Unraveling Plant Ancestry Through Modern Technology
Since Swedish botanist Carl Linnaeus created the modern biological classification system in the mid 18th century, botanists have determined a plant's place among the wild diversity of life on the planet primarily based on its morphology, or form.
But to Bruce Baldwin, a professor of integrative biology at UC Berkeley, what a plant looks like isn't always the best indicator of its species or closest relatives. "When morphological similarity has been the sole line of evidence, it's often been misleading about evolutionary relationships," he says.
Baldwin, whose specialty is native plants of California and the Hawaiian plants that evolved from them, uses DNA testing to resolve plants' ancestry and evolution. The evidence he has unearthed shows that plant evolution can occur more rapidly than once thought, and that plants evolve with remarkable precision to fit sometimes extremely local environments. While botanists have long known that California is rich in native species, Baldwin and his students' work proves that the state's plant life is even more varied and diverse than his predecessors imagined.
Baldwin's most extensive studies have focused on Madiinae, a group of plants in the sunflower family commonly known as tarweeds. The group includes plants that look dramatically different from one another, from the more than 6-feet tall, yucca-like Hawaiian silversword to the tiny California tarweed, Hemizonella minima, often less than an inch in height. "The group has undergone tremendous change for having such a short evolutionary history," says Baldwin.
Baldwin, who is also the curator of the Jepson Herbarium at UC Berkeley, has recently used experimental and genetic methods to explore theories about the evolution of different tarweed species proposed by earlier researchers, including Jens Clausen, David Keck, and William Hiesey, a Bay Area team well known worldwide in botanical circles. The three were pioneers at their time, from the 1930s to 1950s, for incorporating genetics, ecology, and physiology into plant classification. But they lacked the tools necessary to resolve relationships with modern levels of precision.
"There wasn't at that time a way to reconstruct genealogies rigorously, and there certainly wasn't any means of getting at the actual timing of when one species separated from another," says Baldwin.
To determine relationships between different plant species or populations, Baldwin extracts their DNA, then sequences non-coding regions that evolve rapidly enough to provide evidence of recent evolutionary change. Instead of looking for overall genetic similarity between species as in the past, which can be misleading, Baldwin reconstructs relationships based on the fewest mutational steps, or a more explicit model of DNA sequence evolution.
Through these and other techniques, he's been able to resolve questions left unanswered by earlier botanists. One of these questions is the evolutionary history of Layia discoidea ("Discoidea"), also known as rayless layia, a small annual herb.
Unlike other layias, Discoidea has a yellow bloom without rays, or showy petal-like flowers. It lives in serpentine soils, with a mineral composition toxic to many plants, in a small area of San Benito and Fresno counties, and looks so different from other tarweeds that some botanists once thought it wasn't even a member of the tribe.
Clausen's team discovered it was a tarweed and related to Layia glandulosa ("Glandulosa"), or white layia, a ray-bearing plant found in sandy soils throughout the western United States. But the researchers weren't sure which plant came first, and thought that the rayless plant could be an evolutionary relict.
Through genetic testing, Baldwin found that Discoidea split from Glandulosa less than a million years ago. And he discovered an interesting fact about both species. Glandulosa's common name – white layia – is something of a misnomer. While most of the plants have white rays, a rare variety has yellow rays. Baldwin discovered that the rare, yellow-rayed version is more closely related to Discoidea, a separate species, than it is to the white-rayed plants in its own species. That means Discoidea underwent such rapid change that its close relationship to yellow Glandulosa was obscured, except with the aid of molecular data, Baldwin explains. Discoidea even retains a gene coding for yellow ray color, although it no longer has rays, he says.
This example "shows that evolution can proceed at very different rates and in very different ways, depending on ecological circumstances," Baldwin says. California, with its many microclimates and soils, offers a multitude of distinct environments that have shaped plant evolution here, Baldwin says. That means the state likely has many new species yet to be discovered, he adds.
But Baldwin knows if species lose their habitats, they could disappear before being discovered. "If a (species) disappears, we're losing something irreplaceable," he says. "We still aren't knowledgeable enough to say whether these plants have special ecological properties or special medicinal properties," he adds. "But we do know once they're gone, they're gone forever."