The Science of Discworld Revised Edition

stupid and unpleasant political theories.
    Starting from an enormous range of observations of many species of plants and animals, Darwin had become convinced that organisms could change of their own accord, so much so that they could even – over
very
long periods – change so much that they gave rise to new species.
    Imagine a lot of creatures of the same species. They are in competition for resources, such as food – competing with each other, and with animals of other species. Now suppose that by random chance, one or more of these animals has offspring that are
better
at winning the competition. Then those animals are more likely to survive for long enough to produce the next generation, and the next generation is
also
better at winning. In contrast, if one or more of these animals has offspring that are worse at winning the competition, then those animals are less likely to produce a succeeding generation – and even if they somehow do, that next generation is still worse at winning. Clearly even a tiny advantage will, over many generations, lead to a population composed almost entirely of the new high-powered winners. In fact, the effect of any advantage grows like compound interest, so it doesn’t take all that long.
    Natural selection sounds like a very straightforward idea, but words like ‘competition’ and ‘win’ are loaded. It’s easy to get the wrong impression of just how subtle evolution must be. When a baby bird falls out of the nest and gets gobbled up by a passing cat, it is easy to see the battle for survival as being fought between bird and cat. But if
that
is the competition, then cats are clear winners – so why haven’t birds evolved away altogether? Why aren’t there just cats?
    Because cats and birds long ago came, unwittingly, to a mutual accommodation in which
both
can survive. If birds could breed unchecked, there would soon be far too many birds for their food supply to support them. A female starling, for instance, lays about 16 eggs in her life. If they all survived, and this continued, the starling population would multiply by eight every generation – 16 babies for every two parents. Such ‘exponential’ growth is amazingly rapid: by the 70th generation a sphere the size of the solar system would be occupied entirely by starlings (instead of by pigeons, which appears to be its natural destiny).
    The only ‘growth rate’ for the population that works is for each breeding pair of adult starlings to produce, on average, exactly one breeding pair of adult starlings. Replacement, but no more – and no less. Anything more than replacement, and the population explodes; anything less, and it eventually dies out. So of those 16 eggs, an average of 14 must not survive to breed. And that’s where the cat comes in, along with all the other things that make it tough to be a bird, especially a young one. In a way, the cats are doing the birds a favour – collectively, though maybe not as individuals. (It depends if you’re one of the two that survive to breed or the 14 that don’t.)
    Rather more obviously, the birds are doing the cats a favour – cat food literally drops out of the skies, manna from heaven. So what stops it getting out of hand is that if a group of greedy cats happens to evolve somewhere, they rapidly eat themselves out of existence again. The more restrained cats next door survive to breed, and quickly take over the vacated territory. So those cats that eat just enough birds to maintain their food supply will win a competition
against the greedy cats
. Cats and birds aren’t
competing
because they’re not playing the same game. The real competitions are between cats and other cats, and between birds and other birds. This may seem a wasteful process, but it isn’t. A female starling has no trouble laying her 16 eggs. Life is reproductive – it makes reasonably close, though not exact, copies of itself, in quantity, and ‘cheaply’. Evolution can easily ‘try out’ many different possibilities, and discard those that don’t work. And that’s an astonishingly effective way to home in on what
does
work.
    As Huxley said, it’s such an obvious idea. It caused so much trouble from religionists because it takes the gloss off one of their favourite arguments, the argument from design. Living creatures seem so perfectly put together that surely they
must
have been designed – and if so, there must have been a Designer. Darwinism made it clear that a