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Carex lupulina: Hop Caric Sedge

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.

^{Robert Hubner}

“They’re both difficult, but for different reasons,” says botanist Eric Roalson (left). 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.

Some species of African violets in the genus Achimenes have similar flowers despite being distantly related (farther apart on the "family tree" shown here), while species that are more closely related can have flowers that look quite different in color and form. Photos for illustration provided by Eric Roalson.

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.

Different strokes

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.

^{Courtesy Carol Anelli}

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 (left).

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.

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Continued