Darwin's Theory of Natural Selection

Charles Darwin set out his theory of evolution by natural selection in 'On the Origin of Species' (1859), drawing on extensive observations of plants and animals made during his round-the-world voyage. His argument rests on four linked observations. (1) Overproduction — every species produces far more offspring than the environment can support (a Salmon lays roughly 28 million eggs per season; an Elephant, the slowest breeder known, could in principle leave six million descendants within 750 years if none died). (2) Struggle for existence — because resources (food, space, mates) are limited, competition results, which Darwin classified as intraspecific (within the same species), interspecific (between different species), and struggle against the physical environment (drought, flood, climate). (3) Universal occurrence of variation — no two individuals, not even siblings or identical twins, are exactly alike, and useful variations that help an organism cope with this struggle are passed on. (4) Natural selection — nature itself acts as the most powerful selective force, so that in the resulting struggle for existence the fittest organisms survive and leave more offspring ('survival of the fittest'), progressively adapting the population to its environment. Darwinism was later criticised on several counts: it did not explain the underlying mechanism generating variation in the first place; it explained the survival of the fittest but not their arrival; it emphasised small, often non-heritable fluctuating variations; it did not distinguish somatic from germinal variation; and it could not account for vestigial organs or for extreme over-specialised structures such as the tusks of extinct mammoths or the antlers of the extinct I …