Sex spreads good genes faster

A recent study reveals one reason most organisms reproduce sexually, rather than simply making copies of themselves: "good" genes spread more quickly.

Researchers at the University of California in Santa Barbara studied fruit flies.

They had two populations of flies, one that reproduced sexually and one that was modified to reproduce asexually. They tracked a mutation in a gene that codes for eye colour in both populations.

They allowed flies with red eyes to reproduce at a greater rate than flies with white eyes, simulating a trait that gives a fly a 10 per cent better chance to survive.

They found that the "beneficial" red eye trait spread faster in the sexual population than in the asexual population.

Also, in the sexual population, all of the flies eventually had red eyes, meaning the beneficial trait spread completely through the population. In the asexual population, the red eye trait spread but eventually stopped.

The advantage of sexual reproduction over asexual is not immediately obvious. Take, for example, two groups of four organisms, one consisting of four asexual females, and one consisting of two females and two males.

The asexual females can produce four offspring in one reproduction "event," each one an identical copy of its parent. The two couples in the other group can produce only two offspring, each with one half of its DNA coming from each parent.

The asexually producing population will grow faster than the sexually producing one.

Now, consider the situation of the sexual females. They each have an offspring that carries only one half of its genes, while each asexual female has an offspring with all of its genes.

If we assume that the males do not help with the care of the offspring, which is true for most species, the males have passed their genes into the next generation without having to do any work. This is known as the "cost of producing males" and doesn't appear to be a good deal for the sexual females.

Add to that all the effort animals put into finding mates and you can see why scientists wonder why sexual reproduction is so popular.

Since the 19th century, biologists have theorized that the advantage of sexual reproduction is faster adaptation. The authors of this study say this is the first time this has been shown in the lab.

The study appeared in Friday's issue of Science.