Origin Ch.3: In which natural selection is explained

This post is part of a series on The Origin Of Species.  It was originally posted on the old blog in feb 2009, during the Darwin 200 celebrations.

Chapter 3 is the struggle for existence. If The Origin had been written a hundred and twenty years later than it was, around about the 1970s, Mary Midgley would no doubt have pointed out that Darwin was confused, and attributing purpose and emotion to the unconscious. I don’t think I need to point out that he wasn’t.

The chapter accounts for a mere fourteen pages of my printing of The Origin, but less than a page has gone by before Darwin has explained natural selection, and solved one of the greatest problems of science. Blink and you’ll miss it. Here it is: “… variations, however slight, and from whatever cause proceeding, if they be in any degree profitable to the individuals of the species, … will tend to the preservation of such individuals, and will generally be inherited by the offspring.” There! Variations, if they aid survival, tend to get passed on. Darwin goes on to say much the same thing, but from another perspective: individuals in a population multiply beyond that which the environment can sustain, leading to differential survival of those individuals carrying the most useful variations. When you know there is variation and unsustainable growth, you know there must be differential survival; and if there is variation and differential survival, you know there must be evolution. No wonder everyone kicked themselves when the idea was first explained — and why school kids still kick themselves, if and when they learn about it.

Darwin doesn’t just happen to define natural selection in this chapter: there are several ideas that we recognise as important in modern biology. For example, the overall theme is abiotic and biotic constraints on population growth. There are all sorts of checks on population growth, and Darwin mentions climate as an example of a purely physical constraint. But he is much more interested in biological constraints — competition for light in plants, mutual relationships between bees and flowers, and predator-prey relationships. I’m not sure he quite gets as far as describing the runaway evolution of arms-races, but he’s clearly on to an important principle here — that interesting things happen when organisms evolve in an environment of interactions with other life.

That’s not the only modern evolutionary idea that Darwin’s thoughts foreshadow in the chapter. Turn the page, and there he is (almost) considering genetic bottlenecks, before he even had genetics. As one of many examples of the struggle for existence, Darwin considers populations which have great growth, followed by a crash, cutting variation. But lets not get carried away. Turn the page once more, and Darwin is thinking in a “for the good of the species” frame — “X must do Y, else the species will become extinct.” One of the ideas that hasn’t survived quite so well into the modern day.

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