Science of Discworld III

each generation to the next. Both the fossil record and his experience with the breeding of new varieties of plants and domestic animals made that fact plain. But breeding is also a choice imposed from outside, by the breeder, so if anything, domestic animals look like evidence in favour of Paley.
    On the other hand, no human agency ever bred dinosaurs. Does that imply that the agency was God – or did the dinosaurs somehow breed themselves into new forms? Darwin realised that there is another kind of ‘choice’, imposed not by intelligent will but by circumstance and context. This is ‘natural selection’. In the vast, ongoing competition for food, living space, and the opportunity to breed, nature will automatically favour winners over losers. Competition introduces a kind of ratchet, which mostly moves in one direction: towards whatever works better. So we should not be surprised that tiny incremental changes from one generation to the next should possess some sort of overall ‘direction’, or dynamic, with changes accumulating coherently across the aeons to produce something entirely different.
    This kind of description is easily misunderstood as a kind of inbuilt tendency towards ‘progress’ – ever onwards, ever upwards. Ever more complex. Many Victorians took the message that the purpose of evolution was to bring humanity into being. We are the highest form of creation, we are at the top of the evolutionary tree. With us, evolution has arrived; it will now stop, having achieved its ultimate goal.
    Rubbish. ‘Works better’ is not an absolute statement. It applies in a context that is itself changing. What works better today might not do so in a million years’ time – or even tomorrow. Maybe for a time, a bird’s beak will ‘work better’ if it is bigger and stronger. If so, that’s how it will change. Not because the birds know what kind of beak will work better: because the kind of beak that works better is thekind that survives more effectively and is therefore more likely to be inherited by succeeding generations. But the results of the competition may change the rules of the game, so that later on, big beaks may become a disadvantage; for instance, suitable food may disappear. So now smaller beaks will win.
    In short, the dynamic of evolution is not prescribed in advance: it is ‘emergent’. It creates its own context, and reacts to that context, as it proceeds. So at any given time we expect to find some sensible directionality to evolutionary change, consistent over many generations, but often the universe itself only finds out what that direction is by exploring what’s possible and discovering what works. Over a longer timescale, the direction itself can change. It’s like a river that flows through an eroding landscape: at any given time there is a clear direction to the flow, but in the long run the passage of the river can slowly change its own course.
    It is also important to appreciate that individual organisms do not compete in isolation, or against a fixed background. Billions of competitions go on all the time, and their outcome may be affected by the results of other competitions. It’s not like the Olympics, where the javelin-throwers politely wait for the marathon-runners to stream past. It’s more like a version of the Olympics where the javelin-throwers try to spear as many marathon-runners as they can, while the steeplechasers are trying to steal their javelins to turn each hurdle into a miniature pole vault, and the marathon-runners’ main aim in life is to drink the water-jump before the steeplechasers get to it and drink it first. This is the Evolympics, where everything happens at once.
    The evolutionary competitions, and their outcomes, also depend on context. Climate, in particular, plays a big role. In the Galápagos, selection for beak size in Darwin’s finches depends on how many birds have what size of beak, and on what kinds of food – seeds, insects, cactus – are available and in what quantities. The amount and type of food depend on which plants and insects are competing best in the struggle to survive – not least from being eaten byfinches – and breed. And all of this is played out against a background of climatic variations: wet or dry summers, wet or dry winters. Observations published in 2002 by Peter and Rosemary Grant show that the main unpredictable feature of finch evolution in the Galápagos is climate. If we could forecast the