The Science of Sustainability

The High Cost of Sex

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Sex may have evolved to cut down on genetic variation.

No, I don’t mean in the red light district of Amsterdam or at Mustang Ranch. What I am talking about is the high biological cost of sex. In fact, it is so expensive it can be hard to imagine how it ever evolved in the first place.

The main reason sex is so costly is it takes two parents to have a kid. Asexual creatures can do it on their own.

This doesn’t sound like much of an advantage, but it is. Some computer simulations show that a single asexual individual can overtake a population of one million sexual creatures in about 50 generations. That is about 1000 years for people and only 8 years for mice.

So sex needs to have some pretty big advantages to have ever evolved in the first place. Otherwise the first sexual creature would have been quickly swamped out by all of its asexual brethren as soon as it appeared.

In the past, scientists have pointed to variation as one of sex’s big advantage. This probably isn’t the whole story though. Or even most of it.

Sex does create additional variety through the mixing of genes but it probably isn’t enough to explain the rise of sex. You’d have to live in some pretty chaotic times for this variation to offer enough an advantage to an individual to overcome its cost. Eight or a thousand years just isn’t that long in an evolutionary time scale.

A new review out is bringing up an old idea that Muller came up with back in 1932. The main advantage of sex is to provide a safe time to recombine our DNA.

Recombination is simply the swapping of DNA between two identical (or nearly identical) pieces of DNA. For us that means swapping between the chromosomes we got from mom and dad. So DNA is swapped between our two chromosome 1’s our two chromosome 2’s and so on.

This is where part of that variation we were talking about earlier comes from. But more importantly, recombination actually helps repair DNA damage. You can see the effects of no recombination by looking at our sad little Y chromosome which is slowly disappearing because it has no one to recombine with except itself.

But recombination is a double edged sword. Cells need it to repair their DNA but it can cause lots of DNA damage if it isn’t controlled. For example, even with all of our controls in place, 1 in 1000 humans still ends up with one chromosome stuck to another.

You can see what happens with uncontrolled recombination by looking at cancer cells. These cells end up with extra chromosomes, chromosomes stuck together, and lots of other chromosomal problems because they recombine willy-nilly. They do well for themselves but are definitely bad for the individual.

So it makes sense to contain recombination to some easily controlled time. Sex may have arisen and took over the world because it provides a safer way to keep harmful DNA damage in check. The variation that everyone goes on about may simply have been a beneficial side effect.

After just nine generations, the sexual beasts on the left are already being swamped out by the asexual ones on the right.

See the following for more information:

Muller’s Ratchet
Extra chromosomes and new species

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Category: Biology

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Dr. Barry Starr

About the Author ()

Dr. Barry Starr is a Geneticist-in-Residence at The Tech Museum of Innovation in San Jose, CA and runs their Stanford at The Tech program. The program is part of an ongoing collaboration between the Stanford Department of Genetics and The Tech Museum of Innovation. Together these two partners created the Genetics: Technology with a Twist exhibition. Read his previous contributions to QUEST, a project dedicated to exploring the Science of Sustainability.