heritable variation within a population; individuals do not evolve, populations do.
- Hardy–Weinberg: p + q = 1 and p² + 2pq + q² = 1 when no selection, mutation, migration, or drift occurs.
Worked example
A recessive genetic disease affects 1 in 10,000 people in a population at Hardy–Weinberg equilibrium. Find the carrier frequency. Affected individuals are q² = 0.0001, so q = 0.01. Then p = 1 − q = 0.99. Carriers are 2pq = 2(0.99)(0.01) = 0.0198, or about 1 in 50 people. The key is to take the square root before calculating carriers; using q² directly as the carrier frequency is the common error.
Traps
- Calling a response positive feedback merely because it is beneficial.
- Reversing the direction of a feedback loop.
- Treating the 10% energy rule as exact for every ecosystem.
- Claiming an individual organism evolved during its lifetime.
- Mixing up q² (affected) and 2pq (carrier) in Hardy–Weinberg problems.