Science Interviews

Evolution in the Wild - Horny Soay Sheep

Ben - So what is it you’ve found in the Soay sheep?

Alistair - I should say this is work really led by a colleague of mine, Matt who’s gone to Glastonbury today.  Matt has decided he wanted to take a closer look at how natural selection is really acting on these traits of horn size and growth.  As you said, if you’re a male sheep it’s pretty important to have big horns because when it comes to breeding time you actually have to fight with other males to try and get access to other females.  I guess what he expected and what we all expected was that bigger horns, faster growing horns were going to be better.  It turns out that some of the time that is the case.  There’s also another layer of complexity which is that if you want to grow horns very quickly you’ve got to put a lot of energy into doing that.  The Soay sheep live on an island of St Kilda which is characterised by quite strong variation in the environment.  Every year we hit a time when the food runs out and conditions get really, really tough.  It turns out if you put too much energy into growing horns then you’re very unlikely to get through the winter.  There’s kind of a trade-off here and if you grow your horns too quickly there’s quite a high chance of dying if you hit some bad conditions.

Ben - So the sheep are really taking a gamble then because they know that if their horns are as big as they possibly can be then they’re more likely to have the success in later life but they are betting that their first winter will be quite a mild one?

Alistair - That’s it.  You can kind of think of it as different strategies so if you have a good season coming up then it’s definitely better to have gone for it in terms of growing your horns.  If you get that wrong you might end up dead in which case it doesn’t matter how big your horns would have been because you’re not going to get any breeding success the next season.

Ben - Does this mean the sheep are either born with the potential to have big horns or born with the potential to survive the first winter?

Alistair - It’s a little more complicated than that but we’ve been able to find there’s genetic variation for these traits and that translates to genetic co-variation between horn growth and survival.  What that means is selection is acting at the genotype.  It’s not the case of a complete genetic predetermination between these genes will give you fast-growing horns and these will give you slower growing horns in that categorical way.  There are some genotypes that have a tendency to go in one direction or another and it’s this genetic variation that natural selection can act on.

Ben - We’ve just heard about evolution in the lab and it’s very easy in the lab to control the conditions under which your organisms are evolving.  In the wild obviously you don’t have that option.  You can’t say it’s going to be this temperature this winter.  How do you actually study them?

Alistair - It’s certainly tricky.  Obviously we’ve learnt an awful lot and we continue to learn an awful lot by studying evolution in the lab.  Really what we’re trying to do is tackle those problems head-on in changing environments because if we want to discover how evolution works in the real, natural world then we are going to have to work out how we’re going to change all these environmental variables – as I say temperature or population size or anything.  Fortunately ecologists have been working for decades on exactly what these variables do and exactly how they affect organisms such as Soay sheep.  The challenge we’re facing is perhaps trying to integrate what geneticists can do in the lab with what the ecologists are telling us and to try and integrate these sources of complexity rather than avoid them as you would do in a laboratory.

Ben - Can you take a genetic profile of your population and then try and use that to estimate how well they will cope with say, a bad winter coming up.  Can you actually use the genes to predict the success?

Alistair - Well, I think we may not quite be there yet but that’s I guess what we’re working towards.  We’re trying to get an understanding of how changes in the environment will affect natural selection and also the genetic variation of the different traits on which the selection can act.  I think it’s important if we can start to understand how environmental change in any form can affect those parameters.  Then we can actually start to build changing environments into predictive models on phenotypic evolution:  evolution of traits.

Ben - Ok.  If the climate changes as we are predicting the climate should be warming up would this have an effect?  Would we expect to see bigger horns on these sheep as the weather gets hotter?

Alistair - Certainly a starting prediction might be that if the weather warms up we get fewer and fewer of these bad years.  The net result of that is going to be that selection is always going to favour fast growth in horns rather than sometimes favouring slow growth.  If that’s he case then we’re going to have an increase in, if you like, the net selection for faster growth.  We would expect evolution toward faster growing horns, yes.

Ben - What does this tell us about the way that selection pressures balance each other?  Obviously we have the sexual selection here which is that you need to have big horns to compete with the other males around and to actually breed.  We then have the external natural selection.  Does one drive the other?

Alistair - I guess you can look at this problem in different ways.  For me, I see it all as being different components of the total selection.  The key point is that selection can act in different ways through what we might call different components of fitness.  Whether that be reproduction or survival in your first year versus survival in your second year.  The idea really is, if we can get a handle on how these individuals are behaving throughout their whole lives across a range of environments then we can start to see how selection trades off either across different environments, different ages or through different components like survival and reproduction.  We can bring all of that together to try and have an idea of the total picture.

Ben - So when we say an animal is fit, and we’re looking at survival of the fittest, it could be that they are very fit in one aspect but that would, in turn, make them very unfit in another aspect.

Alistair - That’s it exactly and it can be a real problem.  Sometimes the things you can measure might be survival.  It may be much more difficult in your system to measure the number of eggs produced or something like that.  You can get quite a misleading idea of what selection is doing if you’re only looking at one aspect of fitness at a time.  The challenge is to try to get a whole, complete picture of an individual’s life perhaps – how its fitness is achieved through both surviving and reproducing.

June 2008