The Science of Discworld IV

that happen to exist now. The current body-plans are just a contingent, accidental collection that happened to survive. Aliens, even the highest ones, would most likely be very different in design from us, whatever world they evolved on. Including a reboot of ours.
    The old view of the role of genes in Darwinian evolution emphasised mutations: random changes to DNA sequences. However, at least in sexual species, the main source of genetic variability is actually recombination: mix-and-match shuffling of gene variants from the parents. New mutations are not needed to innovate; new combinations of existing genes are sufficient. The diversity of available gene variations can be traced back to much older mutations, but you don’t need a mutation
now
to change an organism.
    All biologists now agree that the body-plans of organisms are not built up piece by piece, mutation by mutation, but have been selected by recombination. Instead of mutations to new genetic variants, we find recombinations of many ancient mutations. These are sorted from kits of compatible parts in every generation, not put together higgledy-piggledy and expected to work. If, as seems plausible, only a few developmental trajectories can lead to larvae that can feed and grow into working adults, compared to the huge number that can’t, then it is to be expected that the successful designs are all separated, without intermediate forms bridging the gaps. ‘Missing links’ need not be missing – or links – because continuous variation is not required in a discontinuous process.
    By looking at so-called
r
-strategists, animals like plaice and oysters whose larvae comprise only a few developmentally competent ones among a majority that aren’t, we can see how this is achieved today. What it does not tell us – what distinguishes the Morris and Gould views – is whether the successful designs are out there in some Platonic organism-space, waiting to be found, or whether the organisms have all invented their own, unpredictable, designs as they went along. Morris, a Christian, believes the former: the appearance of design is the revelation of transcendental attractors in God’s design-space of possible organisms. We, however, believe that there are so many possible ways of being a successful organism, so many effective designs, that the drunkard’s walk of evolution keeps finding them, even though they are sparsely embedded in the vast majority of failures.
    In particular, we think that intelligent design focuses too narrowly on the evolution of
specific
structures found today, such as the precise molecular configuration of haemoglobin or the bacterial motor. In retrospect, these structures seem highly improbable; if nature were to aim for them again, it would almost certainly miss. But evolution selected these structures when it encountered them. What matters is how likely it is that evolution could find
some
such structure, not that
specific
one. If there are many suitable structures, then a process that automatically homes in on anything that seems to be an improvement has a good chance of finding one of them.
    Think how improbable
you
are. If two genomes had not combined just so, if that egg and that sperm had not come together, if your father hadn’t met your mother at the dance, if the wartime bomb in the harbour had hit your grandfather instead of being a hundred metres away, if Napoleon had won the Battle of Waterloo, or if victory had gone the other way in the American War of Independence, if the nascent Earth had not acquired an ocean, or the ripples in the Big Bang had been slightly different … you wouldn’t be here.
    The odds that you exist are infinitesimal.
    No. The odds that you exist are certain,
because you do
.
    The processes that led to you are robust, and at each stage would have led to something similar, albeit different, if run again. No complex process ever produces the same result twice. But if it produces a similar result instead, that makes its consequences certain, not utterly unlikely. Only fine details will be different, second time around. The lottery of life is quite different when seen through the eyes of the eventual winners, rather than those of a random competitor before the contest has happened.
    It’s tempting to assume that the evolution of technology can tell us about organic evolution, or vice versa, but these processes lead to apparent design in very different ways. However, there is a grand overall