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Why plant names change

Catclaw acacia, a large native shrub known botanically as Senegalia greggii used to be called Acacia gregii. The genus name was changed after botanists discovered that it was not closely related to Australian acacias.

Plant names can seem intimidating to new gardeners. Common names are usually vague, and can be confused with other plants. The Latin names can seem complicated — more so when they get changed around. But there’s a method to the madness.

People first started classifying plants thousands of years ago. Initially, it was mainly for keeping track of plants with medicinal uses. In the Western world, plant categorization, or taxonomy, began to come into its own after the exploratory voyages of the 17th and 18th centuries. Botanists brought back many plants new to Europeans, and there was a need to start consistently naming these plants.

In 1753, Carolus Linnaeus was the first to publish a system that was adopted by many — the Linnaean system. He categorized plants based on their morphology — that is, what features they had. For example, plants could be grouped together based on the number of stamens their flowers had. Linnaeus also introduced the idea of using two Latin names — one for the genus and one for the species — referred to as the binomial system — and is thus considered the Father of Modern Botany.

Charles Darwin’s theory of evolution had an enormous effect on the classification of living things. After its publication, taxonomists realized that they needed to classify living things based on their evolutionary history rather than how they look. From that point on, plants would be put into the same genus on the basis of how closely they were related.

Michelle McMahon, director of the University of Arizona Herbarium at the School of Plant Sciences, likens the genus to a last name.

“You're related to your close family members that share that same last name; well, same thing for plants but the genus is first,” she says. As an example, think of members of the Rosa genus such as Rosa woodsii (Woods’ rose) and Rosa californica (California wild rose). “The fact that they’re are all in that [Rosa] genus means that the people who study these things think that they're all each other's closest relatives.

“What we're really after is understanding which groups are on a separate evolutionary trajectory."

When it comes to defining a plant species, McMahon says, things are more complicated. “If you want to see a room full of biologists, disagree, ask them what's a species,” she jokes.

In the animal world, animals that can interbreed are considered of the same species. This is called the biological species concept. “It doesn't work so well in plants because some plants are pretty willing to hybridize ... they just don't hybridize because they're in different areas. And so people use geography to help define species,” McMahon explains. “Most botanists ascribe to a concept [of a species] that's more like a separate evolutionary lineage, meaning it is ... exchanging genes within the species, but not exchanging genes outside — which doesn't mean that it couldn't if you put them in the same greenhouse.”

Plant DNA

Traditionally, botanists looked at flower and leaf morphology, a plant’s biochemistry, and similar characteristics to determine relationships. With the advent of rapid DNA testing in the 1980s, the process of plant taxonomy was yet again turned on its head. Today, scientists can sequence the genes of a plant and determine how closely it’s related to another plant in its genus. Over the past three decades, this has led to a flurry of re-classifications. Entire families have been shifted around once botanists have realized their true relationships.

Reclassifications have also happened at the genus level — for example, our native acacias were thought to be related to the ones in Australia, but once DNA studies were performed, “we realized the [acacias] we have here are not at all closely related to the stuff in Australia,” says McMahon. As a result, in 2005, the International Botanical Congress decided to apply the genus name “Acacia” only to about 950 Australian species, with other acacias being reclassified to new genera. Thus, Acacia greggii, or catclaw acacia, is now called Senegalia gregii.

Sometimes plant species change as well. Usually this is due to a naming error — the same species given two names by two different botanists, for example. The species name with the earliest publication gets chosen.

But who cares what name a plant has? It turns out that it’s important for a number of reasons. First, we need to know what name to call a given plant, and that name needs to be the same whether you’re talking to someone from Australia or someone from South Africa. Common names of plants are notoriously confusing, and many different plants can have the same common name. So the binomial system is a way to standardize the process so we all know what we mean. This has great significance not only for scientists but also for the horticultural trade.

Categorizing plants accurately also tells us a lot about what properties they might have — whether they are good candidates for creating pharmacologically important substances, for example, or whether they are likely to hybridize together to create a new horticultural specimen. Finally, the taxonomic system allows us to keep track of what plants are growing in any given place. We can identify and protect endangered plants, determine what plants are used by native wildlife, and collect specimens and seeds for future preservation. 

If you're interested in learning more (a lot more!) about plant taxonomy, you can take a look at the Angiosperm Phylogeny Group website. You can also check out the most recent code of nomenclature from the International Association for Plant Taxonomy.

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Do you have any gardening topics you'd like to see covered in the Tucson Garden Guide? Email me at dheusinkveld@tucson.com with your suggestions and questions. Thanks for reading!


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