Evolution and Complexity
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[edit] Introduction
There is a mistaken perception in the public mind that evolution necessarily entails an increase in complexity. In this article we shall discuss why this is not true.
[edit] The theory of evolution and complexity
The reader familiar with the theory of evolution will readily recognize that there is nothing in the theory which provides a drive to greater complexity as such. On the one hand, we have random variation provided by such mechanisms as mutation, sexual recombination, and lateral gene transfer; and on the other hand we have the law of natural selection, which selects those variations which are best suited to the environment.
The innate direction of evolution is therefore towards adaptation to the environment. If greater complexity is adaptive, more complex variants will be favored; if greater simplicity is adaptive, simplicity will be favored.
Indeed, the effect of natural selection must be in accordance with Einstein's dictum: "Everything should be made as simple as possible, but no simpler". Consider that every feature of an organism has a cost: it has a metabolic cost in producing and maintaining it, and carries a risk of injury and infection. This means that if any feature of an organism is superfluous, natural selection will favor the reduction or removal of this feature: it will dispense with whatever can be dispensed with. So, for example we would expect to find, and do find, that a species which has lived for a long time in complete darkness will have lost its eyes, or that its eyes have become vestigial when compared to related species that don't live in the dark. For more information on this aspect of evolution, see our main articles on Vestigial Structures and Vestigial DNA.
It is plain, then, that the theory of evolution does not predict that a lineage must increase in complexity over the course of its evolutionary history.
[edit] Evolutionary history and complexity
But, the reader may be thinking, surely the history of life does show a broad trend towards complexity. The first simple cells of which we have evidence were much simpler, by any measure, than a hippopotamus or a hummingbird. Ecosystems, too, have become more complex, with greater specialization and diversity of life.
Now, this is quite true. But that does not mean that there is an innate tendency in the mechanisms of evolution to produce complexity: it is just a consequence of the fact that life started off very simple. If life had begun with massively overcomplicated species, each provided with more useless specialized features than a Swiss Army Knife, and if those species had been of such diversity that there were several species competing for the occupancy of every environmental niche, then we should have seen exactly the opposite pattern: the early history of life would have seen a reduction of species diversity, as competing species went extinct, and the surviving species would have undergone simplification of form and function as natural selection whittled away their superfluous features.
To use an analogy, we might liken the history of life is to the history of an orphaned child found naked on a doorstep. We would predict that thirty years later he would have more possessions than he started with: not because there is some law of economics which says that people have to get richer, but simply because he can hardly end up any poorer than he started.
The increase in complexity and diversity (and, indeed, in size) amongst Earth's organisms since the origin of life can therefore be seen as a consequence of the theory of evolution plus the fact that the first life was small, simple, lacking in diversity, and well-suited to only one environmental niche. When you start at the bottom, the only way is up. In the same way, we note that the average species is now larger than the first life, not because there is an innate evolutionary tendency towards increased size, but just because you can't get smaller than the original single-celled organisms.
[edit] Creationist nonsense
The panels to the right are taken from Jack Chick's justly famous creationist cartoon, "Big Daddy". In this scene, the earnest young creationist dismisses part of the excellent evidence for the evolution of whales from land animals on the grounds that loss of their hind limbs is "the opposite of evolution" (he also doesn't know what "vestigial" means, but this is a subject for another article).
In the world of Jack Chick's cartoon, the creationist confutes the biology professor. But there is a reason why this sort of thing never happens in real life. For, as we have discussed, the theory of evolution is all about adaptation to the environment, and may produce either complexity or simplicity as a by-product of this adaptation. In the case of the hind-limbs of whales, the adaptation is clearly adaptive for two reasons.
(1) As we have discussed above, the loss of any useless appendage is adaptive: whales no longer pay the metabolic cost of growing and maintaining hindlimbs, nor can these vanished limbs now be injured or become sites of infection.
(2) The loss of the hindlimbs also produces a positive benefit: they make the whales more streamlined.
The "opposite of evolution", or, to be more precise, the opposite of how the theory of evolution says that evolution should proceed, would be an evolutionary trend towards decreased adaptation to a species' way of life, such as the addition of useless excrescences to whales. This, of course, does not happen.
