1The four ingredients

Natural selection happens whenever four things are true at once. Learn them as a chain, because exam answers lose marks when a link is missing.

1. Variation. Individuals in the population differ in a trait: beak depth, fur colour, the time a plant flowers, how fast an enzyme works.
2. Inheritance. At least part of that variation is genetic, so offspring tend to resemble their parents in the trait.
3. Overproduction and competition. Populations produce more offspring than can survive. Resources such as food, water, nesting sites and mates are limited, so individuals compete for them.
4. Differential survival and reproduction. Because of their phenotype, some individuals survive and reproduce more than others. "Differential" means "different for different individuals", and it is the key word.

Put together: individuals with phenotypes better suited to the current environment leave more offspring; those offspring inherit the alleles behind the phenotype; so the alleles, and the phenotype, become more frequent in the next generation.

Remove any one link and selection stops. If the variation is not inherited (a deer's broken antler, a muscle built in the gym), it is not passed on, however much it helps. If every individual survives and breeds equally, there is no differential success. If there is no variation, there is nothing to choose between.

Natural selection acts on individuals, but it is the population that evolves: an individual's genes do not change during its life, and only the mix of alleles in the next generation does.

2Fitness means reproductive success

Evolutionary fitness is an individual's contribution of offspring to the next generation, compared with other individuals in the same population. It is not strength, size, speed or a long life. Those things matter only if they lead to more surviving, fertile offspring.

An invented example. In a population of field crickets, two males differ in their song.

Cricket 2 died sooner, perhaps because a parasitic fly that hunts by sound found him. But a loud song also attracts more females, and he left more than twice as many breeding offspring. He has the higher fitness. If song loudness is inherited, the alleles for a loud song will become more common, even though they shorten life.

This is why the AP readers warn against using "fitness" as a buzzword. A sentence such as "the trait increased fitness" earns nothing on its own. A sentence such as "individuals with the trait survived the drought and produced more offspring, so the allele was passed on to more of the next generation" earns the point, because it says what fitness is made of.

3Competition and differential survival in action

An invented example on an invented island. A population of seed-eating birds feeds on two kinds of seed: small soft ones and large hard ones. Beak depth varies between birds and is strongly inherited. Deeper beaks crack hard seeds more easily.

A drought kills most of the plants that make small soft seeds. The large hard seeds remain. Over the dry season the number of birds falls from 1,200 to 300. The birds that survive are mostly the deep-beaked ones, because they could still feed.

Read the table as evidence for each link.

• Variation: the survivors' mean differs from the whole population's, so beak depth varied.
• Competition and differential survival: the population fell by three-quarters, and survival was not random: the survivors had deeper beaks on average.
• Inheritance: the offspring's mean (9.7 mm) is close to their parents' (9.8 mm), not to the old population mean (9.2 mm).

The change from 9.2 to 9.7 mm across one generation is evolution by natural selection. No bird's beak grew deeper because it needed to. The shallow-beaked birds died, the deep-beaked ones bred, and the next generation inherited deep beaks.

4A changing environment changes the direction of selection

Environments are not fixed. Biotic factors (living ones: predators, prey, parasites, pathogens, competitors, mates) and abiotic factors (non-living ones: temperature, rainfall, salinity, light, soil pH) shift from season to season and year to year. When they shift, the phenotype that leaves the most offspring can shift too.

Carry the island birds forward. Two years after the drought come several wet years. Small soft seeds are now abundant, and shallow-beaked birds, which handle small seeds faster, raise more chicks. The mean beak depth drifts back down over the following generations.

So selection has no fixed goal. It favours whatever works now, and different genetic variations can be favoured in different generations. Two consequences matter for the exam.

• Rate. A sudden, severe change (a drought that kills three-quarters of a population) produces a large change in one generation. A mild or slow change produces a small one, and a stable environment may produce almost none.
• Direction. If the environment reverses, the direction of change can reverse. A trait that rose can fall again.

Variation is what lets a population respond at all, an idea that returns in topic 7.11.

5Reading a graph of selection

Many Section I questions show a trait as a frequency histogram, or a smooth curve: the trait value along the x-axis, the number or proportion of individuals up the y-axis. Two curves, "before" and "after", show what selection did. Look for three things.

• Did the peak move? A peak that moves to the right means larger values were favoured; to the left, smaller ones.
• Did the curve get narrower? A narrower curve with the same peak means individuals at both extremes did worse than average ones.
• Did one peak become two? Two peaks means both extremes did better than the middle.

Textbooks call these three patterns directional, stabilizing and disruptive selection. The CED does not ask for the names, but you must be able to describe what a curve shows and explain it in terms of which phenotypes left more offspring.

When you describe a graph, give values. "Mean body length rose from 14 mm to 17 mm between 2018 and 2022" is a description; "it went up" is not.