Darwin's Theory of Evolution: What Natural Selection Is and Isn't

A naturalist aboard a brig

Charles Darwin signs on as naturalist aboard HMS Beagle, a Royal Navy brig on a surveying mission that leaves Plymouth Sound on 27 December 1831. The voyage is supposed to last two years; instead it stretches to almost five, and the Beagle does not return to England until 2 October 1836. Of that time, Darwin spends roughly three years and three months on land — exploring South America and the ocean islands, including the Galápagos — and about 18 months at sea.

The zoological and geological collections, and the fossils gathered during the expedition, become the empirical foundation on which Darwin will build his theory of evolution in the years that follow.

Finches that don’t look alike

In the Galápagos, in 1835, Darwin collects a group of finches — some 13-14 species, by later estimates — that at first glance look like little more than ordinary birds. Studying them, he realizes they all descend from a single common ancestor: scattered across different islands, with no contact between them, they had adapted to eat whatever food was locally available, seeds, insects, cactus flowers, in one case even the blood of other birds. Each species’ beak took a different shape depending on the food it had to handle.

It is the textbook example of adaptive radiation: a single species that arrived on the islands and, over generations, split into distinct species, each suited to its own ecological niche. The mechanism that explains how this happens is natural selection.

The mechanism: three simple ingredients

Darwin’s theory rests on three components, and none of the three, on its own, would explain much.

Ingredient	What happens
Variation	Individuals within a species differ from one another by chance
Inheritance	An individual’s traits can be passed on to its offspring
Struggle for existence	Those with traits better suited to their environment are more likely to survive and reproduce

Put together, these three ingredients let advantageous variants spread through later generations, while less advantageous ones tend to disappear. The Natural History Museum in London sums it up this way: natural selection is neither a conscious choice nor a force with a purpose, but simply the statistical consequence of the fact that whoever is better adapted to the local environment leaves, on average, more fertile offspring than others. Anyone curious about how the handing down of traits from one generation to the next actually works — the inheritance mechanism Darwin observed without knowing its details — can read the Recap on DNA and the one on mitosis vs meiosis.

A letter that changed the plan

Darwin has been working for years on a much longer manuscript about the transmutation of species, at his home in Down, Kent, when in 1858 he receives a letter from Alfred Russel Wallace. Wallace, an English naturalist who has led expeditions to Brazil (1848-1852) and the Malay Archipelago (1854-1862), sends him an essay written during the second voyage: it sets out a mechanism of species change strikingly similar to the one Darwin has been working on for years.

The coincidence pushes Darwin to move faster. First comes a joint announcement, read to the Linnean Society of London in July 1858; then the book, «On the Origin of Species», published fifteen months later, in November 1859. Wallace himself later acknowledges that his own contribution had been to push Darwin to write and publish right away, while the bulk of the work — working out the theory’s details — remained Darwin’s, who had been at it for years before that letter ever arrived. The two keep up a relationship of mutual respect, even after their views later diverge over how natural selection applies to the human species.

	Charles Darwin	Alfred Russel Wallace
Working on the idea	For years, in a long manuscript	During the Malay Archipelago expedition
In 1858	Receives Wallace’s letter	Sends his own essay to Darwin
Publication	«On the Origin of Species», November 1859	Co-author of the 1858 announcement

The moth that proves the theory, live

You don’t have to wait millions of years to see natural selection at work: the case of the peppered moth (Biston betularia) shows it happening within a few decades. The species exists in a light form, dominant before the Industrial Revolution, and a dark form, which appeared and spread during British industrialization, roughly between 1760 and 1840. In the areas most polluted by factory smoke, the dark form comes to make up as much as 80% of the local population.

Practical example: researchers placed 500 fake moths on trees in an unpolluted woodland, in Cornwall and Hampshire, and observed them for 48 hours. The light form had a survival rate of 64%, against 43% for the dark form. Models of predatory birds’ vision confirm why: against lichen-covered bark, the light form is nearly invisible, while the dark form stands out at once; pollution, by killing lichens and blackening bark, flips that advantage.

After Britain’s Clean Air Acts, in the 1950s, pollution drops and the frequency of the light form climbs again, while the dark form declines. The phenomenon that made the peppered moth famous isn’t just observable: it’s reversible, as environmental conditions change. Anyone curious about the historical backdrop against which the dark form appeared can read the Recap on the Industrial Revolution.

What natural selection is not

Several persistent misunderstandings swirl around Darwin’s theory. The most common is that “survival of the fittest” means the strongest, or that evolution aims at progress, at an ever more perfect or complex form. It doesn’t: natural selection acts only on the variation already present in a population at a given moment, it doesn’t design ideal solutions from scratch, and “fit” simply means suited to that particular environment, not superior in any absolute sense.

The phrase itself, “survival of the fittest,” doesn’t originate with Darwin: philosopher Herbert Spencer coined it in 1864, in his work «Principles of Biology». It was Wallace who suggested Darwin adopt it, to reduce confusion around the original term “natural selection”; Darwin only inserted it in the fifth edition of the «Origin», in 1869, meaning it in a relative sense — the fittest for one’s own environment, not the strongest in any absolute way. Spencer, for his part, used it to justify, in his own social thinking, economic laissez-faire and a minimal state: that is the origin of what’s called social Darwinism, a later ideological application, distinct from Darwin’s biological theory, which dealt solely with the natural processes by which species adapt.

When the book comes out, in November 1859, its success is immediate and wider than Darwin himself expected: he says as much in a letter to Charles Lyell a few weeks later, surprised by a reception he hadn’t foreseen. In the decades that follow, the theory establishes itself as biology’s central explanation, backed by lines of evidence Darwin could not even have imagined at the time, from fossils to genetics.