Cache la Poudre River in Rocky Mountain National Park. Photo by Inkknife2000, Wikimedia Commons
(Click on this link to listen to an audio version of this blog ….Taxonomic hierarchies
Last week I wrote about the very human need to put names on things. These names range from somewhat imprecise “common names” to extremely precise “scientific names.” Once an organism is named, though, it then has to be put into its place in the interconnected array of other living things. One way to do this is via the hierarchies of taxonomy: species, genus, family, order, class, phylum and kingdom.
Species: organisms belong to the same species if they can interbreed and form reproductively viable offspring. This is the essence of what is called the “biological definition” of a species. This definition, though, is often very difficult to apply. What if the organisms you are looking at are dead? What if they are fossils? What if they reproduce asexually? What if they are very hard to observe out in their natural habitats?
These difficulties have led to a much more common definition of a species: the “typological” definition. In this definition individual organisms are considered to be the same species if they all “look alike,” and, more precisely, if they “look like” the “type” specimen that is stored in a museum collection and has been very precisely described in the scientific literature.
Now, the line at which organisms pass from being “similar” to being “the same” is incredibly subjective. This line, in fact, is the subject of many professional arguments in the scientific literature and active, vocal disputations at biological conferences and congresses. Lifelong feuds and remarkably immature behavior have resulted from esoteric disagreements about degrees of similarity and difference and the designation of species status. I don’t know of any physical violence that has come of these deep disagreements, but knowing the sizes of some of the egos of the scientists involved, I would not be surprised at reports of anything from fistfights to attempted murder!
Phyllium asekiense. Photo by Dragus, Wikimedia Commons
Life, though, can make this typological species definition difficult to apply. There are many species in which males and females differ greatly in appearance, and, sometimes, these males and females end up being classified as entirely different species! I talked about one of these species complexes back in Signs of Spring 13 (May 27, 2021) when I described the fate of a batch of rare, leaf insect eggs that had been laid by a wild-caught leaf insect (Phyllium asekiense) from Papua New Guinea.
These eggs were hatched and the resulting nymphs were reared by researchers at the Montreal Insectarium. Sadly, only three of the thirteen eggs resulted in fully mature, adult leaf insects. One of these adults was a typical, green, wingless, leaf-shaped P. asekiense individual which was, like all known P. asekience individuals, a female. The other two nymphs, though, grew into stick-shaped individuals and even sprouted wings. Amazingly, these individuals took on the form of leaf-insect species that was from an entirely different genus (Nanophyllium), and they, like all individuals in the genus Nanophyllium, were males.
In a paper published in the journal ZooKeys (September 17, 2020) the Montreal Insectarium researchers and their collaborators at City University of New York described the hatching outcome of the P. asekience eggs and established a new, united species name for the taxonomically split male and female leaf insects: Nanophyllium aeskience.
As The New York Times reported it: “He was a stick, she was a leaf, and together they made history!”
Genus: organisms that are different species may be extremely similar and/or very closely related evolutionarily. These very similar species can then lumped together into a “genus.” Again, the same problems we discussed concerning species apply to the construction of a genus: How similar is similar? How different is different? What are the degrees of variation that cause us to separate individuals into different species but lump them into a common genus? Why aren’t they all one species? No wonder taxonomists argue all the time!
My grandson Ari. An excellent example of Homo sapiens! Photo by M. Hamilton
The scientific name of a species, by the way, utilizes that organism’s genus and species names. We are in the genus “Homo” (along with eight other, now extinct, human species) and have given ourselves the incredibly lofty (and also hopeful) species name “sapiens.” Our scientific name, then, is “Homo sapiens.” The common nightcrawler is in the genus “Lumbricus” (along with 700 other species of earthworms!) and has been given the species name “terrestris.” So its scientific name is “Lumbricus terrestris.”
All of these problems and questions that we have seen in separating species and genera continue with the subsequent, higher taxonomic levels. You have to constantly ask: how similar is similar? How different is different? When do we lump things together, and when do we split them apart?
Family: organisms that are in different genera may be similar and/or very closely related evolutionarily. These very similar genera may then be clumped together into a taxonomic “family.”
Order: a lumped together group of very similar and/or evolutionarily closely related taxonomic families.
Class: a lumped together group of very similar or evolutionarily closely related taxonomic orders.
Phylum: a lumped together group of very similar or evolutionarily closely related taxonomic classes.
Whooping crane with Sandhill cranes (both Phylum Chordata, Class Aves! Photo by USFWS, Public Domain
As you move up into these higher and higher taxonomic levels the features that are lumping organisms together or splitting them apart get more and more fundamental. Phyla (also called “Divisions” in plants) are said to represent some fundamental body plan shared by all of the organisms included within them. There are between 31 and 40 phyla of animals, 14 phyla of plants, 8 phyla of fungi, 19 phyla of protists, 29 phyla of “modern” bacteria and 5 phyla of “ancient” bacteria (the Archaea).
The ultimate goal of taxonomy, then, is to group together organisms in their Phylum, Class, Order, Family, Genus and Species via their increasing degrees of evolutionary closeness and similarity! These taxonomic designations, ideally, represent a flow chart of evolution itself. Most of these evolutionary “trees” have been constructed by experts who consider the similarity of the physical characteristics of the organisms (in other words, by using the same techniques that I described in the “typological species’ definition: they lump together the organisms that “look alike” and split off those that don’t! ). Unfortunately, evolution doesn’t always work linearly on an organism’s appearance!
Evolution fundamentally works on the DNA of an organism. As researchers have been able to more easily and more completely sequence the DNA of an increasingly large number of species, they have begun to construct taxonomic designations based on DNA nucleotide sequences. These gene-based taxonomies are, logically, a much more precise way to visualize evolutionary relationships, but they often do not match up very well with the typological characteristic taxonomies! Appearance, for centuries the gold-standard of taxonomy, then, is not always an accurate reflection of genetic changes!
What a great opportunity for more acrimony in taxonomy!!
Next week we’ll see how different taxonomists lump phyla together into kingdoms! So buckle up your taxonomic seatbelts and get ready for even more controversy!