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remain intact, shuffled about from individual body to individual body as the generations go by, but not blending with one another, never contaminating each other. At any one time, the genes are all sitting in the bodies of individual rats, or they are moving into new rat bodies via sperms. But if we take a long view across many generations, we see all the rat genes on the island being mixed up as though they were cards in a single well-shuffled pack: one single pool of genes.
    I’m guessing that the rat gene pool on a small and isolated island such as Ascension is a self-contained and rather well-stirred pool, in the sense that the recent ancestors of any one rat could have lived anywhere on the island, but probably not anywhere other than on the island, give or take the occasional stowaway on a ship. But the gene pool of the rats on a large land mass such as Eurasia would be much more complicated. A rat living in Madrid would derive most of its genes from ancestors living in the western end of the Eurasian continent rather than, say, Mongolia or Siberia, not because of specific barriers to gene flow (though those exist too) but because of the sheer distances involved. It takes time for sexual shuffling to work a gene from one side of a continent to the other. Even if there are no physical barriers such as rivers or mountain ranges, gene flow across such a large land mass will still be slow enough for the gene pool to deserve the name ‘viscous’. A rat living in Vladivostok would trace most of its genes back to ancestors in the east. The Eurasian gene pool would be shuffled, as on Ascension Island, but not homogeneously shuffled because of the distances involved. Moreover, partial barriers such as mountain ranges, large rivers or deserts would further get in the way of homogeneous shuffling, thereby structuring and complicating the gene pool. These complications don’t devalue the idea of the gene pool. The perfectly stirred gene pool is a useful abstraction, like a mathematician’s abstraction of a perfect straight line. Real gene pools, even on small islands like Ascension, are imperfect approximations, only partially shuffled. The smaller and less broken-up the island, the better the approximation to the abstract ideal of the perfectly stirred gene pool.
    
    
    Just to round off the thought about gene pools, each individual animal that we see in a population is a sampling of the gene pool of its time (or rather its parents’ time). There is no intrinsic tendency in gene pools for particular genes to increase or decrease in frequency. But when there is a systematic increase or decrease in the frequency with which we see a particular gene in a gene pool, that is precisely and exactly what is meant by evolution. The question, therefore, becomes: why should there be a systematic increase or decrease in a gene’s frequency? That, of course, is where things start to get interesting, and we shall come to it in due course.
    
    
    Something funny happens to the gene pools of domestic dogs. Breeders of pedigree Pekineses or Dalmatians go to elaborate lengths to stop genes crossing from one gene pool to another. Stud books are kept, going back many generations, and miscegenation is the worst thing that can happen in the book of a pedigree breeder. It is as though each breed of dog were incarcerated on its own little Ascension Island, kept apart from every other breed. But the barrier to interbreeding is not blue water but human rules. Geographically the breeds all overlap, but they might as well be on separate islands because of the way their owners police their mating opportunities. Of course, from time to time the rules are broken. Like a rat stowing away on a ship to Ascension Island, a whippet bitch, say, escapes the leash and mates with a spaniel. But the mongrel puppies that result, however loved they may be as individuals, are cast off the island labelled Pedigree Whippet. The island itself remains a pure whippet island.