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Numbers trump genetic diversity in survival stakes

Posted on behalf of Ed Yong.

As a species tumbles towards extinction, populations with few members are more likely to die off than those with low genetic diversity. At least that’s the message from a 12-year-long experiment by husband-and-wife team Tim Wootton and Cathy Pfister of the University of Chicago in Illinois.

Understanding the relative importance of these two factors is key to designing effective conservation strategies. It may be common sense to focus on boosting numbers if demographics matter more, and to put together breeding programmes that expand the remaining gene pool if genetics rule, but accurately pointing the finger at the primary cause of decline is rarely a simple task.

The cheetah is the poster child for this problem, Wootton told attendees of the 2012 Ecological Society of America annual meeting in Portland, Oregon, on 6 August. Africa’s most at-risk cat species has lost much of its habitat and its former genetic diversity. Consequently, no one knows which factor will more strongly influence its fate.

He and Pfister wanted to do experiments in which they could manipulate both the genetic structures and the sizes of populations. Cheetahs being impractical subjects, they opted instead for the sea palm, Postelsia, a type of kelp that occurs along the western coast of North America. Postelsia barely disperses at all. It grows in clumps on wave-swept rocky shores, reproducing by dribbling spores onto rocks directly below it. Waves remove the adult plants each year, leaving room for spores to grow. As a result, each clump is often genetically isolated from those just metres away.

Wootton and Pfister bred sea-palm populations with varying degrees of genetic diversity. They transplanted them onto rocky patches of shoreline in batches of different sizes. After 12 years, smaller populations were less likely to have survived than larger ones, and, among the populations that did disappear, smaller ones did so more quickly. Crucially, genetic diversity did not influence the odds of a population’s survival.

Wootton found a cut-off point, between 10 and 100 individuals, where the risk of becoming locally extinct changed dramatically. “The rule of thumb when I was a student was that 50–100 individuals was a viable population size,” he says. “Our experiments support that.”

How widely applicable are the findings? The sea palm is edible and heavily harvested, so the results have direct implications for its management. But it is also stationary, whereas many vulnerable species — speedy cheetahs especially — are mobile.

“It’s hard to know [how broadly applicable the results are] because it’s the first experiment of its kind,” Wootton points out. Meanwhile, Robert Paine, an ecologist at the University of Washington in Seattle, who has worked with Wootton and Pfister, praises the new study, describing it as dealing with a very real question with broad application.

Some other studies have claimed that genetic diversity is the more important factor, but these typically report only a correlation between low diversity and species endangerment, Wootton says. “I’d say if you have a limited budget, you should probably study the ecology and the demographics first rather than doing molecular analysis.”

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    Steven Pelech said:

    Preservation of habitat that is rich in biological diversity seems to be the best solution to ensure survival of most species facing extinction. Some 40% of all animal species today appear to be under threat of extinction, but more than 99% of all of the animal species that have ever swam, walked or flew on this planet are forever gone. It seems that new species emerge when the numbers of the predecessors of these species actually dwindle or become geographically isolated. Apparently, the rate of evolution of new species is accelerated after mass extinctions, such as occurred about 65 million years ago with the demise of the dinosaurs. Genetic evidence points to the near extinction of our own direct ancestors about 200,000 years ago. More recently, whales have made a bit of a come back in the last few decades after their populations were decimated more than 95% by a few hundred years of whaling practices. In these examples, genetic diversity was not really the critical factor for the survival of these species.

    It is intuitive that sheer numbers of individuals within a species would be a critical factor for a species’ survival. With a larger population, it might seems that there would be greater genetic diversity as well, since there would be a more chances for accumulation of natural genetic mutations within the population. However, this is not necessarily true. For example, there is greater genetic diversity amongst individual members of the other primate species than there is in humans.

    The most critical factor for the emergence of new species is likely to be the genetic variation related to the specific locations of genes on individual chromosomes. The vast majority of mutations within the coding sequences are inconsequential for the survival of a species or the emergence of a new one. But chromosomal rearrangements can result in non-viable embryos following fertilization. With dwindling populations and a higher probability of in-breeding, there is improved chances that the similarly rearranged chromosomes from an egg and a sperm can properly align following cell fusion and later reductive cell divisions so that each daughter cell will have two copies of each gene. This becomes most critical for the production of gametes later on, which in their haploid state must have a copy of each gene for the next generation. These considerations are likely to apply equally to both animals and plants that reproduce sexually.

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