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and straightforward. It echoes Newton’s view that the laws he discovered were God’s commands to the universe he created. To reconcile Darwinism and theism, we need to give up a literal interpretation of the Old Testament, but we can still accept one essential tenet of the Abrahamic religions (Islam, Christianity, and Judaism): God created us (and everything else) in his own image—that is, according to a design he had in mind. The means he employed to create us could, for all we know, have been the very process Darwin discovered: blind variation and natural selection. This is almost the official Roman Catholic view. The Pope makes a slight fudge for the human soul, which being nonphysical cannot have been so created and was therefore “seeded” into each member of our species separately.

Even without the Catholic fudge, this reconciliation won’t work. It’s an illusion, based on a subtle misunderstanding of Darwin’s theory and its foundations in the second law of thermodynamics.

The process of natural selection is a matter of probabilities in the same way the spread of gas molecules in the upstroke of a bicycle pump is a matter of probabilities, and for the same reason: the process of entropy increase is probable, not certain. This will be no surprise, given the role of the second law in generating evolutionary asymmetries. But even before this role became clear, the nature of evolutionary probabilities was well understood. The theory tells us that fitness differences between organisms will probably lead to differences in their reproductive success. The Darwinian mechanism cannot guarantee the reproductive success of the fitter of two organisms, lineages, or populations. Therefore, it doesn’t guarantee the evolution of adaptation. If it did claim that fitness differences guaranteed reproductive success, Darwin’s theory would be false. Biologists are well aware of several quite rare circumstances under which, just through bad luck, the fittest among competing creatures do not survive at all, let alone have more offspring than the less fit ones.

The best-understood circumstance under which this happens is when populations are small. If there are only a handful of members of a species of varying fitness, then a bolt of lightning, a forest fire, a flash flood, a sudden earthquake, or an infrequent predator may by chance eliminate the fittest among these organisms before it has had the opportunity to convey its fitness-making traits to any offspring.

The small-number problem is of course much more serious in biology than in other domains, such as chemistry, for instance. Biology deals with organisms that number in the dozens or hundreds or thousands, while chemistry deals with molecules that number in the multiples of millions. Where many millions are concerned, the most probable outcome has a probability fantastically close to 1. Where only thousands are concerned, no outcome’s chances ever reach anywhere near 1. When organisms are further divided up into small populations, lineages, and families interbreeding with one another, occupying different environments, subject to occasional environmental shocks that occur irregularly and infrequently, the highest probability outcome of selection is well below 1. In fact, there will often be several alternative outcomes tied for most probable.

Here is an example that should hit especially close to home. About 200,000 years ago, we Homo sapiens were one of a relatively small number of primate species. Apparently, something not very good had been happening to our species’ lineage for the million or so preceding years. Whatever it was that had been happening, by about 200,000 years ago, there were only something like 5,000 Homo sapiens individuals around. This definitely made us an endangered species, with a set of genes and traits that were quite improbable, given the presumably much larger set of genes shared in the hominin population before the bottleneck reduced us to such a small number. Geneticists know this by comparing mitochondrial and Y chromosome sequence variation among extant