Natural Selection

Natural selection is the engine that drives evolution. It is the process by which living things best suited to their environment are more likely to survive and have babies, passing on their helpful features to the next generation. Over many generations, natural selection slowly shapes populations, gradually making them more and more suited to where they live. The idea was worked out in the 1850s by Charles Darwin and independently by Alfred Russel Wallace, and it is one of the most powerful ideas in all of science.

• Worked out byDarwin and WallaceIndependently, in the 1850s
• Sometimes called"Survival of the fittest"Coined by Herbert Spencer, not Darwin
• Speed of changeSlow to fastYears to millions of years, depending on the species
• Works onExisting variationCannot create new features from nothing
• DirectionAlways localHas no overall goal or "purpose"
• Best demonstrationsAntibiotic resistance, finchesWatched in real time today

The four conditions

Natural selection happens automatically whenever four conditions are all true.

1. Variation: individuals in a population are slightly different from each other.
2. Inheritance: those differences are partly passed on from parent to child through genes.
3. Competition: too many offspring are born for all to survive (not enough food, water, space, mates).
4. Differential survival: the individuals best suited to their environment survive and have more babies.

Notice that all four conditions are true for almost every living thing on Earth. That is why natural selection is constantly working everywhere, all the time.

How it actually works

Imagine a population of green beetles living on a green meadow. By chance, some beetles are slightly darker green than others. Hungry birds eating beetles can see the brighter, lighter green beetles more easily, so those get eaten more often. The darker green beetles survive more often and have more babies. Some of those baby beetles inherit the darker colour and survive even better. After many generations, the whole population of beetles is darker green than it started. Natural selection has shaped the population.

Now imagine that the meadow gets ploughed up and turned to brown earth. Suddenly the darker green beetles stand out, and the few that happen to be brown blend in. Brown beetles now survive more often, have more babies, and within a few generations the population is brown. Same process, opposite result, just because the environment changed.

"Survival of the fittest" does not mean strongest

The famous phrase "survival of the fittest" was actually coined by the philosopher Herbert Spencer, not Darwin. It can be misleading. "Fittest" does not mean "strongest" or "biggest" or "smartest". It means "best fitted to the local environment".

• For a polar bear, fittest means warmest fur.
• For a desert plant, fittest means most water-saving leaves.
• For a hummingbird, fittest means quickest reflexes.
• For a deep-sea anglerfish, fittest means brightest lure.
• For a peacock, fittest means most attractive tail.

What "fittest" means changes from species to species and from place to place. There is no single way to be fit. That is why life is so varied: each environment shapes the creatures in it differently.

Examples we can see today

Natural selection is not just an idea from history. Scientists can watch it happen in real time.

• Antibiotic resistance: bacteria can evolve resistance to antibiotics within years. A few bacteria with random resistance mutations survive when antibiotics kill the rest, then multiply rapidly. This is now a serious medical problem worldwide.
• Pesticide resistance: insects regularly evolve resistance to chemical pesticides within a few generations.
• Galapagos finches: biologists Peter and Rosemary Grant have watched the beak shape of Galapagos finches measurably change from year to year as droughts and rains alter the food supply on the islands.
• Peppered moths: during the Industrial Revolution in 19th-century Britain, the peppered moth evolved from mostly light-coloured to mostly dark-coloured in just a few decades. The reason was that soot from factories darkened the tree bark moths rested on, so darker moths suddenly had better camouflage from predators.

Fact Many of the bacteria that cause infections today are evolving resistance to antibiotics faster than we can develop new drugs. The World Health Organisation calls antibiotic resistance one of the biggest threats to global health. The problem is essentially evolution in action: every time we use an antibiotic, we put selection pressure on the bacteria to evolve resistance. Some bacteria (like MRSA and certain strains of tuberculosis) are now resistant to almost every antibiotic we have.

What natural selection cannot do

Natural selection is powerful but it has limits.

• It cannot create new features from nothing; it can only work with whatever variation already exists or arises through mutation.
• It cannot plan ahead. Each generation is selected for what works now, not what might be useful later.
• It cannot produce "perfect" designs. Most adaptations are imperfect compromises (the human spine, for example, is a famously awkward design because it evolved from a four-legged ancestor).
• It can leave behind useless leftover features (like the appendix, vestigial hip bones in snakes, or eyes in cave-dwelling fish that no longer work).

Did you know? Natural selection is not the only mechanism of evolution. Other mechanisms include genetic drift (random changes in small populations), gene flow (new genes coming in from other populations), and sexual selection (features that attract mates, like a peacock's tail). Modern evolutionary biology takes account of all of these. Natural selection is still the most important by far, but it is part of a richer picture.

Deeper dive: artificial selection, the proof of concept

One of Darwin's most powerful arguments for natural selection was based not on what nature does, but on what humans do to our domestic animals and crops. The process is called artificial selection (or sometimes "selective breeding"), and it has been used by farmers for thousands of years.

Every breed of dog (from chihuahuas to Great Danes), every breed of cow, sheep, chicken, every variety of cabbage, broccoli, kale, every modern apple, every wheat, all came from artificial selection. Humans simply picked the individuals with the most useful features each generation and bred them together, repeating for thousands of years. The result is the enormous variety of breeds and varieties we have today, all bred from a much smaller set of wild ancestors.

Darwin's argument was simple: if humans can shape species so dramatically in just a few thousand years, imagine what nature can do over millions of years. Natural selection is essentially the same process as artificial selection, just with the environment doing the selecting instead of a farmer. Nobody who has seen a chihuahua and a wolf, both descended from a common ancestor only about 30,000 years ago, can deny that selective forces can produce huge changes.

This argument was so powerful that Darwin spent the first chapters of his famous book talking about pigeon breeding (his own hobby) and farm animals. He wanted readers to see the evidence in their own back gardens before he asked them to accept the same process happening in nature.

For the man who worked all this out, see Charles Darwin. For how all of this is recorded in rocks, see the fossil record.