We write a lot about genetic determinism here on MT because unfortunately it's everywhere, but we probably don't turn enough of our attention to its corollary, the assumed certainty that traits are here because of adaptive natural selection. Everyone knows about 'survival of the fittest' and that therefore traits are here because they served a purpose, often if not usually treated as if a specific purpose, in our evolutionary past. Even those who clearly recognize that selection, when it occurs, is usually highly probabilistic, still talk the talk of determinism as if the adaptationist assumption is latent in their thinking.
|Bushy eyebrows (Darwin's)|
But it's fair to say that these kinds of explanations are usually Just-So stories. Made up since there is no direct evidence for the distant past, and perhaps even plausible, but untestable. We've said enough times to get into trouble that really, the most robust selection story we have is probably that of malaria and sickle cell (and other anemias that are protective), that these traits evolved as protection against malaria around 10,000 years ago. Other stories running close behind are lactose tolerance and skin color -- early humans were all lactose intolerant until various groups domesticated dairy animals and adults began to subsist on dairy products, and skin color lightened as humans moved north because of the need for vitamin D.
But even those stories leak a bit. Remember that for a trait to evolve by natural selection, those with the trait had to have more children than those without. For many many generations. But it's at least a bit forced to argue that lactose intolerance systematically lowers the number of children its carriers have. It may cause occasional or even frequent discomfort, but it's rarely lethal. People report becoming accustomed to it. It is argued that in times of food shortage, adults can gain nutrition by drinking milk. But in times of drought what are the cows drinking? And if there's inadequate food for agricultural humans who held cattle, why would there be cattle food? Wouldn't the grass have died too? Whether they or other arguments are true, the issues are not given very close consideration. So even if there is a lot of good-looking circumstantial evidence, should we really conclude definitively that milk drinking was a strong selective force in the not so distant past?
Skin color lightened because of the need to make vitamin D in northern climes, when sunlight isn't as strong for much of the year? But, we're able to store vitamin D for months at a time, so probably don't need to make it all year round. Plus, estimates of required vitamin D levels differ wildly. Further, darker skinned people tend to have lower vitamin D levels than lighter skinned, on average, yet they also have fewer bone breaks, a marker of bone density and a serious consequence of inadequate vitamin D. Could the link between vitamin D and bone mineralization be more complex than we realize, or could there be an additional mineralization pathway, as yet unidentified?
Even if we were to grant that these issues can be resolved and the adaptive stories are correct, it is important to note that of all our traits, and our many thousands of functional genomic elements, there are precious few stories of such genetic adaptation that have persuasive documentation. This is consistent with a much less deterministic view of adaptation, especially if one looks at the gene level.
Normally, estimates of fitness -- very, very difficult to identify directly even in the present, unless involving human activity like antibiotic or herbicide resistance -- are that the difference between the 'fitness' conferred by the better allele at a gene even under rather strong selective pressure is only about 1%. If continuous, and deterministically systematic, a 1% advantage would indeed lead the 'good' allele to replace the 'bad' one. But the advantage is that if I carry the good one, I have 100 children while my bad-allele-carrying neighbor has a mere 99! This is not even testable in most natural human populations, which were in demes too small.
In our book, The Mermaid's Tale, we described natural selection this way:
Natural selection means the systematic differential reproductive success of competing organisms. The idea is simple: if a species over-reproduces so that not all individuals in the next generation can go on to successfully reproduce, and if there is variation in form among that species, and if some forms of an organism do better in a particular environment than other forms, and if the reason for this is included in their heritable genome, and if the environment remains stable long enough over time for this form to be favored persistently, and if the favorable forms are also lucky enough to produce offspring who go on to reproduce, and if they produce more offspring than their competition, then those forms can become ever more common over time at the expense of their competition. If all these contingencies do occur, indeed co-occur, then the more prolific life form will become more suited—better adapted—to the environment in question. If the forms are sequestered from each other by some mating barrier, then they would diverge over time, and this was the explanation Darwin and Wallace proposed for the origin as well as specialization of species.A force is infinitesimally divisible (and this, a kind of Newtonian-force model, was explicitly Darwin's idea), but there is far too much chance that affects survival and fertility for selection to be that kind of force in nature, at least as a rule.
This reasoning is beyond doubt, and is essentially what Darwin and Wallace were suggesting. But it hinges on the many ifs. Clearly, natural selection is always possible, and often important, sometimes over-ridingly so. At the same time, it has been too easy to assume the ifs. But when the selective differences are small, or highly variable over time, selection is not as much like a systematic force of nature as its usual image. A force is forever, and it has both strength and direction. Instead, and aside from the importance of chance, it is more accurate and realistic to view natural selection as more nuanced, and as only one of many contributing ways in which life’s success is determined.
We are all too enamored of simple explanations. We are happy when we learn that this gene is 'for' that trait, and that trait evolved 'for' this purpose. But that is sloppy thinking that is fundamentally inaccurate, and it is not good science, despite its appeal to the media looking for dramatic stories and simple dog-eat-dog explanations, and despite it being a widespread image of life in many health and life sciences.