Natural selection

The four conditions for natural selection

In 1858, papers by Darwin and Wallace were read together at a meeting of the Linnean Society in London, and a year later Darwin published On the Origin of Species. Their idea, natural selection, isn't a force or a plan. It's what automatically happens whenever four conditions are true:

1. 1Variation: individuals in a population differ from one another. Some finches have deeper beaks than others.
2. 2Heritability: at least some of that variation is passed from parents to offspring through genes. Deep-beaked parents tend to have deep-beaked chicks.
3. 3Overproduction: more offspring are born than the environment can support, so not all of them survive and breed.
4. 4Differential reproductive success: individuals whose heritable traits suit the current environment survive and reproduce more than others, so they pass on more copies of their alleles.

Put those together and the result is inevitable: the traits that helped parents succeed become more common in the next generation. Repeat over many generations and the population changes. A trait that has spread because it improves survival or reproduction is called an adaptation.

Notice that each condition matters. If beak depth varied only because of what chicks ate, and not because of their genes, the drought would kill small-beaked birds but the next generation would look just like the old one.

Populations evolve, individuals don't

No finch grew a bigger beak during the drought. Each bird kept the beak it hatched with. What changed was which birds were left alive to breed. Selection acts on individuals, sorting them by how well they survive and reproduce, but the evolution shows up in the population, as a change in allele frequencies from one generation to the next.

Where the variation comes from

Selection can only choose among variants that already exist. It can't produce a new allele because a population needs one. New alleles come from mutation, random changes in DNA. Recombination during meiosis and fertilisation then shuffles existing alleles into new combinations.

Mutations are random with respect to need: a mutation that would help is no more likely to appear just because the environment has changed. But selection itself is not random. Once variants exist, the environment consistently favours some over others.

Antibiotic resistance shows both halves clearly. In a huge population of bacteria, a few cells may already carry a mutation that lets them survive a drug. The antibiotic doesn't cause that mutation. It kills the susceptible cells, and the resistant few multiply to replace them. Within days, the population can be mostly resistant.

What "fitness" really means

In evolution, fitness means how many offspring an individual contributes to the next generation, compared with others in the same population. That comparison is called relative fitness. The fittest individual isn't necessarily the strongest or fastest; it's the one that leaves the most surviving, breeding offspring.

That's why traits that seem to hurt survival can still spread. A peacock's huge tail makes him easier for predators to catch, but if peahens prefer to mate with long-tailed males, the tail raises his reproductive success. Selection driven by differences in mating success is called sexual selection.

Three patterns of selection

Many traits, like beak depth or body size, vary continuously, so a histogram of the population usually forms a bell-shaped curve. Selection changes the shape of that curve in one of three ways:

• Directional selection favours one extreme. The whole curve shifts left or right. Example: the finches after the drought, or peppered moths in industrial England, where dark moths became more common as soot darkened tree trunks.
• Stabilising selection favours the average and removes both extremes. The mean stays put but the curve gets narrower. Example: in a woodland mouse population, mice whose fur matches the brown forest floor are hardest for predators to spot, so very light and very dark mice are picked off.
• Disruptive selection (also called diversifying selection) favours both extremes over the middle. The curve can split into two peaks. Example: if a habitat offered only very small and very large seeds, birds with medium beaks would handle neither well.

To identify the type from a graph, compare the "before" and "after" curves. Did the mean move? That's directional. Did the mean stay but the spread shrink? That's stabilising. Did the middle drop while both tails held up or grew? That's disruptive.

More ways selection works

Artificial selection is the same process with humans choosing who breeds. Breeders turned a single wild plant, wild cabbage, into cabbage, broccoli and kale, and wolves into every breed of dog. Darwin used it as evidence that selection on small differences can add up to big changes.

In frequency-dependent selection, a phenotype's fitness depends on how common it is. When rare types do better, selection keeps several types in the population.

In heterozygote advantage, the heterozygote is fitter than either homozygote, so both alleles are kept. The classic case is the sickle-cell allele. Two copies cause sickle-cell disease, but one copy gives some protection against malaria. Where malaria is common, carriers leave the most offspring, so the allele stays common even though it is harmful in homozygotes.

Similar environments can also select for similar traits in unrelated lineages, which is called convergent evolution. Sharks and dolphins both have streamlined bodies and fins, even though one is a fish and the other a mammal.

Key terms

Natural selection

Individuals with heritable traits that suit their environment leave more offspring, so those traits become more common over generations.

Adaptation

A heritable trait that has spread because it improves survival or reproduction in a particular environment.

Relative fitness

An individual's contribution of offspring to the next generation, compared with others in its population.

Directional / stabilising / disruptive selection

Selection that favours one extreme / the average / both extremes of a trait.

Sexual selection

Selection caused by differences in mating success, often producing showy traits.

Artificial selection

Humans choosing which individuals breed, so chosen traits become more common.

Heterozygote advantage

When heterozygotes are fitter than both homozygotes, keeping both alleles in the population.

Convergent evolution

Unrelated lineages evolving similar traits because they face similar environments.

Check yourself

Try answering in your head before you open each answer.

• 1.A doctor says, "This patient's bacteria mutated to become resistant because they were exposed to the antibiotic." What's wrong with this explanation, and how would you fix it?Show answerHide

  The antibiotic doesn't cause the resistance mutations; they arise randomly, and some cells may already have them before the drug is given. The antibiotic kills the susceptible bacteria, and the resistant ones survive and multiply. Better: "The antibiotic selected for resistant bacteria that were already present."

• 2.In a population of lizards, mean body length stays at 12 cm over ten generations, but the range narrows from 8–16 cm to 10–14 cm. What type of selection is acting, and what might cause it?Show answerHide

  Stabilising selection: the mean doesn't change, but the extremes are removed. For example, very small lizards might be easily eaten or lose fights, while very large ones might need too much food or be easier for predators to spot.

• 3.A weightlifter builds huge muscles. Will his children be born more muscular because of his training? Explain in terms of natural selection.Show answerHide

  No. Muscle from training is an acquired change, not a change in his genes, so it isn't inherited. Only heritable variation responds to selection. His children inherit his alleles, not the results of his workouts.