How Natural Selection Changes Populations Over Time

Natural selection needs variation. Individuals differ because of mutations, genetic reshuffling during reproduction and the combinations of genes they inherit. Some differences affect how well an organism handles its environment, while others make little practical difference. In sexually reproducing organisms, offspring receive new combinations of existing gene versions from their parents. That reshuffling creates fresh mixtures even when no new mutation occurs.

Not every visible difference is inherited. A plant may grow shorter because it lacked water, and that acquired condition is not automatically passed to its seeds. Natural selection acts on traits with a genetic basis because those traits can appear again in the next generation.

Variation does not appear because an organism needs it. Mutations arise without planning, and the environment determines whether a particular difference happens to be helpful, harmful or neutral. A pale coat might help in snow but expose an animal on dark ground. The value of a trait belongs to a particular environment, not to a universal ladder of better and worse.

Imagine a population of insects with several body colours. Birds can spot one colour easily against the local bark, while another colour blends in. Better-camouflaged insects may survive long enough to reproduce more often. If the bark changes because of pollution, fire or a new tree species, the camouflage advantage may also change. Selection follows current conditions rather than planning for the future.

If body colour is inherited, more offspring in the next generation may carry the helpful colour. Over many generations, that colour can become common. Individual insects did not change colour because they tried harder. The population changed because individuals with different traits left different numbers of offspring. Over long periods, many small shifts can produce large differences. Populations may also become separated, experience different pressures and gradually diverge.

Fitness in evolution means reproductive success in a particular environment. It does not simply mean strength or health. A trait that is helpful in one place may be useless or harmful somewhere else, especially if the environment changes. Cooperation, timing, disease resistance, mating success and the ability to raise offspring can all affect fitness. The strongest-looking individual is not automatically the one whose traits spread.

Natural selection becomes easier when you separate its steps:

• •Individuals in a population vary.
• •Some variation is inherited.
• •More offspring are produced than can all survive.
• •Certain traits improve reproductive success.
• •Helpful inherited traits become more common over generations.

Natural selection is often imagined as too slow to see, but biologists have measured it directly. Peter and Rosemary Grant spent decades studying finches on Daphne Major, a small island in the Galapagos. They caught, measured and tagged almost every bird, recording traits such as beak depth. In 1977 a severe drought struck. Small, soft seeds were quickly eaten, and the remaining food was mostly large, hard seeds that were difficult to crack. Many medium ground finches died, and survival was not random.

Birds with deeper, stronger beaks were more likely to cope with the tough seeds and survive. Because beak size is strongly inherited in these finches, the offspring hatched after the drought had noticeably deeper beaks on average than the population before it. In later years, when wet conditions made small seeds plentiful again, the advantage shifted in other directions. The study showed that selection can change a population measurably within a few generations and that its direction depends on the environment, exactly as the theory predicts.

Bacteria offer one of the clearest examples of natural selection working today. A large bacterial population can contain a few cells that already carry a mutation, or a borrowed piece of DNA, that helps them survive a particular antibiotic. When the drug is used, susceptible bacteria die while the resistant ones keep dividing. Bacteria can produce a new generation in under an hour in good conditions, so a resistant strain can become common very quickly. The antibiotic did not teach the bacteria anything. It changed which cells survived to reproduce.

A common misconception is that people become resistant to antibiotics. It is the bacteria that change, and they can then spread from person to person. Bacteria can also swap genes directly, so resistance can pass between different kinds. This is why doctors avoid prescribing antibiotics for viral infections such as colds, which antibiotics cannot treat anyway, and why heavy use in farming worries public health experts. Every unnecessary dose gives resistant bacteria another advantage. The same logic explains why pests can become resistant to pesticides and weeds to herbicides when one chemical is used repeatedly.

Charles Darwin opened On the Origin of Species not with exotic islands but with farm animals and garden plants. He bred fancy pigeons himself and joined pigeon clubs in London, where breeders had produced birds with puffed-up chests, fan-shaped tails and tumbling flight. Darwin was convinced that all these breeds descended from the wild rock dove. If people choosing which birds could breed had produced such variety within recorded history, he argued, nature's own sorting, working over vastly longer periods, could produce far greater differences.

Your vegetable rack holds a striking example of this artificial selection. Cabbage, kale, broccoli, cauliflower, Brussels sprouts and kohlrabi all belong to one species, descended from a wild cabbage that grows on coastal cliffs in Europe. Farmers who favoured big leaves eventually produced kale, while selecting for tight leafy heads gave cabbage, for swollen flower clusters gave broccoli and cauliflower, for side buds gave Brussels sprouts, and for a fat stem gave kohlrabi. The difference from natural selection is who does the choosing. A farmer picks traits deliberately, while in the wild the environment simply decides which individuals leave more offspring.

The peacock's tail troubled Charles Darwin, because a long, heavy train of bright feathers seems to make survival harder rather than easier. His answer, developed in The Descent of Man in 1871, was sexual selection. What counts in evolution is leaving offspring, and a trait can spread if it helps an individual win mates, even when it carries a cost. Sexual selection works in two main ways. Members of one sex may compete directly, which favours weapons such as antlers and large body size. Alternatively, one sex may choose among the other, which favours displays, songs and ornaments.

In a study published in 1982, the Swedish biologist Malte Andersson tested this idea with long-tailed widowbirds in Kenya, whose breeding males grow tail feathers roughly half a metre long. He cut the tails of some males and glued the removed pieces onto the tails of others, leaving further birds as controls. Males given extra-long tails attracted more females to nest in their territories than males with normal or shortened tails. The experiment showed that female choice could favour an exaggerated ornament. The tail does not grow without limit, because longer feathers cost energy to grow and make flight harder, so selection balances attraction against survival.

Natural selection changes populations, not individuals, across generations. Inherited variation creates different outcomes, and traits linked with greater reproductive success can spread. Keep variation, inheritance and reproduction connected, and evolution becomes a clear process rather than a vague story about organisms trying to improve.