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Why organisms reproduce the way they do

New scientific discoveries about plants and snails.

By Moises Velasquez-ManoffStaff writer of The Christian Science Monitor / July 30, 2009

Evening primrose

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Evolutionary biologists have often wondered how and why sexual reproduction came to exist when asexual reproduction seems easier and more efficient.

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Organisms that reproduce asexually have a one-to-one reproductive ratio – a lone organism can replace itself. Compare that with the sexual model: Two individuals provide half the genome for a single offspring. At face value, asexual reproducers would seem to have double the reproductive capacity compared with that of sexual reproducers.

Many organisms – including microbes, plants, and some reptiles – do reproduce asexually. But the vast majority of living things reproduce sexually. Why?

Two new studies offer answers to this question. In one study, sexual reproduction seems to help snails avoid parasites. In another, it helps plants fight off grazers.

When facing certain types of adversity, it seems, a genetically diverse (sexually reproducing) population is more likely to persist than a genetically homogeneous (asexually reproducing) one. Sexual reproduction allows for greater genetic innovation over time than clonal, improving the chances that an organism can solve the problems at hand.
In one study, described in the American Naturalist, scientists looked at potamopyrgus antipodarum, a snail from freshwater lakes in New Zealand. The species has both an asexual and sexual version.

Over a decade of observation, the scientists found that asexual populations dwindled. Some lineages had completely disappeared.

Populations of sexually reproducing snails, meanwhile, remained relatively stable.

What was the difference? Parasites took a heavier toll on the asexually reproducing snails. And it made sense: They were genetic replicas of one another; if one proved vulnerable to parasites, they all would.

On the other hand, individuals in the sexually reproducing populations better staved off parasite infection. Genetic diversity was an asset in a parasite-ridden environment. No single snail was exactly like any other. If one succumbed, just one was lost. In this case, genetic diversity acted as a bulwark against potential parasite pandemics.

In another study in the Proceedings of the National Academy of Sciences, scientists observed a similar pattern in plants – specifically, evening primrose plants. Eighty-five percent of the 259 evening primrose species reproduce sexually. The rest reproduce asexually.

Sexually reproducing primroses fought off caterpillars and other plant-eaters with greater ease than their asexual counterparts. By constantly remixing their genes, they could more easily develop new compounds to repel grazers.

But in this case, there was a twist.

Herbivores come in two types: generalists and specialists. Sexually reproducing primroses fought off generalist herbivores better than asexual primroses did, but not specialist grazers. Indeed, specialists were attracted by those substances that repelled generalists.

The authors hypothesize that specialized grazers coevolved with their food source. They’ve co-opted the plants’ defenses and home in on those very molecules meant to repel them.

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