1Selection acts on phenotypes

A predator eats the beetle it can see. A drought kills the plant that loses water fastest. In each case what decides survival is the phenotype, the trait as it appears. The genotype matters only through the phenotype it produces.

This has a consequence that exam questions use. A recessive allele is hidden in a heterozygote, because the dominant allele sets the phenotype. If the recessive phenotype is harmful, selection removes homozygous recessive individuals but cannot "see" the copies of the allele carried in heterozygotes. That is why a harmful recessive allele can persist in a population for many generations at a low frequency.

Two points keep the picture straight.

• Only heritable variation matters for evolution. A plant that is tall because it grew in rich soil does not pass on its height. Selection may favour it in its lifetime, but its offspring are not taller for it.
• Variation must exist before the pressure arrives. Selection does not produce the variant it needs. If no individual can survive a new pressure, the population declines or dies out.

2Selective pressures change

Environments change on every timescale: day to night, season to season, year to year, and over centuries. When they change, they apply new selective pressures.

Flowering time and a changing climate. For a plant, flowering at the right time is fitness: too early and frost kills the flowers, too late and the soil has dried before the seeds ripen. Field mustard (Brassica rapa) in southern California gives a real case. After a run of dry years shortened the growing season, seeds collected after the drought grew into plants that flowered earlier than plants grown from seeds collected before it, when both were raised side by side in the same conditions. Growing them together is what shows the difference was inherited, not caused by the weather they grew in. Plants that flowered early had set seed before the soil dried out; their offspring inherited early flowering.

A trait is never good or bad on its own: its effect on fitness depends on the environment, so the same allele can be favoured in one place or time and selected against in another.

3One variation, two effects: sickle cell

The sickle cell allele (often written HbS) is a single-base change in the gene for the beta chain of haemoglobin. It swaps one amino acid in the protein, and haemoglobin containing that chain tends to stick together into long fibres when oxygen is low, distorting red blood cells into a sickle shape.

• Two copies (homozygous): sickle cell disease. Sickled cells block small blood vessels and are destroyed early, causing pain, anaemia and organ damage. Fitness is reduced.
• One copy (heterozygous): little or no disease, and resistance to malaria. The malaria parasite spends part of its life inside red blood cells, and infected heterozygous cells tend to sickle and be removed.
• No copies: no sickle cell disease, but full susceptibility to malaria.

So the allele's effect on fitness depends on where the person lives. Where malaria is common, heterozygotes have the highest fitness of the three genotypes, and the allele stays at a fairly high frequency in the population even though it harms homozygotes. Where malaria is absent, heterozygotes gain nothing, homozygotes still suffer, and the allele is selected against. Same allele, same protein change; opposite effect on fitness, set by the environment.

4One variation, two environments: insecticide resistance

From the 1940s, the insecticide DDT was sprayed widely against mosquitoes and other insects. Within a few years, many insect populations were resistant. Here is the logic, step by step.

1. Before spraying, a few insects already carried alleles that made them less sensitive: for example, an altered version of the nerve-membrane protein DDT binds to, or a more active enzyme that breaks DDT down.
2. Without DDT these alleles gave no advantage, and some carried a small cost, so they stayed rare.
3. Spraying killed most susceptible insects. Resistant ones survived and bred, and their offspring inherited resistance.
4. After a few generations, most of the population was resistant.

Note what did not happen: DDT did not cause the resistance mutations. They were already present. The insecticide changed the selective pressure, and the existing variation was sorted.

5Variation in molecules within cells

Underneath most phenotypes is a difference in molecules. The CED asks you to connect two kinds of molecular variation to fitness.

Different types of molecule. A population may carry two or more alleles for one enzyme, coding for versions of the protein that differ by an amino acid or two. Those versions can have different shapes, and so different temperature or pH optima, or different affinities for their substrate.

An invented example. A fish species lives along a coastline where the water temperature ranges from 6 °C in the north to 22 °C in the south. One of its respiratory enzymes comes in two forms, coded by two alleles.

In the cold north, fish with form C run their respiration faster, release energy for swimming and feeding faster, and so are more likely to escape predators, catch food and reproduce: the allele for form C should be more frequent there. In the warm south, form W is slightly faster, and its allele may be favoured. A species carrying both alleles can live along the whole coast.

Other cases of the same idea:

• Membrane lipids. The proportion of unsaturated fatty acids in membrane phospholipids affects how fluid the membrane stays in the cold (Unit 2). Variants that make more unsaturated lipids keep membranes working at low temperature.
• Pigments. Plants and algae with more than one kind of light-absorbing pigment capture a wider range of wavelengths (Unit 3), which helps in shade or deep water.
• Defence molecules. The proteins an immune system uses to recognise invaders vary widely between individuals, so a new pathogen rarely finds every individual equally vulnerable.

Different numbers of a molecule. Fitness can also depend on how much of a protein a cell makes. Some insecticide-resistant mosquitoes carry several extra copies of a gene for an enzyme that breaks insecticides down; their cells make much more of the enzyme and detoxify the poison faster. More molecules of a transport protein, a pigment or an enzyme can likewise raise the rate of a process that limits survival.

In a written answer, link the levels: allele → protein (type or amount) → rate of a cell process → phenotype → survival and reproduction. Each arrow is a step a reader is looking for.