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Comment to the reader.
In the previous entry I described the molecule of inheritance — how its four-letter sequence is copied across generations with very high but imperfect fidelity, and how the small errors that creep in during copying are the source of new hereditary variation. The previous entry closed with the observation that anything which helps a gene get copied into the next generation will, over time, become more common, and anything that gets in the way will, over time, fade.
This entry is what follows from that observation, applied with patience to a planet of finite resources for four billion years. It is the third in the How We Came About series. The mechanism described here — natural selection — is the explanation our era now accepts for how the variety of life on Earth came to be. The mechanism is small. Its consequences are everything alive.
I write in the spring of 2026 of the Common Era. The basic argument is one hundred and sixty-eight years old at this point, and the molecular detail that fills it in is largely the work of the last seventy. I have set out the picture as it is broadly understood at this moment, noting where the picture is settled and where parts of it are still being argued over.
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Where does the variety of life come from?
Look at any patch of forest, any reef, any pond. The number of distinct kinds of organism is staggering. Roughly two million species have been formally described and named; estimates of the true total run from around eight million upward, and many times higher still once microbes are included. Each is recognisably different from the others, and yet each is recognisably an organism — built of cells, running on DNA, descended from parents.
For most of human history, the variety was taken to be original: each kind a separate creation, fixed in form. The fossils unearthed from the rocks during the eighteenth and nineteenth centuries — sequences of organisms unlike any alive today, lying in strata of clearly different ages — made that account harder to maintain. By the middle of the nineteenth century, several thinkers had begun to suspect that species changed over time, and that present-day life was descended from earlier forms. What was missing was a mechanism. Without a plausible account of how one species could become another, evolution was only a description of a pattern in the rocks.
The mechanism was provided in 1858, in a paper read before the Linnean Society of London.
The argument was made independently in the middle of the nineteenth century by Charles Darwin, working in England, and Alfred Russel Wallace, working in the Malay Archipelago. Their joint paper appeared before the Linnean Society of London in the summer of 1858 [1]. Darwin's much fuller treatment, On the Origin of Species, was published the following year [2]. Neither of them yet knew anything about DNA, genes, mutation, or chromosomes. The mechanism they described nevertheless turned out to be fundamentally sound, and the genetic and molecular discoveries of the century that followed resolved the inheritance problem that Darwin in particular had been unable to solve. His framework was vulnerable to the long-standing objection that any new variant ought to be diluted away by blending in subsequent generations; his own subsequent theory of pangenesis, advanced in 1868, was wrong and did not solve the problem. Mendelian genetics, when it was rediscovered around 1900, dissolved that objection by establishing that inherited factors are particulate and do not blend, and the molecular detail uncovered after that fitted into Darwin and Wallace's framework and substantially deepened it.
The argument runs as follows.
Take any population of organisms that meets three conditions.
First, the individuals in the population are not all identical. They vary, in size, colour, behaviour, resistance to disease, and a hundred other features. Variation is the first ingredient. The previous entry described where it comes from in detail: copying errors during cell division, the recombination that occurs when sex cells are produced, and the mosaic that results when two parents combine theirs.
Second, at least some of that variation is passed from parents to offspring. A taller-than-average parent tends, on average, to have a taller-than-average child. Heritability is the second ingredient. The previous entry described its molecular basis: the four-letter sequence of DNA, faithfully copied across generations.
Third, individuals in the population do not all reproduce equally. Some leave more descendants than others. The reasons are various — predators, disease, food, weather, mates, accident — but the bare fact of differential reproduction is the third ingredient.
Once those three conditions hold, a fourth thing follows automatically. If the individuals who leave more descendants are, on average, the ones with particular variants of particular traits, those variants will become more common in the next generation, simply because more copies of them get made. After many generations, the population will look measurably different from the population it was.
That is natural selection. It is not a force, in the way gravity is a force. It is not directed. It is not aiming at anything. It is only an arithmetic consequence of three observations about populations of imperfectly-copying replicators on a planet that does not have unlimited room.
The three ingredients are not arranged independently of one another. Variation supplies the raw material. Heritability ensures that whatever differences are present are conveyed forward. Differential reproduction sorts among them. The whole only works when all three are present together. Take any one away — make every individual identical, or make traits non-heritable, or make every individual reproduce equally — and the engine stops.
Darwin's contemporaries found the argument controversial, partly for reasons that have to do with religion (these belong to a later entry) and partly because the variation that natural selection acts upon seemed, at the time, to have no obvious origin. The discovery of genes, mutation, and recombination, in the century after the Origin was published, supplied that origin. The argument has been confirmed and considerably extended since.