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.
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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?