Published online 7 July 2009 | Nature | doi:10.1038/news.2009.628

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Adapt and die?

Beetles that changed their diet by choice suffered in the end.

The ability to adapt to a new environment may not always be beneficial for long-term success — in flour beetles at least. Beetles that were offered and ate a novel food, even with their ancestral food all around them, suffered over multiple generations, according to a study presented last month at the Evolution 2009 meetings in Moscow, Idaho.

The adaptable flour beetle, Tribolium castaneum.MYRMECOS.NET/A. WILD

The work provides some of the first empirical data for whether behavioural plasticity — the ability to adapt immediately to changing environments — helps or hinders the evolutionary success of organisms.

"We saw that these beetles have a massive degree of behavioural plasticity, but that their evolutionary success was hindered due to their adaptability," says Deepa Agashe, a postdoctoral fellow at Harvard University who did the study while at the University of Texas at Austin.

"When the environment changes, the very range of responses that previously constituted 'adaptability' may become an evolutionary liability instead," notes Donna Holmes, an evolutionary biologist at Washington State University in Pullman, who was not involved in the research.

Let them eat corn

Agashe tested Tribolium castaneum, the ubiquitous flour beetle, by offering the beetles wheat flour — their ancestral diet — and maize (corn) flour, which was new to them. The researchers tracked how much of each flour the beetles had consumed by looking at the ratio of carbon-13 to carbon-12 in ground-up beetle carcasses. Wheat and maize have different ratios of these carbon isotopes.

After only two weeks, Agashe found, the beetles' diet shifted to almost 30% maize flour. "This was a never-before-seen food source," she says, "and they were eating plenty of it. This was a major behavioural change."

The researchers let the beetles multiply through six generations, and tracked adult and larval numbers to assess immediate as well as long-term success. "There was only a 4% increase in corn use over six generations," reports Agashe. In other words, the beetles ate a lot of maize initially but then the rate tapered off. Furthermore, the beetle populations that shifted to eating maize most quickly tended to have the lowest population sizes and stability.

Interestingly, the level of genetic diversity of a group did not affect the degree to which it started eating maize. But in groups that were fed only on maize flour, "genetic diversity becomes really important", says Agashe. Some populations died out after about 12–15 weeks on a maize-only diet, whereas those that survived — the more genetically diverse groups — began to recover by the 25th week.

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"The importance of the genetic diversity to the successful use of corn is really interesting," says Michelle Scott, a behavioural ecologist at the University of New Hampshire in Durham. "There must be variations in the metabolic pathway; in some populations, the taste for corn precedes the ability to utilize it efficiently."

Over longer time frames, Agashe says, a genetically diverse population that is also adaptable may have the best chance for evolutionary success in the face of dramatic environmental change.

"How strong does selection have to be for plasticity to be beneficial?" she asks. As a next step, she hopes to search through published literature for other work on how native species respond to invasive species over both the short and long term. 

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