As this is a scientific journal, I imagine its readers will have more than a passing interest in Darwin. It is hardly surprising. Darwinism is a scientific hypothesis of such revelatory brilliance, of such simplicity and such reach, of such sheer explanatory power that it is difficult to remain unmoved by it. That it also, in a revolution of Copernican dimensions, knocked  homo sapiens from the centre – or from the summit – of a divinely ordered universe means that it has also helped define our epoch, the epoch of scientific materialism. Darwin’s intention, of course, was that his theory would account for the origin of species, and this it quite spectacularly did. Few now doubt that speciation is driven by the survival advantages of naturally occurring variations in living organisms. Such has been the success of Darwinian theory, however, that there are few areas of human experience in which its proponents have not sought to wield its explanatory lance. Two of Darwin’s current cheerleaders, Richard Dawkins and Daniel Dennett, seem almost to find within it a theory of everything – as if there is no dimension of our experience that cannot in the end be explained by it.

Being the married father of two young children I am sometimes given to reflecting on sexual morality. Reproductive behaviour, and therefore, by extension, reproductive ethics, is presumably of deep interest to Darwinians, it being the arena in which our genetic material goes about getting itself transmitted. To echo Richard Dawkin’s arresting phrase, it is presumably in bed that our genes are at their most selfish, even where our whispered words are all aflame with our beloved.

Among the many ticklish dilemmas of the reproductive life is the question of whether we should remain loyal to one or whether we should struggle to spread our genetic data more widely, whether we are being more true to the species by being faithful or faithless. (Consider the old Rabbinic joke: Moses comes down from Mount Sinai clutching the ten commandments. The good news, he says to the amassed Israelites, is that I got him down to ten. The bad news is that adultery is still in.)

Is this a dilemma that evolutionary theory can help us with? Presumably for a Darwinian – at least a Dawkinsian Darwinian, if that doesn’t sound too awkward – the question that must be asked of any activity is whether or not it is likely to lead to the maximal propagation of our genetic material, that being, after all, what we are programmed to do, irrespective of our own feelings on the matter. One of the difficulties here is that evolutionary theory can presumably come down on both sides of this dilemma. For slow-maturing primates such as ourselves, monogamy might just tie me into a family long enough to hold my offspring together until they reach sexual maturity. But promiscuity is presumably another reasonably effective way of spreading my genetic data around, passion no doubt making as many babies as careful family planning. It would seem then that evolutionary theory can give a plausible account of the two horns of the dilemma. It is far less clear that it can provide much help to anyone who might be wrestling with it. Something else is required, and that something looks very much like choice.

So here is a problem. If Darwinism can account for both sides of a dilemma, can it also account for the decision we make when confronted by it? According to contemporary Darwinians, human beings are driven by the same gene-replicating urgencies as animals. It is not clear to me, however, that animals actually recognise moral dilemmas. These seem to be reserved for human beings. Although there are times when we will respond instinctively in the face of a dilemma – oaths are straw to the fire in the blood  – there are other occasions when we will stand back and think. And at that point we are doing something different, something identifiably human, and something that cannot, without distortion, be reduced to the fires in our blood: we are engaging in something like moral reflection.

It is tempting to go further: the facts of our condition – that we are embodied creatures capable of reflecting and evaluating – means that we are born, if not quite, pace Job, to trouble as the sparks fly upward, then at least to the capacity to struggle with ourselves. To the best of my knowledge only human beings have this ability internally to contend with conflicting goods. It does not follow, however, that because moral reflection emerges from the conditions of life that it can be reduced to them. Darwinism wonderfully explains the emergence of the human animal from earlier primates. But it is a crude tool with which to make sense of the complexities of our moral lives.

Julian Sheather is ethics manager, BMA. The views he expresses in his blog posts are entirely his own.