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My lab is on the lowest floor of the building. All of the windows are blacked-out, so it never sees the sunlight. In all fairness, it’s in Scotland; we don’t get much sunlight anyway. The lab is kept at a temperature of around 10°C – and, being packed wall-to-wall with aquaria, it always smells faintly of fish. It’s not exactly the most Hollywood rock-star of scientific laboratories.

But don’t be fooled.

It may not look glamorous, but down in my freezing-cold basement lab, I’m hunting demons.

No, I’m not talking about The Exorcist; I’m talking about Darwinian demons: hypothetical organisms that have “nailed” evolution – that have top fitness in every possible situation and can live forever and produce infinite offspring.

Of course, these organisms do not exist. There are many reasons for that: the laws of physics, for one. You can’t be two things at once – you can’t be simultaneously small enough to conserve energy and big enough to win fights, or bright and colourful to attract mates but camouflaged to avoid predators. You can cheat a bit… but when it comes down to it, universal laws will win every time.

So what I study is the ability of organisms to get a bit closer to being a demon. For example, plants that grow different leaves depending on how much light they get; foxes that swap out a winter coat for a summer one when the days get warm; butterflies that switch their wing colours based on whether they hatch in the dry or the wet season. This ability – the trick of altering their form to suit the environment they find themselves in – is called ‘adaptive phenotypic plasticity’. And it carries with it the potential for an organism to be its best self in every situation life throws at it. In other words… to embrace its inner Darwinian demon.

Except that it doesn’t. Because even with adaptive phenotypic plasticity, there aren’t any organisms which are fit in all environments. But why not, if they can adapt to changing conditions? What’s the catch?

That’s where my research comes in. I’m looking for the mechanisms which set the limits to adaptive phenotypic plasticity. And to do this, I’m studying a fish.

At first glance, the threespine stickleback doesn’t seem to have much going for it. It’s small, common, and it’s food for basically everything that encounters it (except humans, unless we get really hungry). It’s the fish for small kids standing in streams to net and put in jam-jars.

It also just so happens to be the perfect animal for this research. That’s because it has a neat little trick: the threespine stickleback can survive in both salt- and fresh-water. That’s pretty impressive for a fish – it basically involves turning the salt-regulating cells inside their gills inside-out. And while the ‘default’ stickleback uses this on a regular basis (coming into freshwater streams to breed, and living in the sea the rest of the time), there are some which have made their ways into lakes, lochs, and reservoirs… and then got stuck.

These fish no longer have to worry about coping with salt; unlike their cousins in the sea, they never experience it. This means that – since threespine stickleback have moved into new freshwater habitats repeatedly at different time points over the last 20,000 years – it’s possible to watch as they become less and less familiar with saltwater environments over time.

If they were Darwinian demons, it wouldn’t matter. They’d never lose their plasticity; they’d still be able to cope with saltwater 40,000 years after the last time their ancestors swam in it.

But they aren’t demons. They do lose the ability to cope with saltwater over time. Even though there seems to be no cost to keeping it, and it would perhaps be useful to hold on to that plasticity, they get slowly worse and worse at surviving in saltwater conditions. Understanding why and how this happens can help us shed light on all sorts of questions: from how we can expect organisms to cope with new environments, to where new traits come from.

It looks like even Darwinian demons have demons of their own.

Illustration is by the author, Helen Spence-Jones. 

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