We tend to assume that all characteristics of plants and animals are adaptations that have arisen through natural selection. Many are neither adaptations nor the result of selection at all.
Why do so many of us plonk ourselves down in front of the telly with a microwave meal after a tiring day? Because it’s convenient? Or because TV meals are “the natural consequence of hundreds of thousands of years of human evolution“?
Stop laughing. You’ve probably made similar assumptions. For just about every aspect of our bodies and behaviour, it’s easy to invent evolutionary “Just So” stories to explain how they came to be that way. We tend to assume thateverything has a purpose, but often we are wrong.
Take male nipples. Male mammals clearly don’t need them: they have them because females do and because it doesn’t cost much to grow a nipple. So there has been no pressure for the sexes to evolve separate developmental pathways and “switch off” nipple growth in males. Some people claim thefemale orgasm exists for the same reason as male nipples, though this is afar more controversial idea.
Then there’s our sense of smell. Do you find the scent of roses overwhelming or do you struggle to detect it? Can you detect the distinctive odour that most people’s urine acquires after eating asparagus? People vary greatly when it comes to smell, largely due to chance mutations in the genes that code for the smell receptors rather than for adaptive reasons.
Yet other features are the result of selection, but not for the trait in question. For instance, the short stature of pygmies could be a side effect of selection for early childbearing in populations where mortality is high, rather than an adaptation in itself.
Another reason why apparent adaptations can be side effects of selection for other traits is that genes can have different roles at different times of development or in different parts of the body. So selection for one variant can have all sorts of seemingly unrelated effects. Male homosexuality might be linked to gene variants that increase fertility in females, for instance.
A non-adaptive or detrimental gene variant can also spread rapidly through a population if it is on the same DNA strand as a highly beneficial variant. This is one reason why sex matters: when bits of DNA are swapped between chromosomes during sexual reproduction, good and bad variants can be split up.
Other features of plants and animals, such as the wings of ostriches, may once have been adaptations but are no longer needed for their original purpose. Such “vestigial traits” can persist because they are neutral, because they have taken on another function or because there hasn’t been enough evolution to eliminate them even though they have become disadvantageous. Take the appendix. There are plenty of claims that it has this or that function but the evidence is clear: you are more likely to survive without an appendix than with one.
So why hasn’t it disappeared? Because evolution is a numbers game. The worldwide human population was tiny until a few thousand years ago, and people have few children with long periods between each generation. That means fewer chances for evolution to throw up mutations that would reduce the size of the appendix or eliminate it altogether – and fewer chances for those mutations to spread through populations by natural selection. Another possibility is that we are stuck in an evolutionary Catch-22 where, as the appendix shrinks, appendicitis becomes more likely, favouring its retention.
Wisdom teeth are another vestigial remnant. A smaller, weaker jaw allowed our ancestors to grow larger brains, but left less room for molars. Yet many of us still grow teeth for which there is no room, with potentially fatal consequences. One possible reason why wisdom teeth persist is that they usually appear after people reach reproductive age, meaning selection against them is weak.
For all these reasons and more, we need to be sceptical of headline-grabbing claims about evolutionary explanations for different behaviours. Evolutionary psychology in particular is notorious for attempting to explain every aspect ofbehaviour, from gardening to rape, as an adaptation that arose when our ancestors lived on the African savannah.
Needless to say, without solid evidence, claims about how, for instance, TV dinners “evolved” should be taken with a large pinch of salt.