Natural Selection: Does Evolution Pull or Push?

24/07/2015 11:22 BST | Updated 23/07/2016 10:59 BST

One evolutionary concept everyone gets wrong sometimes:

When we first start thinking about evolution, most of us are prone to a temptation when we try to explain biological traits.

By this stage, we've already shaken off the notion that the appearance of design implies a designer. But there's another problem. In casual conversation or even academic discussion, we find it almost impossible to stop talking as if specific traits evolved for specific purposes.

After all, when we look at a human eye, it seems clear that its eyelid and blink reflex adapted to protect the organ beneath. This type of explanation seems helpful when we're confronted with some tough questions:

Jesse: Why did the whale and the shark evolve similar 'torpedo' body plans if they're on separate branches of the evolutionary tree?

Lish: Well, the shark and the whale share similar environmental pressures underwater, and torpedo body plans are extremely hydrodynamic (think of submarines). Their bodies adapted to help them cut through the water easily.

But there are a few problems with only using this adaptation-centred approach to explaining biological traits.

First, the explanations are all ad hoc hypotheses - an adjustment to a theory aiming to explain an apparent problem. These are generally worrisome in science since you can't show them to be false. Note that this doesn't mean that they are necessarily false. But we should be careful about using them this way, since we run the risk of reducing natural selection's explanatory power to series of unsupported claims tagged onto the end of the theory. (Darwin would judge you).

Geneticist Richard Lewontin likened this approach to Just So Stories, named after Rudyard Kipling's collection of children's tales fancifully explaining cases such as how the leopard got its spots.

Secondly, consider that an adaptive trait is an evolved trait which currently has a role in the life of an organism. Take feathers on modern birds, for example. When we see modern buzzards soaring so easily on thermals, it's attractive to assume feathers adapted for flight. But we know that early feathers almost certainly aided in regulating temperature during the reptilian-to-avian transition (cold-blooded to warm blooded, crudely speaking).

But, in attempting to explain buzzards' adaptation of feathers only in terms of their current, adaptive role, we lose a crucial part of the explanation.

Gould suggests we think of it way. A spandrel is an architectural feature caused by placing a dome on top of arched columns. In ancient places of worship, these would often get decorated with frescoes depicting the gods looking down from the heavens. Using the 'Just So' approach, we might conclude that spandrels were built-in specifically as places to paint these. But we'd ignore the rest of the story: they were a feature of the geometry and simply turned out to be a pretty neat place for some beautiful paintings.

Concluding something similar, a friend recently asked, "Why are mammals' brains at the top of their body? It seems like a pretty dangerous place for it to evolve, being so exposed to possible injury". They'd hypothesised ad hoc that there must be some beneficial reason an organism would evolve a top-mounted brain that outweighed the potential risks.

But, as Dennett et al often remind us, natural selection flies blind rather than pulls from above. It can't foresee some imagined "ideal trait" ad hoc. It can only proceed by selecting potential novel mutations from pre-existing type. This often leads to apparent "engineering" mishaps.

Whilst Gould's analogy is abstract, we can use it to fix the attractive assumption underlying my friend's question. Consider the following alternative. The forward-facing brain can almost certainly be traced back to the development of neural nets in all extremely simple organisms with bilaterian (symmetrical) body plans.

The kinds of simple electro-signalling cells that would have comprised these crude nervous systems tend to move towards their stimuli (light, chemicals etc.). Gradually, over millions of years, adaptations developed around these sensory "centres" according to environmental pressures. Much more recently, in the case of humans, our ancestors developed bipedal gait, resulting in the "brain at the top".

This explanation zooms out from the specific trait and accounts for its adaptation in the context of the organism. We can see that mammals' brains didn't adapt to be at the top. Structure adapted with and from crude sensory cells tending towards stimuli, resulting in a particular body plan.

We might use this slogan: natural selection pushes adaptations from behind blindfolded. The above allows us to see that adaptive explanation an important part of a trait's story. But we shouldn't assume that it can explain the entire adaptation as it figures into the whole organism. When we do, we often start talking as if the blindfold's off.