Science of Discworld III

justify each ingredient.
    Even though different species seem to stay pretty much the same – think lions, tigers, elephants, hippos, whatever – it is actually rather obvious that, in general, species are not fixed for all time. The changes are relatively slow, which is why we don’t notice them. Butthey do happen. We’ve already seen that in Darwin’s finches, evolutionary changes can be and have been observed on a timescale of years, and in bacteria they occur on a timescale of days.
    The most obvious evidence for the variability of species, in Darwin’s day and ours, was the domestication of animals – sheep, cows, pigs, chickens, dogs, cats …
    … and pigeons. Darwin was rather knowledgeable about pigeons, he belonged to two London pigeon clubs. Every pigeon-fancier knows that by selectively breeding particular combinations of male and female pigeons, it is possible to produce ‘varieties’ of pigeons with particular characteristics. ‘The diversity of the breeds is something astonishing,’ says Darwin in the first chapter of Origin . The English carrier pigeon has a wide mouth, large nostrils, elongated eyelids, a long beak. The short-faced tumbler has a short stubby beak like a finch. The common tumbler flies high up in a tightly knit flock, and has an odd habit of falling about in the sky, whence its name. The runt (despite its name) is huge, with a long beak and large feet. The barb is like the carrier but with a short, broad beak. The pouter has an inflatable crop and can puff out its chest. The turbit has a short beak and a line of reversed feathers on its chest. The Jacobin has so many reversed feathers on its neck that they form a hood. Then there are the trumpeter, laugher, fantail … These are not separate species: they can interbreed, to produce viable ‘hybrids’ – cross-breeds.
    The enormous variety of dogs is so well known that we don’t even need to mention examples. It’s not that the dog species is exceptionally malleable, just that dog-breeders have been unusually active and imaginative. There is a dog for every purpose that a dog can carry out. Again, they’re all dogs , not new (albeit related) species. They can mostly (barring really big size differences) interbreed, and artificial insemination can take care of mere size. Dog sperm plus dog egg makes fertile dog zygote, and, eventually, dog – independently of breed. That’s why pedigree pooches need a pedigree, toguarantee that their parentage is pure. If the different varieties of dog were different species, that wouldn’t be necessary.
    In modern times, it has become clear that cats are just as malleable, but the cat-breeders have only just got going on exotic cats. The same goes for cows, pigs, goats, sheep … and what about flowers? The number of varieties of garden flowers is immense.
    By avoiding the creation of hybrids, the breeder can maintain the individual varieties over many generations. Pouter pigeons breed with pouters to produce (a substantial proportion of) pouters. Carriers mated with carriers produce (mostly) carriers. The underlying genetics, about which Darwin and his contemporaries knew nothing, is complicated enough that apparent hybrids can sometimes arise from what seems to be pure stock, just as two brown-eyed parents can nonetheless have a blue-eyed child. So pigeon-breeders have to eliminate the hybrids.
    The existence of these cross-bred varieties does not, of itself, explain how new species can arise of their own accord. Varieties are not species; moreover, the guiding hand of the breeder is evident. But varieties do make it clear that there must be plenty of variability within a species. In fact, the variability is so great that one can readily imagine selective breeding leading to entirely new species, given enough time. And the avoidance of hybrids can maintain varieties from one generation to the next, so their characters (biologese for the features that distinguish them) are heritable (biologese for ‘able to be passed from one generation to the next’). So Darwin has his first ingredient: heritable variability.
    The next ingredient was easier (though still controversial in some quarters). It was time. Oodles and oodles of time, the Deep Time of geologists. Not a few thousand years, but millions, tens of millions … billions, in fact, though that was further than the Victorians were willing to go. Deep Time, as we’ve previously observed, is contrary to the biblical chronology of