Why evolution isn't perfect
Most of the time, evolution seems to do a pretty good job of turning out animals with adaptations that help them survive and thrive. But sometimes it doesn鈥檛 entirely seem to make sense.
One misconception about natural selection is that, over time, evolution 鈥榮elects鈥� the features of an organism that are most perfectly suited to its environment. The may be partly due to the term 鈥榥atural selection鈥� itself, which conjures up parallels with, say, a dog breeder 鈥榮electing鈥� for desirable traits in their animals. In fact, nature isn鈥檛 actually 鈥榮electing鈥� anything鈥攏atural selection is a process, not a conscious force.
There are good reasons why the process of natural selection may not always result in a 鈥榩erfect鈥� solution. Firstly, selection can only act on the . A cheetah, for example, can鈥檛 evolve to run faster if there is no 鈥榝aster鈥� gene variant available.
Secondly, the body has to work with the materials it already has. It can鈥檛 make something out of nothing鈥攖hat鈥檚 why winged horses are the stuff of myth.
Evolution also has to work with the developmental patterns established in distant ancestors, and the results sometimes seem very strange. For instance, you鈥檇 think it鈥檇 make most sense for the nerve that goes from the voice box to the brain in a giraffe to take the most direct route鈥攁 length of around 10 centimetres. But because the giraffe鈥檚 body plan was established in an ancestor that had no neck, the nerve goes all the way down the neck, around the heart and back again鈥攁 distance of four metres!
Then, of course, there are those times that really leave us scratching our heads. Like, what鈥檚 with wings on flightless birds, or eyes in blind snakes? And why do men have nipples? Evolution is all about creatures gradually adapting to their environment, right? And doesn鈥檛 鈥榮urvival of the fittest鈥� mean a move towards better and better adapted creatures? Couldn鈥檛 evolution have come up with a better solution? And why don鈥檛 these oddities simply disappear?
First, it鈥檚 important to recognise that . For instance, some non-adaptive, or even detrimental, gene variants may be on the same DNA strand as a beneficial variant. By hitching a ride on the same DNA strand as the useful variant, a non-adaptive gene can quickly spread throughout a population. In other words, just because a certain trait is there doesn鈥檛 necessarily mean it鈥檚 useful.
In addition, some features may simply be a result of chance, spreading through a population via what鈥檚 known as 鈥樷��. As we鈥檝e seen, DNA in all organisms can be subject to copying errors. Some of these mutations will be harmful, and will probably be eliminated by natural selection. Others, though, will be 鈥榥eutral鈥�: neither harmful nor beneficial. Most of these will die out, but some will spread throughout a population. Although the chance of neutral mutations spreading is very small, genetic drift is nevertheless a significant force, especially in small populations, because of the enormous number of genetic mutations in each generation.
Genetic drift can also result in gene fixation in a population. This occurs when all other possible variations of a gene (alleles) are lost forever, so that only one allele remains available to pass on to future generations. For that particular trait, the lone surviving allele then becomes the only possible variant of that gene.