Read an extended version of this article, and many more evolution myths, in our online special
Use it or lose it. That old adage applies to evolution as well as everyday life, and explains why cave fish are eyeless and parasitic tapeworms gutless.
Until recently, such examples were considered the exception, but it seems we may have seriously underestimated the extent to which evolution likes to simplify matters. There are entire groups of apparently primitive creatures that are turning out to be the descendants of more complex organisms. For instance, the ancestor of brainless starfish and sea urchins had a brain; why their descendants dispensed with a brain is still unclear.
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Despite this, there is no doubt that evolution has produced ever more complex life forms over the past four billion years. This is usually assumed to be the result of natural selection, but recently some biologists studying our bizarre and bloated genomes have turned this idea on its head. They propose that, initially at least, complexity arises when selection pressure is weak or absent. How could this be?
Suppose an animal has a gene with two different functions. As a result of mutation some offspring may get two copies of this gene. In a large population where competition is fierce and selection pressure strong, such mutations are likely to be eliminated because they do not increase an individual鈥檚 fitness and are probably slightly disadvantageous.
In smaller populations where selection pressure is weak, however, these mutations have a small chance of surviving and spreading as a result of random genetic drift (see 鈥淣atural selection is the only means of evolution鈥�). If this happens, the duplicate genes will start to acquire mutations of their own. A mutation in one copy might destroy its ability to carry out the first of the original gene鈥檚 two functions, while the other copy might lose the ability to perform the second function. Again these changes don鈥檛 confer any advantage 鈥� such animals would still look and behave exactly the same 鈥� but these mutations might also spread by genetic drift. So the population would have gone from having one gene with two functions to two genes with one function each.
This increase in genomic complexity would have occurred not because of selection pressure but despite it. Yet it can be the foundation of greater physical or behavioural complexity because each gene can now evolve independently. For example, either can be switched on or off at different times or in different tissues. And as soon as any beneficial mutations arise, natural selection will kick in.
It seems there are opposing pressures at the heart of evolution: while complex structures and behaviours, such as eyes and language, are undoubtedly the product of natural selection, strong selection 鈥� as in large populations 鈥� blocks the random genomic changes that can throw up greater complexity in the first place.
Read an extended version of this article, and many more evolution myths, in our online special
