How the Elephant Got His Trunk Oct24th2011

Consider the word ‘adaptation’.

Adaptation has a particular meaning in evolutionary circles. A trait is ‘adaptive’ if, when it comes under selection due to some quality of the environment, it conveys a positive advantage to an individual in terms of survival and reproduction compared to other traits in other individuals in the same environment.

Note the use of the present tense.

The adaptiveness of a trait is measured in the present. It reflects a ‘trade-off’ between the costs to an individual of a particular trait and its benefits. If the benefits outweigh the costs then the trait is adaptive. But it is only adaptive in the current circumstances. A change in circumstances might change the balance. If the costs become too high to be worth the benefits then the same trait can quickly become non-adaptive.

What then is an adaptation?

An ‘adaptation’ is any trait that was adaptive and, because of this, was passed on to succeeding generations as a result of differential reproductive success. The assumption is that individuals in possession of adaptive traits will have survived longer and produced more surviving offspring than individuals without those traits. Consequently, over time, those traits will have been reproduced and eventually become fixed in the wider population.

So all fixed traits are adaptations, right?

Not necessarily.

Some evolutionists do believe this. They think all such traits – so-called ‘universals’ – do ultimately represent adaptations and exist because they were once adaptive even if they are not so now. They assume that any trait that was not adaptive would have been quickly weeded out by the unremitting harshness of a continuously selective environment. We call this view the ‘adaptationist paradigm’.

However, other evolutionists disagree. They accept that adaptation is one mechanism of evolutionary change but they doubt that it is the only or even the most important mechanism. One reason for their doubt is that environments are unlikely ever to be continuously selective. That being so, there are times when a high proportion of traits will have a pretty much equal chance of being passed on, even when they confer no reproductive advantage. Moreover, even if one particular trait is under selection, no one trait is passed on alone. Others go with it in the course of reproduction. These are then benefited by the same rules of differential reproductive success as the trait that is under selection. Given enough time, these traits too might come under selection but until they do they have a good chance of becoming fixed in a population without being an adaptation. This non-selectionist view of evolution has its origins in the so-called ‘neutral theory of molecular evolution’ advanced by the Japanese geneticist Motoo Kimura. However, there is a logic to the argument that appeals to some evolutionists at the species and behavioural levels too. The question then arises: how can we tell fixed traits that are adaptations from fixed traits that derive from neutral evolution? The answer is with difficulty.

Allow me to introduce you to the concept of ‘equifinality’.

Equifinality is essentially a restating of the old adage that there is more than one way to skin a cat. Put simply, it means that no matter what you observe, no matter how clear it seems to you that the things you observe ought to be interpreted in a certain way, you can bet your bottom dollar that there are other ways of explaining the data that are equally plausible and can be argued equally convincingly.

This is especially so when the data set is not complete.

It never is in evolution. As a rule, we don’t get to see evolution in progress. We only get to see it in hindsight. This means that we are reliant for our information on bodies of evidence that are patchy and partial. This has the potential to bias our outlook. But it is all we have. Evolutionists of all stripes seldom get to play with a full deck.

What do we do to mitigate this?

We milk the data for all it’s worth. In the field of human evolution there isn’t just one discipline gathering data but several. Each one follows a different set of threads and devises ever more cunning ways to extract information from it, whether it comes from primate studies or the human brain or game-theoretic experiments on economic behaviour or evolutionary genomics or palaeoanthropology or the residues of human activity. We combine the data sets, checking one against the other for correlations and contradictions. We seek as much supporting evidence as we possibly can, destruct testing hypotheses until we are reasonably sure that we have laid to rest the dreaded spectre of equifinality. Then we publish or speak at a conference. This is never the end of it because somebody somewhere will have a valid objection. But sometimes we get lucky and all the evidence seems to be pointing in a single direction. This happened in the case of one particular body of evidence that I suspect you will have heard about if you watched the first episode of BBC 2’s Origins of Us on the 17th October.

I refer to a suite of anatomical traits that appears to have co-evolved in the human lineage over the course of about four million years. It turned our ancestors from bipedal hominins into increasingly specialised distance runners. By around two million years ago, when the African version of Homo erectus (Homo ergaster) first made an appearance, this suite of traits had already become fixed in the human phenotype. From that time on, all humans were committed to upright posture, long, straight hind limbs, flexible waists and, some say, lack of body hair. At the same time they were committed to weak backs, arthritic knees, narrow hips that restricted the birth canal and serious disability if they broke a leg. It is these high costs, combined with very persuasive experimental evidence that shows these traits acting in concert, that allow us to state with some certainty that what we see is not an accident brought about by neutral evolution but a genuine, bona fide adaptation. So that’s it, isn’t it? Case closed. The human lineage is adapted to run.

Well, not quite.

There is also the issue of why we are adapted to run. To put it another way, we have to ask what it was about distance running that was so beneficial in terms of survival and reproduction that it was worth the high costs noted above. What made running such a killer app?

This is where the problem of equifinality raises its head again. The prevailing hypothesis is that running enabled our ancestors to escape from danger and, perhaps more importantly, to exploit the vacant niche of the savannah at noon and get a head start on competing meat eaters. It sounds good. It could even be true. But the sad fact is we don’t know that it’s true. However plausible, it has still to be verified. Crucially, we need to be able to demonstrate (a) that our ancestors did exploit this niche, and (b) that the benefits were worth the costs. People are working on this but until they get there it remains a Just So Story.

Hence the title of this piece.

To find out more about the perils of equifinality read my next post: The Salutary Story of Raymond Dart.

P.S. I know the real name of the title story is The Elephant’s Child.

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