In the first of an occasional series on up-and-coming researchers, Graham Lawton talks to biologist Armand Leroi.

Why are Dutchmen taller than Pygmies? Why are elephants bigger than mice? According to one evolutionary biologist - who also happens to be a Dutchman - nobody really knows. These questions, he says, represent one of the biggest remaining problems in the theory of evolution: how does one species evolve into another?

Eight months ago Armand Leroi left the dreary surroundings of the Albert Einstein Medical Centre in New York City for the arboreal splendour of Silwood Park, Ascot, Imperial College's out-of-town research centre. He is hoping to make a major contribution to the theory of evolution by cracking the problem of body size.

"At the moment there is no quantitative theory for how genes control development, much less how those genes change in the course of evolution to cause morphological change," says Dr Leroi. "I can't even give you a coherent account of why two humans are different in height. It's very curious that we know so little about this."

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Curious indeed. An explanation of the genetic basis of evolution must form a central component of any satisfactory Darwinian theory. So why do we know so little?

To understand the omission requires a little knowledge of the history of evolutionary biology. Most present-day biologists prefer a brand of Darwinism known as the neo-Darwinian synthesis. This theory was formulated in the 1940s at a time when classical Darwinism was in decline for want of an explanation of the mechanism of heredity. The new synthesis rescued Darwin's theory of natural selection by adding a vital ingredient - genetics.

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Neo-Darwinism opened the way to a coherent explanation, at the genetic level, of how one species is transformed into another. But it simultaneously pushed the search for that explanation into the background. "In order to know anything about how organisms evolve you first have to know how organisms are put together. Before you can ask how a worm changes you have to know how to build one," explains Leroi.

Thanks to 20 years of research on the growth of the roundworm Caenorhabditis elegans, that knowledge is now available. Every cellular change that occurs in a C. elegans as it develops has been recorded exhaustively. Like biblical scholars, researchers know which cell begat which and where they all live. "With that information in hand," says Leroi, "we can go back to the evolutionary problem."

That problem is one of difference. By definition, evolution involves change. The best way of studying those changes is to identify differences between species. "The most obvious way that organisms differ from each other is in body size," says Leroi. "If you look at mammals, you've got mice on the one hand and elephants on the other. And look at humans - we go from the Dutch who are very tall to the Pygmies who are very short. There is almost no trait in which we differ so much. But we do not understand why and how."

For practical reasons Leroi eschews the use of mice, elephants, Dutchmen and Pygmies in his research. He prefers worms. Using the data on C. elegans as his baseline, he studies the growth of other species of roundworm. In the long run he hopes to discover how differences in body size evolved.

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It could be a very long run. "This aspect of biology is very much in the inductive stage," says Leroi. "We are going out into the wilderness and collecting the first scraps of information about how organisms change during the course of evolution."

Moving to Imperial is something of a gamble for Leroi. At the relatively tender age of 32 he is charged with building a cutting-edge research team in an unfamiliar country on a lower salary than he could have commanded in the United States. But he is confident that he has taken the right decision. "I have always tried to go to places which allow me to do the best science that I can," he says. "Imperial is such a place."