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,
