The study of evolution goes far beyond dinosaur bones and finch beaks. Fueled by advances in technology, research in evolutionary biology has never been stronger or more diverse—especially on the Palouse.
An evolutionary biologist at Washington State University says he often encounters people who are surprised to learn what he does. They have the impression there’s only a handful of scientists in the country who manage to scrape together a few bits of information in support of Darwin’s theory.
Nothing could be further from the truth. Scientific journals publish reams of new data every year about how evolution works. The Palouse alone is home to 80 to 100 biologists exploring the patterns of evolution and the processes that drive it.
And that number is deceptively low. In a way, every biologist is an evolutionary biologist. Carol Anelli, an entomologist who also studies the history of evolutionary thought, says few people realize the importance of evolution in their everyday lives, that the theory of evolution underlies all of modern biology and medicine.
“In drug design, or in taking natural products from animals, there’s an underlying recognition by the scientist that the way that’s operating [in those animals] is the way probably it’s going to work on humans,” says Anelli. And that similarity is due to shared genetic history.
“There are many areas of science where breakthroughs are made using so-called ‘lower organisms’ such as bacteria, roundworms, and fruit flies,” she says—and if we and the model animals were not linked through evolution, “why would we be doing these studies? We wouldn’t. The federal government would not be giving millions of dollars to work on roundworms and fruit flies.”
In our look at evolutionary biology at WSU, we have space for only a few research stories. There’s a lot more where they come from, spanning the range from what’s sexy to salamanders, to how the evolution of a virus can result in an epidemic that kills millions of people. All of the stories are linked by the theme of species adapting and changing to launch their offspring successfully into the world. That’s what evolution boils down to: producing offspring that will be able, in their turn, to thrive in their habitat and have offspring of their own.
How the beetle got his horns
Walk into Laura Corley’s lab, and you won’t notice anything you couldn’t find in any other modern biology lab. But open the door to the walk-in incubator that houses her experimental animals, and you get hit by the aroma of the barnyard.
Corley studies dung beetles, which get their name from their reliance on the droppings of much larger animals, such as cattle or antelope, to nourish their young. Each egg is laid inside a “brood ball” of dung. The female beetle gathers the dung, chews it and mixes it with sand, shapes it into a tidy oval, and places it, with egg inside, in a sort of den she digs in the dirt. When the larva hatches out of its egg, it has exclusive access to its food supply. As it eats and grows, it hollows out the brood ball from the inside. When it finishes growing, it pupates, like a butterfly chrysalis, and then emerges from the ball as an adult.
Despite their small size and humble origins, adult dung beetles are among the most spectacular creatures on Earth. Males of various species possess an array of head ornaments that rival anything seen in the deer family. Some of the males do, that is. Whether a particular male develops horns depends not on his genes but on the ball of dung that nourishes him—how big the ball is, how much he eats, and how big he gets.
“It’s a threshold trait,” says Corley. “If they reach a critical weight, then they make horns. If they don’t reach the critical weight, they don’t.” All the male beetles have the genes to make horns; but those genes are turned on—and they grow horns—only if they get enough to eat as larvae.
Corley is investigating how the horn-development program is controlled. She’s especially interested in the insulin-signaling pathway, by which insulin and other molecules enable the animal to sense its own nutritional state and signal various parts of its body to turn specific genes on or off.
She cautions against the notion that “horns are good.” She’s not keen on the TV-nature-show version of “natural selection” in which every trait and behavior of an animal exists with direct reference to a yes/no, good/bad sort of tally sheet. The situation is more complex, and more interesting.
Corley differentiates between positive, negative, and neutral selection. The payoff in each case is which animals get to pass their genes along to future generations. Positive selection occurs when a genetically controlled trait helps its owner to spawn more offspring. You’re a salmon who can swim upstream for two months without eating, and still spawn vigorously? You’re in. Or rather, your genes are in (the next generation).
Negative selection occurs when a genetically controlled trait diminishes its owner’s chances of passing them on to the next generation. You’re a gazelle who can’t run faster than a hungry lion? You’re outta here.
Then there’s neutral selection, which is less dramatic than the other two, but may be at work just as often. “Neutral” means the trait doesn’t confer enough benefit or harm to influence the reproductive success of its owner. Such a trait may not be a great boon, but if it’s not a big negative, it won’t be selected against. A lot of traits—and the genes that control them—may be passed along this way, riding the coattails of some more essential trait.
In Corley’s beetles, the key trait—the selected-for trait—may not be those impressive horns, but the plasticity, or flexibility, to grow them or not. It’s a way for the beetle to cope with a patchy environment. Horns account for up to 15 percent of a beetle’s total weight; in an environment with minimal food, spending calories to lug them around could consume energy better used in other ways. But in an environment with abundant food, maintaining the horns is not a problem. In that situation, it might be worth having the ornaments, because males with horns have better luck with the ladies than those without them.
“What I think is just absolutely, one hundred percent cool, is that these individuals are the same, but they’re different,” says Corley. “The first time I ever found out about phenotypic plasticity—that you have the opportunity to be short or tall, or have a horn or not have a horn, or be yellow or white, or make a spot or not make a spot—and that it’s almost purely based on the environment, I completely flipped. How does that happen? And I’m still searching for the answers to that question.”
Corley’s research is part of the emerging field of “evo-devo,” which combines evolution and embryology. Only about a decade old, the field already has provided stunning evidence about how different body plans can evolve.
“Do you need a whole bunch of genes to change and be subject to natural selection? Or do a few key regulatory genes do it?” asks Mike Webster, who co-teaches the course in evolution for biology majors. “Some of the most exciting results that are coming out of this area of research are that if you just change the timing of when the genes are turned on and off, you can get a very radically different body plan.”
He describes research showing that an embryonic invertebrate can be made to develop into something that looks like a spider (with two main body segments), an insect (three segments), or a centipede (many segments), depending on when a certain gene is turned on.
Still, environment can’t do the job alone. In order for a dung beetle to make horns, he must have the genes to do so. Genes remain central to evolutionary study, and changes in genes—mutations—are still thought to be the main source of differences among species. And mutations, we now know, happen disturbingly often.
“People think you have to
get zapped by something,” says Charlotte Omoto, who teaches evolution as part of a genetics course for non-biology majors. “No. You know why there are mutations? Every time a new cell or new organism is produced, the genetic material has to be copied. Mother Nature’s wonderful, has all kinds of checks and balances; it’s very important to make sure things don’t change [too much]. But we have three BILLION of these letters that have to be copied every time a new cell is made. So little mistakes are made.
“It’s no different than the monk copying the Bible by hand,” she says. “And people have done this—we can see how errors have propagated in manuscripts because of writing errors. Well, exactly the same thing happens in cells.”
And in cells, those mistakes—those mutations—can boost an organism’s chances to reproduce, or ruin them completely.
Let’s get botanical
Superficially, sedges and African violets couldn’t be more different. Sedges resemble grasses, except their stems are triangular rather than round. “Sedges have edges,” as the botany teachers say. Their minimal flowers make identifying species a challenge even for experts. African violets have beautiful blossoms; identifying the species is fairly easy, but determining how they’re all related to each other is not.
“They’re both difficult, but for different reasons,” says botanist Eric Roalson. It’s the difficulty that appealed to him when he first studied sedges as an undergraduate and African violets as a postdoc. “I was amazed at how complex and poorly understood they were,” he recalls. “That was one of the things that drew me in to studying them.”
Evolutionary biologists generally work on either the processes of evolution—like Corley’s evo-devo experiments—or the patterns of evolution—the family trees.
“I tend to start from the pattern side,” says Roalson, adding that the pattern of relatedness can often shed light on the processes that led to the species being the way they are today.
The difficulty with sedges, other than their tiny, drab flowers, is that they seem to disregard the rules of chromosome behavior that guide other organisms. Any given species may contain chromosomes that have been duplicated, fragmented, or rejoined, in various combinations. Nobody knows yet how the plants survive with all that turmoil at such a basic level of cell structure. What’s clear is that these chromosomal hijinks provide a lot of opportunity for species to try new (mutated) forms of genes without paying the price of extinction if they don’t work out. A duplicated chromosome gives a plant a “free” copy of hundreds or thousands of different genes. Since the plant still has its original, “correct” copy of all the genes, mutations in the extra copies may not hurt the plant. It’s a great way to experiment. Like a writer saving a copy of a first draft, if the next draft isn’t good, you can go back to the original.
Roalson is hoping the family-tree approach will help him understand how the variations in chromosomes might have led to the formation of new species of sedges, and help him untangle the confusing state of affairs among African violets. With thousands of species in the group, and a vast array of flower forms and colors, the African violets have sparked many a late-night debate at botanical conferences.
Distantly related species can have very similar flowers, while closely related species often have very different kinds.
“And that is nonintuitive,” says Roalson, “if you just think that similarity should convey some idea of relationship.”
A big question lurks in those statements. How does he know how closely related two species are, if their flowers are so different?
He knows because of their DNA. Roalson figures out the family tree by sequencing multiple genes of the species he’s interested in. New technology enables him to spell out the instructions on the DNA-the exact sequence of A, T, C, and G-and compare it to the same genes in other species. There will be fewer differences in the DNA of two species that are closely related than between two species that are more distantly related. It’s like a person doing genealogical research finding the “family resemblance” in an uncle or cousin rather than a great-great-grandmother. The DNA sequences provide the family tree; then he can look at flower form and other visible characteristics and see how they fit within that pattern.
Roalson suspects the variety of flower forms says a lot about how the different species evolved—how one species might have split to form two.
“If you have variation in flower form, then you could have selection for different kinds of pollinators, and that could easily drive speciation,” he says. One population of a species could favor a hummingbird as pollinator, gradually evolving a longer, narrower flower tube with a cache of nectar at its base; a neighboring population could favor bees as pollinators, and evolve a broader flower form that would offer bees a stable landing platform. Over time, as the differences in the flowers became more pronounced, the two populations would no longer be able to share the same pollinators—which means they could no longer interbreed. At that point, they would be different species. Still very closely related, still living next to each other, but no longer sharing genes and co-parenting offspring.
The traditional view—Darwin’s view—of how new species form was that when two populations of a species become geographically isolated and no longer interbreed, they may over time become so different from each other that they are no longer the same species.
But even in Darwin’s day, a few odd cases didn’t fit that scheme. They seemed to show speciation—the origin of a new species—can occur without geographic barriers. One famous case happened right here in North America in colonial times, says Carol Anelli.
When European settlers first arrived on the continent, one species of apple maggot infested the haws, or apple-like fruits, of hawthorn trees in the Hudson River valley. The adult flies mated on the hawthorn tree and laid their eggs in the young fruit. Most of the flies only visited hawthorn, but a few took a liking to the apple trees planted by European newcomers. By the mid-1800s, the valley was home to two types of apple maggot flies: the original, still at home on hawthorn, and an emerging species that infested apples.
This discovery, and others like it, led biologists to amend Darwin’s theory of how new species arise. Geographic isolation is still regarded as the most common route to speciation, but we now know that other forms of isolation can be just as effective at preventing two populations from interbreeding.
“In this case, these insects could be very close to one another geographically, but they’re separated from one another because of host-plant preference,” says Anelli. All it took was for a subgroup of the original species to develop a preference for apple over hawthorn, which separated them from haw-preferring flies, and they were on their way.
Who loves ya, baby?
Having more offspring than your competitors is the key to evolutionary success, but it’s not always easy to tell which adults produce which young. Although close observation can reveal who spends time with whom, recent advances in DNA profiling show that time together doesn’t always mean what we think it means. Take, for instance, a pair of songbirds working hard to feed their clamoring youngsters.
“The assumption up until a decade or so ago was that when you see a male and a female at a nest together raising young, that all the young in that nest belong to that male and female,” says Mike Webster. “And then somebody decided to test that genetically—and surprise, lo and behold, not all the young in that nest belong to those parents!”
It turns out that birds, long held up as models of dutiful monogamy, are in fact randy little rascals, and many bird societies are cauldrons of adulterous hanky-panky. Most kinds
of birds engage in at least an occasional extra-pair mating. Some mate with others more often than with their partner. In the Australian fairy wrens Webster studies, nearly half—half—of all chicks are sired by a male other than their mother’s social mate.
Surprising as it was, that discovery helped explain something that has puzzled evolutionary biologists ever since Darwin-the bright plumage, dazzling songs, and flashy courtship displays so common among male birds. The puzzle was how birds could evolve traits that would make them more conspicuous to predators.
Webster says Darwin came up with the idea of “sexual selection,” a form of natural selection in which competition for mates drives the evolution of key features. If bright feathers and loud songs enable male birds to have enough offspring to make up for the greater risk of being eaten by a predator, those traits will evolve.
There was a problem with that idea, though. Most songbird males don’t seem to compete much for access to partners. In Webster’s wrens, some of the males have bright red or orange plumage; others are drab brown. Both kinds get a social mate without much fuss. So where is the sexual selection?
The discovery of extra-pair matings may have solved that puzzle. The males do compete—after they’ve found a social partner. And it is quite a competition, complete with offerings of flowers to the object of their desire.
“It’s a really beautiful display,” says Webster. “The males present flower petals to the females. They’ll pick a flower petal, and they’ll fly in and present it to the female. They fluff up their feathers, and they dance around. It’s just spectacular.”
Webster wants to understand the competition from the perspective of both the females and the males. Using observations of the adults and paternity tests on every chick in a population, he’s found that female fairy-wrens don’t mate with just any extra-pair male that comes along. Rather, they show a strong preference for more brightly colored males. Webster thinks a female accepts her first suitor as her social mate, regardless of what he looks like, just to get started on a family. Later she copulates with higher-quality males if her partner is not such hot stuff. Webster doesn’t know yet why females prefer the colorful males. Bright color may indicate better food-finding skills or better resistance to disease; or the preference may have arisen by chance.
For male wrens, stepping out seems more straightforward. Males who mate with more females sire more offspring and pass more of their genes on to the next generation. During one season, one male Webster’s group studied sired 15 chicks, 10 with females other than his social partner.
Extra-pair mating may also be a form of insurance for the males.
“If a male has all his eggs in one basket, so to speak, or all his young in just his social mate’s nest, and a predator gets that nest, he has zero reproductive success for that year,” Webster says. “From that perspective, I don’t think they care which female they mate with. They just want to get their young out in several nests.” And if flashy colors help them do that, there will be strong selective pressure to go for the glitz.
Following the questions
Horned beetles that don’t always grow horns; sedges with mixed-up chromosomes; African violets fitting their flowers to their pollinators; adulterous fairy-wrens. None of these are finished stories. Their evolution, and the research into it, continues. There is no final answer, only deeper questions—many of which lead back to the far-seeing work of Charles Darwin.
The difficulty with Darwin’s ideas, among the scientists of his own day, stemmed from the fact that no one had a clue how natural selection could work. “Darwin didn’t know anything about genetics,” says Webster. “The scientific debate over whether evolution had occurred or not died within a few years of the publication of that book [On the Origin of Species]. The debate that continued on was how. What were the mechanisms?”
When Mendel’s work on the laws of inheritance was discovered in the early 1900s, biologists finally could “put teeth into the theory,” as Webster says. Since then, evolutionary biologists have found that even without knowledge of genetics, Darwin got the general outlines of the story, and many of the details, absolutely right.
In recent decades an explosion of new molecular techniques has enabled researchers like Corley, Roalson, and Webster to pursue many of Darwin’s old questions, as well as pose some new ones. If they run into a door they can’t open, a question they can’t answer, they may shift their focus to another organism or another problem. But they will keep the hard question in mind; and when they learn of a new technique that could help, they’ll return to it. Science runs on curiosity and patience. “Unanswerable” questions are an invitation to further thought; they are never a reason to stop searching.
“There’s a difference between ‘We don’t know yet‘ and ‘This is unknowable,'” says Eric Roalson. “Sometimes people interpret uncertainty as, well, this is something we won’t ever be able to figure out. These are very complex systems, and we have made great strides over the last hundred years in trying to understand what’s going on. There’s still a lot that we don’t understand fully. But we have ideas and pieces of the puzzle.
“I think that if we want to know it, eventually we can know it.”