The [R]Evolutionary Work of a Leading Scholar
One of the world's foremost classical evolutionary biologists says he likes to work on the "neglected problems" of the field.
By Scott Hauser
In an age when molecular biology has defined most species right down to the base pairs of their DNA, H. Allen Orr prefers the classical comfort of the tried and true fruit fly.
The tiny insect, better known to most as a habitué of garbage cans,
has long been the Ford Fairlane of genetics-a fine vehicle, to be sure, but
not terribly glitzy.
Over the past decade, the one-time philosophy major turned Rochester biology professor has been quietly altering notions that many in evolutionary biology had thought were resolved more than half a century ago.
"I like to work on the neglected problems, the ones nobody else seems to be exploring," Orr says. "For the past 50 years or so, up until the late 1980s, speciation was pretty much an intellectual backwater in evolutionary biology. It really just got left behind."
"Adaptation was pretty dead, too," he says. "You think of adaptation as the name of the game in evolution, but it had become neglected because everybody thought they knew the answer."
Neither speciation-the study of how one species branches off to become a separate species-nor adaptation-the evolutionary process by which organisms become best suited to their environments -are backwaters now, thanks to Orr.
Over the last five years, he has presented and honed a new explanation of adaptation, the first such postulation in the past 70 years, tweaking the work of one of the giants in evolutionary biology, R. A. Fisher. In other work, Orr has proposed what's considered an unusually elegant solution to a problem first identified by the great British scientist J. B. S. Haldane: Namely, why is it that when two species cross, the male offspring are more likely to be infertile than the female offspring?
Orr and his former Ph.D. advisor Jerry Coyne of the University of Chicago are credited with re-energizing the field of speciation with their late 1980s analysis of how species of Drosophila become separate from one another. Their work is so well known that their most-cited paper, "Patterns of Speciation in Drosophila," made a cameo appearance in last year's David Duchovny movie Evolution.
That's quite a buzz for a specialist whose experimental methods are, at heart, unchanged from the mid-19th century when Gregor Mendel first mapped out the fundamentals of heritable traits by crossbreeding pea plants.
Much in biology has changed since then, of course. Most notably, advances of molecular biology have allowed scientists to see ever finer detailed pictures of the mechanics behind genetics.
By the time Orr began pursuing his Ph.D. in the 1980s, the focus of most evolutionary biologists had moved to mapping chromosomes, genes, and DNA. The fruit fly had lost some of its luster.
"The molecular biologists had taken over," says Coyne, who admits he took Orr into his lab on the less-than-wholehearted recommendation of their common mentor, biologist Bruce Grant of the College of William and Mary. "He is what we would call a classical evolutionary biologist. As am I."
Working together, Coyne and Orr have provided the field's most compelling analysis for estimating how and when one species branches off to become a separate species. Their 1989 paper (updated in 1997) analyzing the genetic changes that take place in crosses of fruit flies has given evolutionary biologists a breakthrough tool for understanding the process.
One of the principal phenomena of evolution, speciation is a neverending process that, although it occurs on the order of hundreds of thousands of years, continues apace. Biologists consider a new species to have been established if members of the new species can no longer mate with members of their parent species. They either refuse to do so (or cannot because of geography), or when they do mate, their offspring are sterile or stillborn.
While speciation attracted a lot of interest from scientists after Darwin first identified natural selection in 1859, the topic had faded from the front burner in the 20th century until Coyne and Orr resuscitated it.
Coyne, a professor in Chicago's Department of Ecology and Evolution, is quick to point out that while Orr may work in classical genetics, he is anything but old-fashioned in the way he approaches his work.
He is something of a modern hybrid unto himself, making major contributions to his field as an experimentalist and as a theoretician who can find connections across areas of evolutionary biology, Coyne says. When his interests have taken him in new directions that have required new skills, Orr has plunged right in.
He's a polymath," Coyne says. "And he's an autodidact when it comes to mathematics. The adaptation work, in particular, requires a complex mathematical armament, and he taught himself the tools that he needed.
"I've never seen a person enter a field and take it over the way he has, considering where he was when he began."
Orr begins most of his groundbreaking work where classical geneticists have long launched their studies-in a lab full of fruit flies. His Hutchison Hall laboratory is stocked with several varieties of Drosophila, including the run-of-the-mill kind that crashes picnics as well as several exotic species from Asia and the Pacific.
The three-millimeter long insects are ideal for experiments that require analyses across generations because a fertile pair can produce offspring in about two weeks. As an added scientific benefit, the insects have only four pairs of chromosomes (as opposed to the 23 pairs found in humans), and their genome is only about 165 million base pairs long, containing about 14,000 genes. (The human genome contains about 3 billion base pairs and about 70,000 genes.)
"They're cheap, they're quick, and you can get them in huge numbers," Orr says. "They're easy live animals."
Orr's laboratory team crosses members of the species with different characteristics-such traits as white eyes or curly wings-puts them together in flasks and lets nature take its course. Two weeks later, he and the members of his lab have a new generation of flies.
They then selectively separate the offspring and breed them again until they have flies that consistently display a chosen set of characteristics. By examining the genes of the crossbred flies, they can identify which and how many genes are involved in each generation's divergence.
"In that sense, it's pretty low-tech science," Orr says.
But the analyses are not.
In his work on adaptation, Orr challenges what has come to be known as the "micromutational view"-Fisher's notion that evolution is driven by lots of small, random mutations that each have the same 50-50 chance of contributing to species survival.
"That's really been gospel for 70 years or so," Orr says. "Now, there's a group of us questioning that."
The more Orr questioned, the more he realized that the facts didn't seem to fit the theory. Instead of marching in a relatively steady line from one spot to another on an evolutionary path, species in nature frequently did not diverge through a concatenation of a lot of small changes. As Orr analyzed the data, he discovered that sometimes very large genetic changes occur, making the path to the environmental optimum more exponential than linear.
"What I wanted to ask is, Are there genes of very large effect?" Orr says. "Are there real whoppers? Are there single genes that if you put them on a genetic background, would they cause hybrid sterility or hybrid inviolability? And the answer is, sometimes, yes."
It was an "Ah-ha" moment that Orr remembers well, but he also remembers the harder part: "Once I realized that, it was a long slog of doing computer simulations and a little bit of algebra to figure out what the answer is. That took more than a year."
Orr now is trying to re-create his prediction at the molecular level, and so far, the predictive power holds.
"I think this is a very robust model."
How does he feel taking on a giant in the field like Fisher?
"It's easier to do something like that if you are working on ignored problems," Orr says. "It's just that nobody has worried about this for the past 70 years.
"I don't want to take anything away from Fisher," Orr says. "He got most things right. He just misinterpreted the math."
The approach of looking at neglected problems will continue, he says.
"My lab tries not to do what everybody else is doing," Orr says. "We're consciously trying not to work on things that everyone else is working on."
As for Orr, he planned to finish a book this summer on speciation that he has been writing with Coyne. The first full volume written on the subject in decades, the book will summarize the developments over the past decade-several of them their own.
"Speciation is actually getting kind of crowded right now," Orr says with a look that indicates he's on the lookout for another challenge. "Adaptation has a little more elbow room."
Scott Hauser is editor of Rochester Review.
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