Populations and genes (A Level) flashcards

Topic 3: Populations and genes

Habitat and niche

Habitat: the place where an organism lives. Niche: the role of a species in its habitat, set by its adaptations to biotic and abiotic conditions. Habitat is where; niche is how.

Why does a predator population peak after its prey?

More prey means more food for predators, so predator numbers rise after a delay; more predators then reduce the prey, and predator numbers fall in turn.

How does primary succession lead to a climax community?

Pioneer species (lichens, mosses) colonise bare ground; their remains build soil. Each stage changes the environment, making it suitable for newcomers with different adaptations, which may out-compete earlier species, until a relatively stable climax community forms.

Photolysis of water in the light-dependent reaction

2H₂O → 4H⁺ + 4e⁻ + O₂. Electrons replace those lost from chlorophyll; protons combine with NADP to form reduced NADP; oxygen is released.

Light-independent reaction in outline

CO₂ + RuBP (5C) → 2 GP (3C), catalysed by rubisco. GP is reduced to triose phosphate using ATP and reduced NADP. Most triose phosphate regenerates RuBP; the rest makes useful organic substances.

Respiratory quotient (RQ), and typical values

RQ = CO₂ produced ÷ O₂ consumed. Carbohydrate about 1.0, protein about 0.9, lipid about 0.7.

Net production of a consumer

N = I − (F + U + R). I: energy in ingested food; F: in faeces; U: in urine; R: respiratory loss.

Nitrification and denitrification

Nitrification: ammonium oxidised to nitrite, then nitrate, by nitrifying bacteria; aerobic. Denitrification: nitrate to nitrogen gas by denitrifying bacteria; anaerobic, e.g. waterlogged soil.

Chi-squared test: degrees of freedom and decision

χ² = Σ(O − E)² ÷ E, using counts not percentages. Degrees of freedom = categories − 1. Below the critical value at p = 0.05 (3.84 for 1 df): not significant, the null hypothesis stands.

Conditions for the Hardy–Weinberg principle

A large population, random mating, no mutation, no migration (gene flow) and no selection.

Genetic bottleneck

A severe reduction in population size, caused by an environmental event or human activity, which reduces the genetic variety of the gene pool.

Stabilising, directional and disruptive selection

Stabilising: mean phenotype favoured; mean unchanged, spread narrows. Directional: one extreme favoured; mean shifts. Disruptive: both extremes favoured; spread widens, two peaks.