A friend texted me today to share a ‘crazy’ biology fact – that ‘a slug is not a critter but a fungus’. Of course, I had to protest immediately. A slug is not a fungus, it is a mollusk! (Which I guess would colloquially be a critter?) This interchange made me think about how that kind of statement translates for an evolutionary biologist. When we say, a [species] is a [taxonomic group], we are saying, that species belongs to a clade, a branch of the tree of life, that we have given a name. So it’s really a statement about evolutionary history. A slug does not belong to the clade that we call fungi. A slug is in the mollusk clade, which is a branch of lophotrochozoans, which is a branch of animals, which is a branch of eukaryotes, etc. So a slug *is* all of those. The limit on the number of these statements is only the number of nodes (and corresponding clades) which we have assigned names. Since all species share common ancestors back in time, at some point, the slug’s history will intersect with the fungus; that is, we will eventually arrive at a node which sits along both of their evolutionary histories. Currently, this node is the opisthokonts, whose descendants are eukaryotes with a single posterior flagellum. Naming these nodes give us a handy way to remember traits that the species belonging to that clade possess (although some may have subsequently lost those synapomorphies). So then you might wonder, how do we decide which nodes to name? Anyone who publishes a phylogeny is free to name any node they wish, although typically only nodes that are well-supported by various sources of evidence would be named. Among those with good support, authors often pick those that correspond to some previously named group (like a genus) in order to carry that information (whatever characteristics originally resulted in the group's description as a separate genus). Many nodes however don’t contain members that had a traditional name (pre-phylogenetics), but nonetheless correspond to a group with some distinctive features, like the Opisthokonts. Thus, taxonomy and classification return to the same principles that have long guided the field – that names function to convey information about organisms – about their evolutionary history (their relatedness to other organisms) and their traits. In this sense, naming is a choice -- about which information we think is interesting or important to communicate. I always think if bacteria were doing the taxonomy instead of humans we would have many fewer named nodes within the animal clade and many more along branches in their part of the tree.
P.S. The title 'naming nodes' will only be entertaining to those of you who are also fans of Best in Show. Taxonomy is a far more valuable endeavor than naming nuts, but certainly less funny.