Prying Into the Sex Life of Wild Radishes : A UC Riverside Biologist’s Experiments May Alter Long-Held Beliefs About Natural Selection--and Even the Purpose of Sex

On a grassy, eastward-facing hillside in Riverside County, between a citrus grove and a dusty basin, fenced in by a sagging string barrier and a couple of crooked stakes, grow half a dozen wild radish plants. Standing as they do among foxtails and wild sunflowers, the radishes, three feet high with pink or white blossoms and green seedpods, look more like vigorous flowering weeds than garden vegetables. But, says Norman Ellstrand, a UC Riverside evolutionary biologist who has studied them for the last three years, the seeds from these undistinguished pods may turn out to have major consequences in ecology and sociobiology.

Until now, some scientists considered evolution to be almost solely the work of natural selection--the “survival of the fittest” theory that organisms best adapted to their environment survive more readily than ill-adapted ones, thereby naturally weeding out undesirable traits from a particular species. Ellstrand, however, has conducted a relatively simple experiment with his radish plants that suggests that evolution may in fact be more random than previously thought. The implication is that human behavior may not be, as some ecologists and sociobiologists believe, inexorably fixed in our genes and unchangeable.

It is quite understandable, Ellstrand says, why in a chaotic world many people are drawn to the concept of natural selection. For one thing, it is virtually the only aspect of evolution most people learn in school. But most important, he says, may be that natural selection is comforting to many people in that it offers an “organized, deterministic, predictable way of looking at the world. We are better than dogs and dogs are better than goldfish and goldfish are better than protozoa. Natural selection rules like God.” And its “highest manifestation” is man.

This is not to say that Ellstrand doesn’t believe in natural selection. “Obviously it works or we wouldn’t be here,” he says. It’s that contrary to the semi-mystical powers attributed to it by some people, natural selection doesn’t so much bring out the very best in us as it allows us to just get by. “This is why,” Ellstrand says, “we have bag ladies and diseases and things like that.”

Ellstrand, 34, has a high-pitched voice, an informal manner and a droopy-nosed smile reminiscent of comedian Robin Williams. He seems almost too mild-mannered to go around smashing evolutionary icons. Although trained as an evolutionary geneticist, he teaches population ecology at UC Riverside, where his weed-covered experimental plots look so little like research sites that a developer once inadvertently razed one to build a condominium.

Ellstrand didn’t begin his radish experiments with the intention of challenging sociobiological theory. In 1980, he says, one of his graduate students was “thrashing around for a Ph.D. project. She was interested in flower-color variation and what the consequences were with regard to pollination biology.” So Ellstrand sent her to look at wild radishes, which have flowers that routinely come in many different hues. The student brought back research that showed that, at the biochemical level, wild radishes made good experimental plants because they had highly variable genes.

Ellstrand found that interesting but thought nothing more of it until a year later, when he was lecturing on the importance of gene flow (the movement of genetic information from one population to another) as an evolutionary force. “And we were getting frustrated because there was no direct data on what gene flow was in plant pollinations,” Ellstrand says. “I said there must be some way of measuring this directly.” What was needed was a plant with highly variable genes. Then it struck him--the wild radish. As the purpose of the experiment was to measure gene flow, which in this case meant nothing more than the average distance pollen travels to fertilize each plant, Ellstrand sent his graduate students out into the field to locate and tag every radish plant in the study area out to a radius of 1,000 meters. When the plants matured, the seeds were collected, and Ellstrand’s research associate, Janet Lee, performed a sophisticated biochemical technique to determine their genetic composition.

Because Ellstrand already knew the genetic contribution of the mother, he could use a simple technique to determine which plant in the study area was the father. And that, in turn, told him the minimum distance the pollen had to travel to fertilize the blossoms.

In the past, Ellstrand says, biologists generally assumed that pollen travels an average of one to two meters. “And it was anticipated that the gene flow at 100 meters would be 1% or less.” Yet, Ellstrand’s experiment showed that the pollen travel at 100 meters was closer to 20%. For 1,000 meters it was 9%.

For Ellstrand such results were absolutely stunning. “For the last 20 years, ecologists and evolutionists have been saying: ‘We don’t need to talk about gene flow because it is not important. And the only thing we should emphasize is natural selection.’ ”

Now, suddenly, here was someone challenging that assumption. Furthermore, at least for this particular experiment, Ellstrand also was in effect saying: “I can prove it. Here is a force that in these circumstances is at least as important as natural selection, and as a result we can no longer assume that all characteristics are adaptive. With high gene-flow rates, it is entirely possible that some characteristics are maladaptive and counterproductive. Which is another way of saying, just because some human behavior is genetically determined, this doesn’t mean it is natural and good. Natural selection, as important as it is, is not the only evolutionary force of any consequence. Mutation, genetic drift (chance) and gene flow may also have to be taken into account.”

Ellstrand’s interest in the sex life of plants goes back to postdoctoral work he did at Duke University where, under the direction of his adviser, Janis Antonovics, he looked into the question of why sex exists in the first place. Although that was hardly a new question--”the idea has been kicked around almost since the time of Darwin,” Ellstrand says--it wasn’t until the mid-1970s that people seriously started asking: Is there a cost to sexual reproduction? “Given that an organism has an option of being sexual or asexual in an evolutionary sense, which would be the most fit, all other things being equal? And it turns out that the organism that reproduces without sex contributes twice as many genes to the next generation as one that reproduces sexually.”

There is nothing very complicated about this. In sexual reproduction, each parent contributes half of the genes of the offspring, whereas in asexual reproduction, the mother contributes all the genes. This raises the question why, if the goal of reproduction is to contribute as many genes as possible to the next generation, sex exists at all.

Although many biologists overlook it, Ellstrand says, asexual reproduction is common in the plant world. Virtually all strawberries grown in California come from just a couple of clones. All seedless navel oranges originally came from one tree in Brazil. About 5,000 species of wasps reproduce asexually, plus about 50 species of fish, 20 to 30 species of lizards, 5 species of frogs, 1 species of snake and even a clonal turkey developed by the Department of Agriculture.

Surprisingly, Ellstrand says, before his experiments, no one had really done any field research to find out why sex exists. In an effort to shed some light on the matter, Ellstrand and Antonovics devised a simple test with vernal grass (a kind of clump grass). They planted about 1,000 tillers (small plugs from individual clumps) in an experimental plot. Having come from the same clump, the tillers were all clones of one another. Into the matrix of these identical tillers they planted relatively rare tillers from other clumps of vernal grass from outside the immediate area.

What they found was that the rare tillers were not only much healthier but that, on average, they produced twice as many seeds.

Ellstrand came up with two possible explanations for this: The first was that the numerous clonal tillers had to compete with each other for water, light and nutrients, while the rarer tillers with their slightly different requirements faced no direct competition at all.

The other possibility was that predators were exerting pressure on what is common and, in the manner of predators, missing what is rare. “If there is a bird that is tuned in to eating red bugs,” Ellstrand says, “and one bug tends to produce purple, green and yellow bugs, then that bird will miss those colors.”

As it turned out, Ellstrand says, the predator scenario best fit the facts. Many soils are rife with funguses, viruses, bacteria and nematodes. On their home soil, nematodes were well adapted to identifying a certain type of plant, but when that type of plant was moved as little as 30 feet, its survivorship and seed set was much greater. This suggests that a locally adaptive disease will identify and attack a common type of organism but not attack a rare kind.

That, in turn, Ellstrand says, suggests that the reason sex exists is to produce rare progeny. The reason rare is good, he says, “is because it helps you avoid things that are going to eat you or diseases that are going to hit you.”

As an example of how his work relates to human populations, Ellstrand cites a longstanding paradox of natural selection: “If evolution works continuously to favor reproduction, how do we account for those individuals who give up their opportunity to reproduce? Why does a soldier jump on a hand grenade? Why do some women become nuns? Why are there gay men?”

“An extreme sociobiologist would argue that there is a gay gene. The reason a man becomes gay is because he gives up his own reproduction for the purposes of helping his brothers and sisters (raise) their own children.”

Although the explanation that gays exist in order to help raise their nieces and nephews seems unlikely on its face, Ellstrand says, these are the kinds of explanations you are driven to when you believe, as the more extreme sociobiologists do, that natural selection is the only significant evolutionary force worth considering.

As Ellstrand sees it, there are two reasons why natural selection has held “not only center stage but the entire stage” in scientific circles since the turn of the century. The first is its ability to explain the obvious adaptation of organisms to their environment. “A fish has fins. A bird has feathers. A lizard has scales. Eyes help us see. It is a very powerful way of looking at how an organism matches its environment.”

The second reason, Ellstrand says, was the deeply felt need of ecologists and sociobiologists to give their disciplines a solid theoretical underpinning. Before the late ‘50s, ecology was a descriptive science that simply measured plant and animal populations and arranged them in hierarchies of who eats whom. But then ecologists Robert MacArthur and G. Evelyn Hutchinson seized on natural selection to transform ecology from a descriptive science into a predictive science that gave meaning to a lot of otherwise unrelated data. “If you can hang your data on some underlying theme,” Ellstrand says, “then it is a lot more powerful.”

Ellstrand doesn’t object to this. Uncovering fundamental principles is the reason science exists. The problem, he says, was that MacArthur and Hutchinson had so persuasive an influence on ecology that it turned the students into “born-again natural selectionists” who threw mutation, genetic drift and gene flow right out the window.

Nor was ecology the only discipline affected. Ten or 15 years later, the same thing happened in sociobiology, a field that takes the premise that human social behavior has a biological basis. “Here were people studying the behavior of animals and humans,” Ellstrand says. “All of a sudden here was a new, fancy, very powerful theory, that human behavior is not necessarily based on Freudian paradigms but on the laws of natural selection. Now they had a way to make their science more predictive. They could attribute all sorts of human traits to natural selection.”

As a result of their misunderstanding of genetics, Ellstrand says, many sociobiologists came to believe that human behavior was simply caused by genes and was therefore beyond the reach of human intervention.

At this point, Ellstrand says, it’s too early to tell what impact his paper in last December’s American Naturalist (one of the two leading evolutionary biology journals) will have on the thinking of ecologists and sociobiologists. But when he submitted it, two of the reviewers, he says, were “strongly enthusiastic and excited and thought it was a strong contribution.” A third reviewer was “extremely negative and said, ‘This paper should not be accepted for publication.’ ” Since then, he says, the informal response has been guardedly positive.

As Ellstrand sees it, the realization that there are forces beyond natural selection can lead us to a very different sort of world view. “Exploring alternatives to natural selection frees us from the tyranny of natural selection. We are not constrained by some sort of externally directed force that is pushing us inexorably toward some ultimate end, but we are responsible for what we do.”

It is no surprise to Ellstrand that most sociobiologists are conservative politically while he, an ardent Democrat, finds himself on the other side of the issue. Scientists, like everyone else, see the world through the distorting lens of deeply held biases. “Although,” he says, “there is a tendency for scientists to sometimes believe they hold the truth in their hands, in reality truth is a sort of slippery thing. It’s like the six blind men and the elephant. Everybody gets a little piece of the truth.”

Sociobiology won’t dry up and blow away because one researcher at one school generated some contradictory data. Nor would Ellstrand really want it to. Every new bit of data helps refine the theory. All that gene flow and wild radishes do, he says, “is give us one more piece of the truth.”