In fact, natural selection often leads to ever greater simplicity. And, in many cases, complexity may initially arise when selection is weak or absent.
If you don’t use it, you tend to lose it. Evolution often takes away rather than adding. For instance, cave fish lose their eyes, while parasites like tapewormslose their guts.
Such simplification might be much more widespread than realised. Someapparently primitive creatures are turning out to be the descendants of more complex creatures rather than their ancestors. For instance, it appears the ancestor of brainless starfish and sea urchins had a brain.
Nevertheless, there is no doubt that evolution has produced more complex life-forms over the past four billion years. The tough question is: why? It is usually simply assumed to be the result of natural selection, but recently a few biologists studying our own bizarre and bloated genomes have challenged this idea.
Rather than being driven by selection, they propose that complexity initially arises when selection is weak or absent. How could this be? Suppose an animal has a gene that carries out two different functions. If mutation results in some offspring getting two copies of this gene, these offspring won’t be any fitter as a result. In fact, they might be slightly less fit due to a double dose of the gene. In a large population where the selective pressure is strong, such mutations are likely to be eliminated. In smaller populations, where selective pressure is much weaker, these mutations could spread as a result of random genetic drift (see Natural selection is the only means of evolution) despite being slightly disadvantageous.
The more widely the duplicated genes spread in a population, the faster they will acquire mutations. A mutation in one copy might destroy its ability to carry out the first of the original gene’s two functions. Then the other copy might lose the ability to perform the second of the two functions. As before, these mutations won’t make the animals any fitter – such animals would still look and behave exactly the same – so they will not be selected for, but they could nevertheless spread by genetic drift.
Use your mutations
In this way, a species can go from having one gene with two functions to two genes that each carry out one function. This increase in complexity occurs not because of selection but despite it.
Once the genome is more complex, however, further mutations can make a creature’s body or behaviour more complex. For instance, having two separate genes means each can be switched on or off at different time or in different tissues. As soon as any beneficial mutations arise, natural selection willfavour its spread.
If this picture is correct, it means that there are opposing forces at the heart of evolution. Complex structures and behaviour such as eyes and language are undoubtedly the product of natural selection. But when selection is strong – as in large populations – it blocks the random genomic changes that throw up this greater complexity in the first place.
This idea might even explain why evolution appears to speed up after environmental catastrophes such as asteroid impacts. Such events would slash the population size of species that survive, weakening selection and increasing the chances of greater genomic complexity arising through non-adaptive processes, paving the way for greater physical or behavioural complexity to arise through adaptive processes.