Drawing the tree of life notes on biological taxonomy and the species concept Biological taxonomy is the science of grouping organisms (plants, animals, anything alive) and establishing relationships between the groups. In biology, such groups are called taxa. The term taxon encompasses a group at any level of generality in the biological hierarchy: in the Linnaean system, these levels include species, genus, phylum, and so on. Ereshevsky describes four different ways of accomplishing this task, each of which has adherents in the scientific community. In evolutionary taxonomy, there are two potential means by which new groups, or taxa, can be generated, cladogenesis and anagenesis. In cladogenesis, a species A is split into two groups, A and B, probably when the predecessor of B entered a new environment. B branches off from A. In anagenesis, an entire species A evolves throuh mutation into species B; B’s predecessors remain in the same environment as A, and yet A disappears. Cladogenesis results in monophyletic taxa; each new group includes an ancestor and all and only its descendants. (A leads to A and B, or to B and C.) Anagenesis can lead to paraphyletic taxa, which include an ancestor and some, but not all of its descendants. Here’s an example: We have a species, ProtoReptile. Three groups branch off from ProtoReptile and become new species: ProtoLizard, ProtoSnake, and ProtoBird. ProtoBird finds itself in some very different environments and situations from ProtoLizard and ProtoSnake, and over time all the ProtoBirds evolve into a whole new series of species, Birds. For evolutionary taxonomists, Birds are so different from Lizards and Snakes that they aren’t part of the Reptilia taxa that groups Lizards and Snakes, even though ProtoLizard, ProtoSnake, and ProtoBird are all in the same lineage, descending from ProtoReptile. Evolutionary taxonomists make this claim that Birds are differentiated from Lizards, despite their common ancestor, on the idea that ProtoBird entered a totally different adaptive zone from Lizards, one that led to radically different adaptations (e.g., wings). (One way of looking at this is to say that evolutionary taxonomists establish taxa and relationships between taxa based on environmental changes as well as changes in heredity.) In contrast, process cladists are not convinced that changes in adaptive zones are sufficient to result in new taxa. For process cladists, the only process that creates new taxa is cladogenesis, and thus only monophyletic taxa are allowed. For process cladists, Birds, Lizards, and Snakes are equally related to ProtoReptile, no matter that Birds live in a totally different environment from snakes and have thus become very different organisms. Cladists decide how to relate taxa by determining the most recent common ancestor. They do this by isolating shared derived characters, or synapomorphies. A synapomorphy is present in an ancestor and all of its descendants, but not in the ancestor’s ancestor. For example, let’s say that ProtoLizard’s tail could regenerate, a feature not originating in ProtoReptile and thus not shared by ProtoSnake or ProtoBird. Tail regeneration is a shared derived character, or synapomorphy, in ProtoLizards and Lizards (and in other descendants of ProtoLizards, say ProtoCrocodiles). All biologists believe that evolution is the best explanation for the progress of species over time. However, evolutionary taxonomists and process cladists look to the mechanisms of evolution as the justification for their classificatory principles. If the point of taxonomy is to illustrate the history of different species (in order to predict what may happen with current species, perhaps), then taxonomy should be informed by what we know about evolutionary processes. Pheneticists, however, aren’t sure that our knowledge of evolution is sufficient to make correct decisions about the existence and relationships of taxa. Pheneticists, therefore, want to establish and relate taxa based on comparison of any characteristics. (Process cladists only consider synapomorphies; evolutionary taxonomists consider any homologies, or any trait passed down by ancestors, even when these aren’t shared amongst all child taxa; pheneticists consider homologies and homoplasies, traits that occur in different lineages but aren’t passed down from ancestors, for example if Lizards and Snakes independently evolved forked tongues, even if ProtoReptile didn’t have one.) Finally, pattern cladists, like cladists, only use synapomorphies as evidence for the establishment and relationship of taxa. However, pattern cladists do not assume that these shared derived characters are the product of specific evolutionary mechanisms. They just want to say that the taxa they describe are “out there” and not how or why they are out there. These different approaches to taxonomy influence different ideas of what constitutes a species, the most specific form of taxa. Ereshevsky defines six of these. The biological species concept states that an interbreeding group of organisms is a species. The recognition species concept states that that a species is a group of organisms that would recognize each other as potential mates. (This concept includes groups that might be geographically isolated from each other.) The phenetic species concept states that species are groups of organisms that are most similar to each other. The ecological species concept states that species is a distinct lineage that occupies a specific adaptive zone. The evolutionary species concept states that species are groups of organisms in a single lineage that “have their own evolutionary tendencies,” which may be caused by a variety of processes: interbreeding/gene flow, ecological forces, etc. Phylogenetic species concepts require that species be monophyletic taxa. (If two lineages evolve to enable interbreeding, they would be the same species under the biological species concept, but not in a phylogenetic species concept.) Whew! Let’s see if we can have some fun with this. True Blood Taxonomy The television series True Blood includes some variations on human organisms. Are any of these new or different species? Let’s discuss! (Note: I haven’t read the books and I’m not being too careful about fidelity to the tv universe either...just saying.) Vampires. Vampires “reproduce” asexually, by draining a human of blood and then replacing it with vampire blood. Vampires occupy a different adaptive zone than humans: they do not eat human food, they only drink blood. They burn up if exposed to sunlight. They have different physical capabilities than humans; they are stronger and faster and have incisors that pop out when they are about to bite someone. Also, they do not age. Oh yes, and silver burns them. It is not clear what the differences in genetic material are between vampires and humans. Are vampires their own species? On what basis? Are vampires human? What are they? Werewolves. Werewolves can interbreed with humans; however, a human/werewolf pair produces a human, while a werewolf/werewolf pair produces a werewolf. Werewolves look like humans but can turn into wolves at will. Werewolves have different sensory capabilities (enhanced smell, etc) and their own social structures (packs). They occupy a similar adaptive zone as humans; they live in the same conditions, eat the same food, etc. However, they retain their own communities distinct from humans as well. Are werewolves their own species? On what basis? Are werewolves human? Fairy humans? Sookie Stackhouse has a fairy ancestor. Sookie has some fairy characteristics: she can read human thoughts, her blood is extra-tasty to vampires, and she has strange powers where weird rays come out of her hands. These characteristics have led Sookie to form relationships with vampires and werewolves and to be involved with their communities. Sookie’s grandfather had these characteristics, but her parents (dead) and brother (living) do not. Other humans with these fairy characteristics also seem to exist, outside of Sookie’s family. Let us grant that fairies are a different species from humans. What about Sookie? Is she human or a new species? What about her brother? Taxonomic relationships in True Blood Shapeshifters are like werewolves, but they can turn into any animal they want, and they don’t have the pack behavior that werewolves do. What are potential relationships between humans, shapeshifters, and werewolves, if we assume that all are species? What taxa might be established and arranged, and under what grounds?