02/26/92
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To answer this question, Stanford biological sciences Prof. Jonathan Roughgarden turned to, of all things, his computer.
Roughgarden simulated 5,000 generations of two hypothetical species to solve a puzzle that has intrigued biologists for 50 years: on paper, asexual species should have a big head start over species that reproduce sexually, but in nature, sexual reproduction is the norm.
Roughgarden's simulation reveals the long-term evolutionary advantage of sex - an advantage that overturns a common assumption of biology. It's not the ability to generate new combinations of genes that makes sexual species more resilient in case of change, he found. Sex triumphs because it preserves relatively steady amounts of existing gene combinations.
From an ecological perspective, sex seems like a losing proposition. As biologist Maynard Smith pointed out 20 years ago, if a population reproduces asexually, every individual is capable of independently making offspring. With sex, only half the individuals bear young, so the potential growth of the population is halved. Biologists have repeatedly attempted and failed to show what benefits of sex were sufficient to outweigh this high cost.
Researchers have agreed that the advantage of sex has to do with the combining of genes from two mating individuals.
Those quickly created combinations, many theorized, make the organism capable of surviving dramatic changes in the environment - a sudden temperature rise, for example - that might drive an asexual population to extinction.
But this process is a double-edged sword, Roughgarden noted. Sexually produced offspring can be better suited - or less suited - to the environment than their parents.
Roughgarden cites at least two other weaknesses in such explanations: Biologists often have underestimated the ability of an asexual population to generate new genetic combinations through recurring mutation, and they have neglected the importance of ongoing, small fluctuations in the environment.
Roughgarden described his work in the October 1991 issue of The American Naturalist. With a computer simulation, he compared two populations, identical except for the fact that one reproduced sexually and the other asexually. He modeled the simple case where each individual organism in either population carried one of three possible gene combinations: aa, AA or Aa.
He assumed that all the gene combinations needed for the populations to survive in the fluctuating environment existed at the outset. He then simulated environmental change and reproductive activity for 5,000 generations.
Both populations "experienced" natural selection - the process of interaction between organisms and their environment that results in some organisms having more offspring than others. For each generation, Roughgarden randomly selected the strength of the environmental change and whether it was good or bad for each gene combination or "genotype." For example, a generation might experience an effect that allowed AA and Aa each to have two offspring, but aa only one.
He also simulated randomly occurring mutations. In each generation, some number of genes changed from a to A or vice versa. He randomly "mated" individuals in the sexual population.
At the end of the 5,000 generations, he discovered that the sexual population had maintained its numbers more successfully than had the asexual one.
The asexual population was subject to wide swings in its numbers. An environmental advantage to the AA genotype meant a big increase in AA numbers, but left relatively few Aa or aa individuals to rebuild the population when a new change wiped out most of the AAs.
"Recurring catastrophes mean the (asexual) population is always caught off balance," Roughgarden said.
In contrast, the sexual population shuffled genes at reproduction, and achieved a more equal distribution among genotypes for each generation. This even distribution in turn implies a relatively consistent degree of success from one generation to the next.
In effect, sex buffers the population from changes in the environment, he said.
He drew an analogy to financial strategy. If a person invests $100 in two different ways, one that earns 7 percent over two consecutive years, and one that fluctuates from 1 percent over the first year to 13 percent over the second, he will earn 36 cents more from the steady earner. Such small differences, compounded over many years, can become substantial.
Long-term reproductive yield is similar: Less variance in reproduction from generation to generation results in a higher mean performance over time, said Roughgarden.
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