Darwin's Theory of Natural Selection

Charles Darwin's landmark 1859 book, 'On the Origin of Species by Means of Natural Selection', proposed a specific, testable mechanism — natural selection — to explain how populations of organisms change, and how new species ultimately arise, over the course of evolutionary time. Darwin developed his theory over more than twenty years, drawing on detailed observations he made of the plants and animals of South America and, especially, the Galápagos Islands during the five-year round-the-world voyage of HMS Beagle (1831-1836), and drawing further inspiration from the English clergyman and economist Thomas Malthus, whose writing on human populations growing faster than their food supply could sustain suggested to Darwin a broader biological principle applicable to every species, not just humans.

Darwin's theory of natural selection can be broken down into a small number of closely interconnected observations and logical inferences. First, individuals within any natural population of a species show variation: no two individual organisms within a population, even close relatives, are ever perfectly identical to one another in every one of their characteristics. Second, every species has the biological capacity to produce far more offspring, in each generation, than the environment's limited resources — food, water, living space, shelter from predators — can possibly support to full adulthood; as a direct consequence, only a fraction of the individuals born or hatched in any given generation actually survive long enough to reproduce successfully themselves, a condition Darwin termed the struggle for existence. Third, precisely because individuals within a population vary, some individuals happen, purely by the chance combination of characteristics they were born with, to be better suited — better adapted — to the particular conditions of their environment than other individuals are, and these better-adapted individuals therefore have, on average, a somewhat higher probability of surviving this struggle for existence and of going on to reproduce successfully. Fourth, since many of the specific variations that make an individual better adapted are heritable — capable of being passed on from parent to offspring — the offspring of individuals that survived and reproduced tend to inherit those same favourable variations, so that favourable variations become progressively more common within the population's gene pool with each successive generation, while unfavourable variations become progressively rarer. This gradual, generation-by-generation shift in the relative frequency of different heritable variations within a population, driven purely by differences in survival and reproductive success, is what Darwin termed natural selection — a process the philosopher and biologist Herbert Spencer later summarised, memorably though somewhat imprecisely, as the 'survival of the fittest'.

The evolution of the peppered moth, Biston betularia, in industrial England during the nineteenth and twentieth centuries is among the most frequently cited and best-documented real-world illustrations of natural selection observed acting within a human, historically recorded timescale, rather than over the vastly longer timescales usually associated with fossil evidence. Before the large-scale industrialisation of England, the great majority of peppered moths recorded were of a pale, speckled-grey colour form, which blended in almost invisibly when resting against the pale, lichen-covered bark of trees typical of the unpolluted countryside of the time, giving this pale form excellent camouflage protection against being spotted and eaten by insect-hunting birds; a rare, dark (melanic) colour form of the same species existed at very low frequency, but stood out conspicuously against the pale bark and so suffered heavier predation. As heavy industrial soot pollution spread across many English regions during the nineteenth century, blackening tree bark and killing the pale lichens that had previously grown on it, the relative advantage of the two colour forms reversed almost completely: the once-rare dark melanic form was now the one that blended in effectively against the newly blackened, soot-covered bark, while the formerly well-camouflaged pale form now stood out conspicuously and suffered much heavier predation by birds. Over the following decades, the recorded frequency of the dark melanic form rose dramatically in heavily industrialised regions, becoming the predominant form in many polluted areas, exactly as Darwin's theory of natural selection, applied to a changing selective environment, would predict — and, notably, as pollution controls later reduced soot pollution in many regions during the twentieth century, the pale form once again became more common, providing a further, reversed demonstration of the same underlying selective process responding to a changing environment. …