Evolutionary Advantages of Sex

Chris - To help us understand a bit more about the benefit of having sex in the first place is Robert Foley.  He's the director of the Leverhulme Centre for Human Evolutionary Studies in Cambridge.  Thanks for joining us on the Naked Scientists.  Why have sex at all?  Why aren't we just clone ourselves, reproducing ourselves?  Why don't we just give birth to a copy of ourselves and populate the world with millions of clones like greenfly do?

Robert - Well, sex has been around for millions of years and is one of the ways of evolution operating.  We don't fully understand why having two sexes rather than just one is the best thing.  Once you've got it, it's absolutely critical.  We often think of evolution as simply the survival of the fittest - it's how long we live that matters.  It's not.  It's how many offspring you have, it's how many of your genes you can get into the next generation.  For a sexually-reproducing organism such as any animal it's absolutely critical.  Your mating behaviour and how you bring up your young.  So much selection is focussed on getting it right.

Chris - What is the benefit of having two sexes and mixing genes up the way that sex between two genetically-different individuals does?  There have been a number of theories.  Probably the best one is that it really jumbles up the genes in such a way that it partly allows organisms to adapt to new environments.  As the environment changes you're not stuck with something by endlessly re-scrambling it.  You're raising, in a lottery sense, the chances of something better coming which will help you survive.  The other is that probably the ability to resist disease is one of the critical factors in this.  We know that some of the disease associated with resisting disease are ones that evolve very rapidly and benefit from this reshuffling.

Chris - I suppose nature's good at examples of what happens if you do just clone yourself.  We have potato famines in Ireland, we have the loss of the Cavendish banana because of panama disease because bananas are all clones.  I suppose there is a benefit because we're still here.  We haven't succumbed to some horrible disease because we have sex.

Robert - I think both systems - asexual and sexual reproduction - work very well in the right context.  Once you're into very complex organisms, or which humans, primates, monkeys, animals are some.  It probably just would be completely impossible to deal with this asexually.

Chris - What flies in the face of what you're saying is the fact that I chose my wife, I married her and have two children with her.  If I wanted to mix my genes up as much as possible then I wouldn't just stick with one person.  Why do I want to do that?

Robert - It's easier to look across animals and immediately we see there's an enormous variety of ways of reproducing Monogamy is just one.  It's actually very rare one.  Most animals will mate in a very promiscuous way and move on to the next partner.  If we try and ask the question, 'under what circumstances do animals stay with one another?'  It's probably, broadly speaking where the costs of bringing up offspring are very high.  We know with humans we take a long time to grow, we have a large brain.  It's very expensive for the mother and so somewhere  in our evolutionary path there has been a general tendency to increase the amount of parental care.  That means females have to be very choosy and males have to hang around and help bring up the offspring in some way or another.

Chris - It's interesting though because if you look at those voles that Larry Young was talking about earlier in the programme they have very large litters and the costs are not that high because they're just producing so many offspring there's a chance they're going to survive no matter what.  Does that rule apply there?

Robert - There are a number of different reasons why.  The other reason is where a male might not be able to defend an area or defend a number of females.  In that sense he's forced into monogamy.  I think one shouldn't get too fixed ideas about monogamy and other forms.  Actually, in practise most animals are very flexible.  They take the opportunity.  Even birds that we think of as highly monogamous, turns out that something like 15% of their offspring are actually fathered by another bird floating in and taking the opportunities.