Why have sex?

Why does it take "two to tango" when you need to make babies?
15 February 2016

Interview with 

Professor Laurence Hurst, University of Bath


Graffiti sperm


For most animals, many plants and some fungus, sex is vital if you want to reproduce. But why does it take two to tango? Wouldn't it just be easier, and more efficient, if you could reproduce on your own? This is something of a puzzler to biologists, as Professor of Evolutionary Genetics at the University of Bath, Laurence Hurst, explained to Georgia Mills...

Laurence - If you ask the question "why should any organism have sex?" many people's first response is "well you need it to reproduce don't you?"  And that is true, but when you're thinking about natural selection you also have to think - well what would the alternative be.  And the alternative is what we call asexuality, in that what happens is a female reproduces without the aid of a male and just makes all female progeny. 

Georgia - Key to Darwin's theory of natural selection is survival of the fittest.  If you were slightly worse at doing something like reproducing, you're going to get outcompeted and booted from the evolutionary gene pool.  So when it comes to the two strategies for making babies (sexual and asexual) it seems a no brainer.  In asexual you can do it on your own as a strong independent organism and all of your offspring will be able to too.  For sexual reproduction, you need to actually go out and find a partner and, if you manage to, your offspring are going to be 50/50 male and female so, only half of them will be able to make their own babies.  If it's a game of numbers, sooner or later team asexual should win - right?

Laurence - This nicely captures the central dilemma of the problem of the evolution of sex which is, actually, at first sight it looks as though it's hugely disadvantageous.  So this isn't a small cost that sexual females are suffering, it is a massive cost the sexual females are suffering, but, in the longer term, we see there must be some massive advantage therefore to having sex because it must be able to overcome this immediate disadvantage that you have simply in terms of number of progeny.

Georgia - I was going to say -  do we have any idea of why we're wasting so much time with this?

Laurence - Yes well, as you can imagine, because at first sight this is such an incredible waste of time, at least for females, and so the ideas of largely being dominated by thoughts on two sides.  One is that sex might be a very good strategy to getting rid of bad mutations, and the other is that it's a really good strategy of enabling the spread of adaptive mutations, or advantageous mutations.  But, what both of these have in common is the idea that what is really good about sex is that it generates variety.

Georgia -   When asexual animal make babies, they are essentially clones so, if mummy's bad at maths, all her offspring are bad at maths.  When you reproduce sexually though, you jumble up your DNA with someone else's and this creates variation.  Your children are different from you, and from each other.  So, a couple might be bad at maths but a couple might be very good, especially if you partner was.  This variation is kind of like keeping your eggs in different baskets, and this is a very good idea, especially because we are constantly at war with our parasites.

Laurence - Whatever sort of organism you are, you're always going to have a problem with parasites, and parasites like aids virus, worms, etc; these parasites have a typically reproductive cycle that is faster than ours so they can evolve to us faster than we can evolve to them.  Under these sorts of context, then it actually pays to be sexual because you're trying to get a new set of resistance strategies.  So we think that one of the great advantages of being sexual is that, indeed, it provides some resilience of my progeny to infection.  And some recent work, for example, has shown that, rather nicely, some organisms adjust the amount of sex they have.  They adjust that dependent upon their parasite status; if they're loaded with parasites, if the bug thinks it's got a parasite it actually has more sex than if it thinks it hasn't got a parasite.


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