We know how evolution works. An individual’s physical and behavioural characteristics (or ‘traits’) are determined – at least in part – by their genes, meaning that they are heritable.
New trait variants arise by chance from random mutations in those genes, such that no two individuals are exactly alike. Because there is competition for resources, including mates, those with the most favourable traits will leave the greatest number of viable offspring. In evolutionary language, they will have been ‘selected’.
By this deceptively simple mechanism, a lineage of animals (or plants, or any other life form) gradually acquires the traits that best prepare it for survival in its environment. This has been going on for at least 3.5 billion years, and the result is an explosively diverse assemblage of species that appear perfectly adapted to their own individual ways of life.
Notice the word ‘appear’, however, because all is not quite as it might seem. In reality, natural selection never achieves perfection, frequently seems to be asleep at the wheel, and sometimes even pushes in the opposite direction.
Does evolution ever get it wrong?
Below, I introduce some surprising truths about evolution that don’t tend to make the TV documentaries.
There are often some pretty fundamental problems in an animal’s life that natural selection simply cannot fix. Consider a gannet approaching the enormous breeding colony at Bass Rock, off the south-eastern coast of Scotland.
Somehow, it will pick out its own chick from amongst the teeming thousands of other gannet chicks, all of which look and sound – to our senses, at least – identical. Whatever mechanism they use for this feat of identification, it’s one that’s been magnificently honed by natural selection.
But contrast this image with that of a reed warbler perching on the back of a hulking great cuckoo chick, dutifully shoving insects into the maw of a monster many times its own weight that looks precisely nothing like a warbler chick. How could this bird be so comprehensively duped?
If gannets can tell their own offspring from the 2,000 others in the same featureless hectare of rock, why can’t a reed warbler see that this screaming behemoth is absolutely no relation?
The mystery only deepens when you realise that reed warblers apparently can recognise cuckoo eggs. Indeed, cuckoos have had to evolve patterned eggshells to try and fool the warblers, who have responded in turn with ever more complex patterning, resulting in a classic evolutionary ‘arms race’ of adaptation and counter-adaptation. Why then have the warblers not evolved the ability to recognise cuckoo chicks?
What is imprinting?
The answer probably involves the way that birds recognise members of their own species, and in particular their own family, a process known as imprinting. During a brief window of development, young birds come to identify strongly with whatever animals they see most often. Usually this will be their parents, but they will just as easily imprint on humans if hand-reared from the right age.
Much the same thing happens in reverse for parent birds, who come to recognise their own chicks by imprinting on whatever they find in their nests. This is very efficient, and much better than having an innate ‘picture’ of what chicks ought to look like, because it means that chicks can evolve a different appearance (eg for camouflage), without the parents needing to miraculously evolve a new ‘picture’ at exactly the same time.
Gannets are undoubtedly relying on imprinting, too, and it works well for them. It would work for warblers but only – and this is key – if they can guarantee that the first chick they see will be a warbler chick.
If, instead, a warbler’s first clutch is visited by a cuckoo, then the hapless parent will imprint upon that, and for every year thereafter will abandon a nest of its own chicks, because they don’t look right.
By the dry arithmetic of natural selection, so long as cuckoos remain relatively uncommon, it’s better to have no idea what your chick ought to look like, and waste the odd breeding season rearing a cuckoo, than to risk never rearing any chicks at all because you’ve imprinted on the wrong one. The gannet runs no such risk, for the simple reason that cuckoos do not lay their eggs in gannet nests. For the warbler, alas, there is no good solution.
While we don’t always think of them this way, cuckoos are parasites, taking something from their hosts without giving anything in return (this behaviour earned them a place on our laziest animals in the world list) .
Cuckoos are unusual among birds, very few of which are parasitic, but across the whole animal kingdom, parasitism is undoubtedly the most common way to make a living. Almost all non-parasitic species (and plenty of parasitic ones) have their own parasite species, and some will have hundreds. Humans have parasites, cats and dogs have parasites, spiders and bees have parasites – even bacteria have them.
These days – in richer countries, at least – we are relatively untroubled by parasites, but for most of our history we were afflicted by untold hordes of worms, fleas, lice and other miscellaneous freeloaders. Given how unpleasant they are, it is perhaps surprising that natural selection has not endowed us with immune systems capable of defeating them on first contact.
To understand why this hasn’t happened, think of natural selection as a filter, letting only the best adapted individuals through. The filter’s mesh can vary in size, depending on the importance of the characteristic being filtered; a wide mesh lets most individuals through, while a narrow one only lets the very best through.
If you’re a wolf in Alaska, there’s a very finely meshed filter for the thickness of your fur, because if yours is thin, you aren’t going to survive long enough to reproduce. For the colour of your fur, however, the mesh is much larger, since it doesn’t have much impact on your future reproductive success; as long as it’s not bright orange or pink, it’ll be perfectly alright.
As a result, Alaskan wolves all have very thick fur, but the colour of it varies quite a bit. In technical language we’d say that there is a strong selection pressure on coat thickness in wolves, but only weak selection pressure on coat colour.
Now think about something rather less fluffy – a tapeworm living inside your intestines. For you, the tapeworm is an irritation, taking a small measure of food away from you, and perhaps giving occasional abdominal pain.
There is therefore a selection pressure for humans to evolve a defence against tapeworms, but not a particularly strong one. Things are very different for the tapeworm; if your immune system is able to kill them, the result is catastrophic, so the filter is extremely narrow, only allowing the most resilient individuals through. The selection pressure to evade destruction is therefore massive, and because of this imbalance in the consequences of failure, tapeworms will always tend to stay one step ahead of our immune systems.
Much the same asymmetry explains why even the most specialised predators usually fail on any given hunt. The Greek storyteller Aesop had it exactly right when he said, “The rabbit runs faster than the fox because the rabbit is running for its life while the fox is only running for its dinner.” With higher stakes for the rabbit, the selection pressure is more severe, and the balance of power – paradoxically – lies with the prey.
Can evolution get it wrong?
The idea that species can evolve to become ‘worse’ over time is counter-intuitive, but the phenomenon is not especially unusual.
Small fish called swordtails are a great example. In each of the several species, male swordtails have a greatly elongated lower part of their tail fin (hence the name), which makes them very pretty, but also makes them demonstrably worse at swimming than they would be without it.
As some (ethically dubious) experiments have demonstrated, the sword means that males need to maintain a 19 per cent higher metabolism to swim at any given speed than they would do without it. What’s more, swordless males can manage a top speed 30 per cent faster (I’ll leave you to guess how this was discovered).
So, if the sword is a clear impediment to the daily business of being a fish, why has natural selection allowed it? The answer lies in mate choice. At some point in their evolutionary history, female swordtails evolved an arbitrary preference for males with slightly longer tails than the average.
We can’t know how this happened, but a good guess is that small fish are already sensitive to lateral patterns in the environment; they will often indicate the presence of another fish, which might well be a larger one that wants to eat you. A male swordtail that happens to have a slightly longer tail than his fellows may be slightly more conspicuous to females.
If all other things are equal, being more obvious to females could be a real advantage for a male, so the longer-tailed males achieved better reproductive success, and hence became more common. Females with the arbitrary preference also enjoyed better long-term reproductive success, because their own sons inherited the longer tails and hence were more attractive. Over time, however, a tail needed to be longer and longer in order to stand out, and a process called ‘runaway selection’ set in.
Fast-forward a few thousand generations, and males now have tails so long that their survival is negatively affected. It’s too late, however, to stop the backslide – the problem is locked in. A male with a ‘normal’ tail will definitely enjoy greater survival prospects, but he won’t pass on those fast-swimming genes because he’ll never attract a mate.
This version of natural selection, wherein reproductive success is controlled not by the environment, but by mate choice, is called sexual selection, and it frequently leads to a situation where individuals alive today are actually worse at surviving than their direct ancestors would have been.
Natural selection has indisputably led to the wonders of the natural world, but it doesn’t have any purpose or foresight, and it cannot solve all of a species’ problems. Alongside the evolutionary masterpieces, such as bioluminescent octopuses, are some obvious failures – like birds that don’t know what their own chicks look like, and fish that evolve to be worse at swimming even though their survival depends on it.
However, we must never regret nature’s lack of perfection. The whole project of evolved life occurred – and continues to occur – because of mistakes made in the copying of DNA molecules. Imperfection is the very stuff of life – without it there’d be nothing but dust.