Sunday, December 15, 2013

Why plant names change

This article written by Lucinda McDade was originally posted in Oak Notes, the newsletter of the Volunteers of RSABG. Because it answers questions that come up frequently, we thought we'd share it here too.

Why do those annoying scientists keep changing the names of plants? In this month’s column, I 
am going to attempt to convince you that name changes reflect progress and should be embraced with enthusiasm owing to the information that they convey. First, though, let me agree: names changes are annoying. I rather like Senegalia greggii (formerly Acacia greggii), and I am warming up to Erythranthe (for the sweet little monkeyflowers that grad student/conservation botanist Naomi Fraga studies), but Acmispon and Hosackia (both formerly Lotus) just don’t roll off the tongue smoothly and still leave me a little cold. Nonetheless, I too must  learn to say Acmispon and Hosackia with grace. 

So why do scientists keep changing plants’ names? The answer is actually pretty simple: because we have learned more about the “ family tree” (i.e., evolutionary relationships) of the plants in question and seek to reflect that knowledge in our scientific naming of the plants. We could of course keep knowledge about relationships and names separate, but biologists have almost unanimously agreed that we should use the scientific names of organisms to convey information about relationships. Interestingly, by following a few simple rules in assigning scientific names to organisms, we can do this quite readily. Here we have the culprit: because we are directed to depict patterns of evolutionary relationship when naming organisms, names sometimes must change as our knowledge of relationships grows. Thus the fact that the names of our monkeyflowers have changed to Erythranthe and Diplacus alerts us  that modern work to understand the evolutionary  history of the genus Mimulus has revealed that all plants with monkeyflower-like flowers are not each other’s closest relatives. The evolutionary tree for Phrymaceae indicates that the plants that have been referred to as Mimulus occur in three different branches of the tree. More closely related to each of these evolutionarily independent monkeyflowers are plants that have never been referred to as Mimulus and that have flowers that are quite different from what we expect monkeyflowers to look like.

When advancing knowledge demands name changes, in order to avoid total chaos, we follow the rule that the scientific name stays with the type species. Think of the type species as providing a sign post for where the genus name belongs. In this case, the type species, M. ringens, occurs in eastern North America and species on that branch of the evolutionary tree retain the name Mimulus. The two branches where the western monkeyflowers are placed are now referred to as Diplacus (the woody perennial subshrubs like D. aurantiacus) and Erythranthe, which are the diminutive annuals that Naomi studies. It is notable that both of these genus names far pre-date the phylogenetic results that are now our basis for using them again: our predecessors had an inkling that these groups existed! Knowledge of the patterns of relationship among these plants tells us that the suite of floral traits that makes a monkeyflower look like a monkeyflower may well have evolved multiple times rather than just once. 

The story of California fuschia, formerly referred to as Zauschneria, is the converse of the monkeyflower story. Here, the results of modern evolutionary studies show us that the handful of species that had been called Zauschneria are just a small twig on the large phylogenetic tree that is the genus EpilobiumFixing this problem of mismatch between names and evolutionary patterns could involve breaking Epilobium (with as many as 200 species) into a large number of genera, but it would be challenging to point to traits that would let us tell one from the 3 other, which is almost as annoying for users of taxonomic names as is changing names! Also note that this path would create a lot of new genus names. The alternative path has been chosen: Zauschneria has been moved into Epilobium. This change should  lead you to understand that the remarkable large red, hummingbird-pollinated flowers of Epilobium canum likely evolved from less charismatic flowers that mark most species of Epilobium. This is interesting and makes us want to know more 
about the evolutionary history of these plants.

The take home message is that change 
is annoying, but learning is good (as I know that the many of you who are teachers will agree!), and the fact that knowledge is advancing rapidly is great. I am convinced that it is well worth our while to learn these new names because they instruct us about evolutionary relationships of the plants we love. The other simple fact is that for people just starting to learn plant names now and for all those who come after them, new names are not in fact “new” but are just names!

 I am going to change gears now and fill in a bit of the background on the science that is behind the discoveries that are behind these name changes. If this is too much information for you, feel free to skim/skip. Also, let me know whether you like or do not like this sort of information. That will help me make these columns useful and interesting to you. These are very exciting times in the branch of  biology—systematics—that has responsibility for documenting and understanding biodiversity—the living organisms that share planet Earth with us. Since the time of Darwin, it has been understood that our goal is to unravel evolutionary relationships. Organisms are related to each other to different degrees, owing to evolutionary patterns—to recency of shared ancestry. Thus, just as you and your siblings are more closely related to each other than to any other human beings owing to sharing the same parents (i.e., very recent common ancestors!), so oaks are more closely related to other oaks than to ashes or alders because they stem from a common ancestor that lived much more recently than did the ancestor shared by oaks, ashes and alders. 

Despite this widespread agreement that we seek to unravel evolutionary relationships, until about 50 years ago, it was not clear how to discover these patterns of relatedness—how to organize our observations of similarities and differences between organisms to reveal degrees of common ancestry. A very clever German entomologist, Willi Hennig, figured it out in the 1960s and this sparked a burst of energy in systematics. Next, during the late 1980s and into the 1990s saw the development of methods to gather DNA sequences (yes, the exact order of the Cs, Gs, Ts, and As in genes—happy to show you in our molecular lab how this works!). This provided incredibly powerful information for deducing relationships. By the way, both the principle contributed by Hennig and the DNA methods are elegant to the point of simplicity—the sort of thing that, once you understand, you  feel that you could and should have figured out on your own! A final ingredient “stirred in” over the last couple of decades has been advances in analytical methods. The field has recruited many extremely talented mathematicians who are devising analytical methods that follow Hennig’s principle while taking advantage of the huge amounts of data that can now be amassed in the molecular lab. That fertile mix has yielded a revolution in our understanding of how organisms are related to each other. 

Those of us contributing to the revolution undertake research that yields tree-like, branching diagrams that depict patterns of relationships among the organisms that we study. As noted above, by following some simple rules when we assign scientific names to organisms based on relatedness, we can convey information about relatedness in our biological classifications. The most important of these rules is that any taxon (e.g., family, genus) must include all of the descendants of a common ancestor. These are very exciting times in the branch of biology—systematics—that has responsibility for documenting and understanding biodiversity—the living organisms that share planet Earth with us. only the descendants of that common ancestor. The knowledge that Zauschneria is a twig on the tree that is Epilobium can be restated more “formally” as: Zauschneria shares a common ancestor with some species of Epilobium (those on the same branch of the Epilobium tree) more recently than any common ancestor that ALL Epilobium share. This  means that we cannot recognize both Epilobium and Zauschneria because this would break the “all  descendants” rule, and we will not be able to  understand the relatedness of these plants from the classification. I hope that makes sense – it is actually easier to “get” from a diagram and I’d be happy to go over that with any of you who might be interested. 

 One more point before I leave this topic: because the name changes that we are now experiencing are based on the above described three major advances in our field, there is every reason to believe that they will achieve a level of stability that we have not had in the past. Although we can’t promise that this round of name changes will be the last, there is every reason to believe that they are a major step toward achieving stability

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