This sounds logical. After all, if your population is just a few dozens, as can happen when a species is desperately near extinction, you would want all of the members of the population to be genetically represented in the new generation. After all, that's the way to prevent inbreeding, right?

But is that analysis backwards?

That's what Lisa Marie Meffert, a research assistant professor at the University of Houston, thinks. She's spent years studying small populations of a glamorous, buzzingly expressive animal.

Right. Houseflies. Meffert actually breeds these annoying critters.

Oddly enough, she's not looking for a miracle "suicide gene" that would lure these noisome nuisances to flypaper, or cause them to stand still for an onrushing flyswatter. In other words, she's not trying to cause an extinction of these unlovable arthropods. Instead, she's interested in what happens when a fly population is forced through a genetic bottleneck that restricts the diversity of genes passed to subsequent populations.

And her findings threaten to rattle some of the axioms of conservation biology. For one thing, she says (gasp!) that more genetic diversity is not always better than less.

Why is this heretical? Because biologists usually say that more genetic diversity will enable organisms to cope with new situations. If a new leaf-eating ant evolves, genetic diversity is what will enable a plant to evolve new defensive chemicals against it. If the climate warms up, diversity will enable a mouse to evolve subtly different enzymes that work in those warmer conditions.

Without genetic diversity, a population also runs the threat of inbreeding depression -- the loss of vitality and reproductive fitness when organisms breed among themselves.

Meffert does not deny that some genetic diversity is useful. The genes that determine the immune system, she says, may need diverse components to combat diverse opponents. Nor does she deny that inbreeding can be harmful, since it can lead to disease, lethargy and deformity.

Buzz on down to the single's bar?
But sometimes, she says, predictability might be better than change. "You can imagine that you would not want to be the first on the block to demonstrate a new way to mate," she says, since your potential mates might not understand your actions.

Similarly, variation in the chemicals that transmit signals between nerve cells could prevent their transmission.

Like fly, like human?
After 15 years of studying small groups of houseflies, Meffert has found a way to combat inbreeding depression: let the flies decide who gets to mate. Her technique is to compare flies that are deliberately forced to breed at random to flies that are left alone and choose for themselves. In the latter groups, what she calls "studly males" shoulder aside less-fit males at mating time. (Some of the research has been done by doctoral student Jennifer Regan.)

After 10 or 20 generations of inbreeding, she allows each population to multiply freely, and the flies that were forcibly bred at random do quite poorly, she says: "You get a nightmare, the population crashes." But the self-selected "superflies" do much better, Meffert says, and quickly rebuild a large population.

She says these findings offer advice that is "completely opposite" to the typical practice of conservationists who breed tiny populations of endangered species. Animals whose populations have plunged to tiny remnants include the whooping crane, the California condor and the cheetah. In these cases, breeders, to retain maximum genetic diversity, try to induce every individual to breed, sometimes even using in-vitro fertilization -- when the egg and sperm meet in a glass dish.

That may be getting the picture backwards Meffert says. "What if you know a certain individual is a wimp? I'd say we should not force it to mate. I'd argue that mother nature is trying to tell us something."

Bottlenecks can actually help a species by pruning out unfit organisms, she argues. "When you perturb [disturb] the genetic structure, it can have increased potential. A bottleneck can purge harmful genes from a population."

The big question that remains, she says, is "how much pressure do you need to apply? We could lose genetic diversity that we'd want later if you put too much pressure on the animals. It's a tricky question."

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