Why Do Plants And Animals Need To Have Sex ?

13 February 2005
Posted by Chris Smith.

Steve - One thing about sex is that it is extremely expensive. It is particularly expensive for the female sex, as they do a very generous job of copying the male genes. Why do they do it? We know that a very tiny difference in the ability to copy your own genes can have dramatic effects.

So why do the females go into this altruistic business of copying somebody else's DNA ? The answer is we don't really know. There are a number of theories: one which is really popular is that it's like a Red Queen's race, which is like running up an escalator. Once somebody has started having sex, you've got no choice but to join in. If that somebody is a parasite or disease, once it takes up sex, it can reshuffle its genes very quickly. This is what sex is all about: recombination. If it can reshuffle its genes, then you as a host need to do the same thing otherwise it ill overwhelm your genetic defences. Therefore, we have a type of arms race going on. This may be as much an argument for the maintenance of sex as it is for its origin.

There's an interesting parallel between sex and DNA repair. DNA is actually damaged all the time. Many people think that mutations are rare and extraordinary things that hardly ever happen, but it isn't true. Even by the time people have finished listening to me talking, they will not be the same people. They will have undergone lots of changes and mutations; many of which will be very harmful. DNA is a very complicated chemical. To a chemist it seems almost impossible that it can maintain itself. Biology has come up with a very complicated system of repairing this damage with repair enzymes. A lot of this repair takes place when sex happens. Sex involves cutting and splicing DNA into new combinations and a lot of the mutational damage is fixed then. That may well have been how sex began.

There's an interesting spin on that, which is most the damage is actually done by males, because males make small sperm all the time. At each division, there is room for error, so most mutations happen in the male line. Females actually do a lot more DNA repair than males do, so it's the males' fault and the women put it right.

Chris - If we turned the clock back in time, were there males and females then, or was it something that came later?

Steve - I wasn't there at the time and it's fatally easy to speculate about what happened long ago, but it's very clear that you can imagine a universe that is entirely female. Indeed, there are plenty of species today that are entirely female. You can't imagine a universe that was entirely male as it's not one that would last for more than one generation. So I think it's pretty clear that for much of life, males weren't around. Thank god we made it!

Chris - But you don't think we'll hang around?

Steve - The way we're made could change, but once we're here, we're embarrassingly hard to get rid of. There are plenty of creatures out there, especially in the plant world, which historically had two sexes and have gone on to only having one. What's interesting is that in nearly all cases, those lineages without males are at the tips of the branches of the evolutionary tree. Once you give up males, you lose your ability to adapt and evolve quickly. Maybe our job in life is to help females to evolve.

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