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Sex Speeds Up Evolution, Study Finds

Stefan Lovgren
for National Geographic News
March 30, 2005
 
What is the point of sex?

Scientifically, it appears to make little sense. It can be time-consuming and exhausting. It's not even essential to reproduction, at least not to lower organisms such as microbes and certain fungi, which can multiply without sex.

But a new study shows that sex leads to faster evolution.

To demonstrate this, a team of scientists created a mutant strain of yeast that, unlike normal yeast, was unable to divide into the sexual spores that allow yeast to engage in sexual reproduction. Yeast can reproduce either sexually or asexually.

When testing this mutant strain in stress-free conditions, the scientists found that it performed as well as normal yeast. In more extreme conditions, however, the normal yeast grew faster than the asexual mutants.

This shows "unequivocally that sex allows for more rapid evolution," said Matthew Goddard of the School of Biological Sciences at the University of Auckland in New Zealand.

Goddard led the study, which is reported in this week's issue of the science journal Nature.

An Evolutionary Function

Going back to the 19th century, scientists have proposed that sexual reproduction makes natural selection more effective because it increases genetic variation.

However, since then, evolutionary biologists have struggled to find a simple and general explanation for why sex comes in handy when it comes to the survival of the fittest.

"Experiments with sex have been very hard to conduct," Goddard said. "In an experiment, one needs to hold all else constant, apart from the aspect of interest. This means that no higher organisms can be used, since they have to have sex to reproduce and therefore provide no asexual control."

Goddard and colleagues instead turned to a single-celled organism, yeast, to test the idea that sex allows populations to adapt to new conditions more rapidly than asexual populations.

But first the scientists had to create a mutant strain of yeast. That's because when there are sufficient nutrients available, normal yeast reproduces asexually.

A scarcity of nutrients, on the other hand, stimulates yeast to produce spores. The spores are produced via meiosis, a cell-division process that halves the number of chromosomes—a process that is sexual. (Chromosomes are structures that contain all, or most of, a life-form's genes.)

"This experiment has for the first time removed this problem by making mutant yeast that sporulate when starved, but remain asexual," Goddard said. "Both sexual and asexual populations may therefore be treated identically."

The results suggest that a sexual population evolves faster than an asexual population when challenged by a new environment.

In a harsher environment, the sexual strain reached an increase in growth rate of 94 percent, but the asexual strain only 80 percent.

"This experiment provides clear and unambiguous evidence that sex allows faster adaptation to novel conditions," said Rolf Hoekstra, a professor of genetics at Wageningen University in the Netherlands. Hoekstra, who was not involved in the new study, wrote an accompanying commentary in Nature.

"The idea that this could be the case [has been] around for a long time. But now it has been supported by rigorous experimentation," Hoekstra said.

Genetic Shuffling

At first blush, sex appears to have more disadvantages than advantages.

"Think, for example, of the logistical problem of finding a mating partner, which is a big problem in many species where mobility or population density is low," Hoekstra said.

Scientists have also argued that the genetic recombination brought about by sex means that favorable combinations of genes that have accumulated by selection are forever at risk of being broken up.

The new study, however, suggests that the genetic shuffling from sex allows a greater chance that a useful suite of genes will come together.

But the study is unable to answer some questions. For one thing, yeast has no males and females. So the problem of the so-called twofold cost of sex does not apply.

Explaining the twofold cost, Hoekstra said, "If males do not contribute much to the upbringing of the offspring, the sexual females waste half their reproductive capacity to sons and should be outcompeted by asexual females, who produce exclusively daughters and thus have a twofold higher reproductive rate."

Also, though the experiment shows adaptation in the sexual case is faster than in the asexual case, the findings do not uncover why it is faster.

"We are still far from a definitive answer to the question of why sexual reproduction is so common," Hoekstra said.

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