Animals evolve over time to become perfectly adapted to their environments, right? Well, not always…
ByANDY DOBSONIllustrations byERIC SMITH
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. 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 (left), 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 patter
