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🧬Biology8 min read

Natural Selection

The most powerful idea in the history of biology is also one of the most misunderstood. Natural selection is not survival of the strongest, it does not aim at anything, it cannot plan ahead, and it was discovered by two people simultaneously, neither of whom was famous at the time.

In plain English

Imagine a population of beetles. Some are green, some are brown. Birds eat beetles. Green beetles are easier to spot on brown soil, so they get eaten more often. Brown beetles are harder to spot, so they survive longer and have more offspring. The offspring tend to inherit the colour of their parents. Over generations, the population shifts toward brown.

No one planned this. No one decided that brown was better. The beetles did not try to become brown. The birds did not decide to eat green ones. The environment presented conditions, and the beetles that happened to fit those conditions better happened to leave more descendants.

That is natural selection. In its entirety. Darwin spent 20 years worrying about it before publishing.

The mechanism only requires four things to be true simultaneously:

  1. Variation: individuals in a population differ from each other
  2. Heritability: offspring tend to resemble their parents
  3. Differential reproduction: some variants leave more descendants than others
  4. Struggle for existence: not all individuals can survive and reproduce to their maximum potential

When these four conditions are met, natural selection is not a possibility. It is a mathematical inevitability. It cannot not happen.

Five things to file under "wait, what?"

  • Darwin and Wallace discovered it simultaneously and independently. Alfred Russel Wallace was a naturalist collecting specimens in Borneo in 1858 when he sketched out the theory of natural selection during a bout of malaria. He sent the manuscript to Darwin, who had been developing the same idea since 1838, for forwarding to the Geological Society. Darwin, reading a summary of his own theory from a stranger, had a crisis. Their papers were presented jointly, without either man present, at a Linnean Society meeting in July 1858. Neither became famous that day. One member of the audience wrote in his diary that the year 1858 had passed without any scientific news of note.

  • Selection is the opposite of random. The most common misconception about evolution is that it is "just random chance." Mutation is random: the errors that generate new variation occur without regard to whether they will be useful. But selection acts on that variation systematically: variants that function better in their environment leave more descendants. The result is directional, non-random change. Random variation filtered by non-random selection produces non-random outcomes. The eye, the wing, and the hand are not accidents.

  • Antibiotic resistance is natural selection happening in real time. When a population of bacteria is exposed to an antibiotic, most die. A small number, carrying random mutations that happen to confer resistance, survive. Those survivors reproduce. Within hours, the surviving population is predominantly resistant. This is not adaptation in any intentional sense. The bacteria did not decide to become resistant. The antibiotic created a selective environment, and only pre-existing resistant variants survived. The same process, over longer timescales, explains every other biological adaptation that has ever existed.

  • "Survival of the fittest" does not mean survival of the strongest. The phrase was coined by Herbert Spencer, not Darwin, and Darwin regretted adopting it. "Fittest" in evolutionary biology means best suited to the current environment, not strongest, not fastest, not most aggressive. The "fittest" organism for a dark cave with no light is a blind, colourless cave fish. The "fittest" organism in a high-altitude low-oxygen environment is a yak. Fitness is always relative to context.

  • Dogs are natural selection with a human doing the selecting. All domestic dogs, from the Chihuahua to the Great Dane, are descended from wolves, within the last 15,000 to 40,000 years. The diversity was produced by selective breeding: humans chose which individuals to breed, imposing a selection pressure. The result demonstrates how rapidly natural selection (or its artificial equivalent) can reshape an organism. The process that produced all the dogs in the world is the same process, running on a longer timescale with a different selector, that produced all the other animals in the world.

The full story

The problem of design

Before Darwin, the standard explanation for the complexity of living things was design. The eye, with its lens and retina and ability to focus, seemed too intricate to have arisen by chance. The wing, the claw, the beak, each seemed fitted to its function in a way that implied an engineer.

William Paley articulated this argument in 1802: finding a watch on a heath, you would infer a watchmaker. Finding the complexity of life, you should infer a Creator. Darwin read Paley as a student and found the argument compelling.

What Darwin eventually recognised was that there was a third option between "designed by an intelligent creator" and "produced by pure chance": produced by a process that is neither intelligent nor random. A process that accumulates improvements over vast stretches of time. A process that produces the appearance of design without the designer.

The analogy he reached for was artificial selection, the selective breeding of animals and plants that farmers had been practising for millennia. A farmer breeding horses for speed does not introduce any new genetic material. He selects the fastest horses to breed from, generation after generation, and the population shifts. If a farmer can transform a wild horse into a racehorse in a few dozen generations, what might natural selection accomplish over millions of years?

The tree of life

The implication of natural selection that Darwin found most difficult to commit to paper was the idea that all life on Earth is related. If selection acts on heritable variation over time, and if you go back far enough, all species share common ancestors. Not just humans and apes, though Darwin knew this would be inflammatory, but humans and fish, humans and mushrooms, humans and bacteria.

This is the tree of life. Every species alive today is a twig. Trace back along the branches and they converge: to the first fish, the first animal, the first eukaryote (a cell with a nucleus), the first life.

We now have strong evidence for this in the form of DNA. Every living organism uses the same genetic code, the same molecular language, to translate DNA into proteins. The probability that this occurred independently multiple times is vanishingly small. The simplest explanation is that the genetic code was established once, in a common ancestor, and inherited by everything alive today.

Sexual selection

Darwin identified a second mechanism alongside natural selection: sexual selection. This operates not through survival but through reproduction. Features that increase an organism's chances of mating, even at some cost to survival, will be selected for if the reproductive benefit outweighs the survival cost.

The peacock's tail is the classic example. A large, elaborate tail is energetically expensive, makes the bird more visible to predators, and impairs flight. By the strict logic of survival, it should be selected against. But peahens prefer males with larger, more elaborate tails. Males with bigger tails have more offspring. Over generations, tail size increases, driven not by survival advantage but by mate preference.

Sexual selection explains many features of the living world that survival selection alone does not: the bright colouration of male birds, the size difference between male and female species, the antlers of deer, the elaborate songs of frogs and insects at night. It is selection by attraction rather than selection by death, and it is often the dominant force in shaping a species' appearance.

What it doesn't explain

Natural selection is powerful, but it is not the complete story of evolution. Genetic drift, random changes in gene frequency in small populations, can fix neutral or even slightly harmful traits. Neutral evolution (the accumulation of changes that have no effect on fitness either way) accounts for significant portions of the genome. Mass extinctions reset the selective landscape, giving previously marginal species opportunities that had nothing to do with their fitness. Epigenetics, horizontal gene transfer in bacteria, developmental constraints: the picture is richer than selection alone.

Darwin did not know about genes. He had no mechanism for heritability, just the observation that offspring resemble parents. The synthesis of natural selection with Mendelian genetics (the "Modern Synthesis" of the 1930s-50s) was necessary to put the theory on solid molecular footing. Further revisions continue.

What remains unchallenged is the core mechanism: heritable variation, differential reproduction, time. The rest is detail (important, interesting, contested detail) but the engine is the same.

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