You don’t have to be perfectly adapted to survive, you just have to be as well adapted as your competitors. The apparent perfection of plants and animals may be more a reflection of our poor imaginations than of reality.
It’s a theme repeated endlessly in wildlife documentaries. Again and again we are told how perfectly animals are adapted to their environment. It is, however, seldom true.
Take the UK’s red squirrel. It appeared perfectly well adapted to its environment. Until the grey squirrel arrived, that is, and proved itself rather better adapted to broadleaf forests thanks, in part, to its ability to digest acorns.
There are many reasons why evolution does not produce “designs” that are as good as they could be. Natural selection’s only criterion is that something works, not that it works as well as it might. Botched jobs are common, in fact. The classic example is the panda’s thumb, which it uses to grasp bamboo. “The panda’s true thumb is committed to another role. So the panda must… settle for an enlarged wrist bone and a somewhat clumsy, but quite workable, solution,” wrote Stephen Jay Gould in 1978.
As this example shows, evolution is far more likely to reshape existing structures than to throw up novel ones. The lobed fins of early fish have turned into structures as diverse as wings, fins, hoofs and hands. We have five fingers because our amphibian ancestors had five digits, not because five is necessarily the optimal number of fingers for the human hand.
Many groups simply never evolve features that might have made them even more successful. Sharks lack the gas bladder that allows bony fish to control their buoyancy precisely, for example, and instead have to rely on swimming, buoyant fatty livers and, occasionally, a gulp of air. Similarly, mammals’ two-way lungs are far less efficient than birds’ one-way lungs. And sometimes creatures evolve features that actually reduce their overall fitness rather than increase it, such as the peacock’s tail (see Evolution always increases fitness).
Use it or lose it
Continual mutation also means that if you don’t use it, you lose it. For instance, many primates cannot make vitamin C, because of a gene mutation. This mutation makes no difference to animals that get plenty of vitamin C in their diet. However, when the environment changes, such loss of function can make a big difference, as one primate discovered on long sea voyages.
Evolution’s lack of foresight can produce inherently flawed designs. The vertebrate eye – with its back-to-front wiring and blind spot where the wiring goes through the retina – is one example. Later adaptations have compensated for these problems to a large extent but once natural selection fixes upon a flawed, but workable, design, a species’ descendants are usually stuck with it.
An organism’s fitness is also relative to its environment, which is usually changing. There is a constant arms race going on between predator and prey, parasite and host. Many species have to evolve continuously just to maintain their current level of relative fitness, let alone get fitter. As the Red Queen says in Through the Looking Glass: “It takes all the running you can do, to keep in the same place.”
Humans are not running fast enough. Evolving through natural selection is about time and numbers. The number of new mutations that appear, and the number of chances that natural selection has to eliminate the harmful and favour the beneficial ones, depends on the size of a population, the number of offspring each individual has and on the number of generations, among other things.
Around 10 billion new viral particles can be produced every day in the body of a person infected with HIV. By contrast, the total human population on Earth was no more than a few million until a few thousand years ago.
Furthermore, in a decade bacteria can produce 200,000 generations — about the number of generations of humans there have been since our lineage split from that of chimpanzees. So it’s hardly surprising that in less than a human lifespan we’ve seen the evolution of new diseases such as HIV and numerous antibiotic-resistant bacteria.
Although human evolution has sped up in the past 10,000 years, we are changing our environment faster still. As a result, instead of becoming better adapted we are actually becoming less well adapted to the world we are creating. Think of the huge range of modern afflictions, from obesity and allergies to short-sightedness and drug addiction, we suffer from. Viruses and bacteria might approach perfection, but we humans are at best a very rough first draft.