Meet the Plant Detective

Move over Sherlock Holmes. In mysteries involving the discovery and classification of the flora around us, the legendary fictional detective would be no match for evolutionary biologist Michael Simpson.

Simpson and his students and colleagues at San Diego State University are credited in the field for unearthing six new plant species and two new varieties and for “rediscovering” another species thought to be extinct.

Moreover, his formidable sleuthing talents aren’t limited to botanical analysis. Simpson has served as an expert witness in two local murder trials, a distinction that may be unique among SDSU faculty.

Most days, though, his work doesn’t make the tabloids. Simpson and his gumshoe students use DNA sequencing and high resolution scans to find missing links in the centuries-old stories of plant species migrations.

Evolutionary history

They can reconstruct what are called "phylogenetic trees" of plant groups, which depict the pattern of an evolutionary history that spanned millions of years. From these trees, they can estimate the timing of divergence--that is the splitting of one ancient population into two.

Simpson and one of his former students discovered evidence that species of a plant genus called Cryptantha, more commonly known as popcorn flowers, may have been dispersed thousands of miles in the past, likely carried by birds that migrated long distances in a single flight.

They also found indications that after Cryptantha dispersed from North to South America, some species became perennials and developed a rare type of reproductive biology called cleistogamy.

These flowers do not open up as most cross-fertilized flowers do, but always self-fertilize, perhaps an adaptive response to the initial absence of pollinators in their new South American home.

Back from extinction

“At the heart of our work is the investigation of evolutionary history,” Simpson said. “We ask why species diversified only at certain times. For example, species of the plant genus Pogogyne or mesa mints, three of which are endangered, live only in vernal pools, and so we can infer that its group, known as a clade, didn't diverge until vernal pools were formed.”

Simpson continues to have a special affection for Cryptantha, the popcorn flowers and is currently working on naming and classifiying the North and South American members. This group of more than 100 species is mostly ignored by botanists because identifying them requires meticulous scrutiny of their tiny fruits, called nutlets.

One North American species, Cryptantha wigginsii, was assumed to be extinct for more than 80 years until Simpson and his students discovered specimens close to home--in San Diego and Riverside counties and on Catalina Island. News of their find was published in the journal Madroño.

In the courtroom

Simpson’s extensive knowledge of taxonomy, so valuable in the classroom, occasionally finds an audience in the courtroom as well. Called as an expert witness in a murder case, he was asked to analyze a one-inch sliver of wood found on the victim’s skull.

By studying the wood cell structure under a microscope, he identified it as ash and later learned that the murder weapon was a baseball bat—manufactured from ash.

In a second case, police asked Simpson to identify samples of three desert plants found on the molding and inside the trunk of a car owned by a missing San Diego woman. He told police where the plants grew, they peppered that area with flyers, and hikers found the woman’s body a few days later.

The SDSU Herbarium

Simpson came to San Diego 30 years ago after earning a Ph.D. from Duke University. He has taught Economic Botany, Plant Systematics, and Taxonomy of California Plants in addition to curating the SDSU Herbarium, a treasure trove of nearly 22,000 plant specimens.

The collection, classification, and preservation of plants is a never-ending task for evolutionary biologists. Simpson and his students have searched the length of California and beyond for new and unusual species, and Simpson recently received a grant from the National Geographic Society to extend his search to Chile and Argentina.

The specimens are pressed, mounted, and finally stored in the herbarium, a climate-controlled room containing plants collected over more than a century. The oldest specimen dates from 1874.

Twice annually, the specimens are frozen to kill any insects that may have breached the giant metal cabinets where they’re housed.

"Keep away moisture and insects, and these plant specimens can remain in good shape for hundreds of years, and the DNA inside them can persist for decades," said Simpson.

For each new sample, students create a high-resolution scan, geo-reference it and add it to online databases. Simpson and his crew are also taking high resolution photographic images of the tiny popcorn flower nutlets and posting them online with detailed information. All of these images aid in the identification of species and constitute an important resource for professional botanists as well resource managers.

Given that San Diego County has the greatest diversity of plant species in the country, Simpson and his intrepid students still have a lot of detective work to do. "We have at least three new species waiting to be described," he said, "and many more to properly classify."