In response to “On Being a Fish” (Vol. 2, No. 3).

To the editors:

One may wonder why human nature seems to have an intrinsic need to categorize and group similar items together. Our natural surroundings, with all their splendid diversity of biological organisms, are not exempt from this habit of ours. The fact is that such categorization of all living things into different clades may prove to be more useful than just being a result of the obsessive-compulsive side of our brain.

Carl von Linné, the “father of modern taxonomy,” is omnipresent in my current hometown of Uppsala and his hovering presence makes pondering nomenclature and its importance for understanding evolution an intriguing pastime. The great naturalist strove to divide all of nature’s organisms into characteristic groups, as he understood that this would help us get to the bottom of things. He is the inventor of binomial nomenclature, which gives each species a name composed of two words, one that determines its “genus” or group of similar-looking—Linné was going mainly by appearance—organisms, and one that sets it apart from all the other similar-looking organisms and defines it as its own species. Linné gave species very descriptive names, often reflecting the most typical characteristic of that particular group. However, as Bret Weinstein has very nicely illustrated, these names are often quite arbitrary and could easily be exchanged with another name that would be equally fitting to the actual appearance of the species. But once a species has received a scientific name, it usually is stuck with it—unless the results of the perpetually-turning wheel of science have revealed that the previous placement of a specific species within a certain genus is really quite wrong. Then renaming a species is possible.

Such happened, for example, with the blue tit, which was originally named Parus caeruleus (which directly translates into “blue tit”) and was thought a classic member of the tit family. The onset of the genomic era allowed applying an evolutionary clock to the tree of life and dating the time of the last common ancestor quite accurately, using the genetic code as a time scale. In the case of the blue tit, it was the sequence of a gene shared by all living eukaryotes—the cytochrome b found in our mitochondria—that gave away its secret. It was not a real tit. It was therefore renamed Cyanistes caeruleus, creating the new genus Cyanistes, which includes another two “blue tit” species, making it a group of three.

Now, such details as to which exact genus and clade a species belongs to may be vital to us biologists, but may seem rather trivial from the outside. It may seem that, as long as we can distinguish a songbird from a raptor, that level of detail should be enough. The nitty-gritty of more recent evolutionary events may seem to be of less immediate importance.

This is an interesting point, and recently I was reminded of this argument when attending a seminar on rates of extinction, as we observe them across different taxonomic groups, and in animals in particular. The aim of the particular studies presented in that seminar was to estimate the total amount of evolutionary time lost with each species that goes extinct.

The reasoning was this: If an extinct species, say, of birds, had a very closely related sister species, the time back to the moment since they shared their last common ancestor is relatively short, in evolutionary terms, say, something like two million years. (This really is not much when it comes to evolutionary time units.) If, however, the extinct species is the only member of this particular clade, we may have to go back many millions of years until we can find the last common ancestor with any other bird species. If, then, this bird species goes extinct, we lose an entire clade and hence a large amount of evolutionary history.

To return to my example of the blue tit, if the Eurasian blue tit went extinct, we would still have two of its sister species, the African blue tit and the Azure tit. Were, however, the highly enigmatic South American Hoatzin to go extinct, we would be left with no other species like it, not even remotely related.

We could now embark on a full-blown discussion of the ethics of conservation biology and whether we should focus our efforts on species, like the Hoatzin, that belong to a clade harboring only one or two species, as opposed to those clades that are very species rich. One might argue that, if a species from a species-rich clade went extinct, the hole it would leave in its ecological community could be more easily filled by a related species. But could it really?

I am not sure there is a correct way to argue about which species we should save from extinction, but thinking about clades is certainly a good way to think about the significance of where we divide different organisms into groups and where we would be prepared to cut the tree of life. I am not sure there is a definite answer to that either, but it is worth trying to understand how the tree of life has been growing over time and that it is certainly worth trying to save as much of it as we can.

Simone Immler

Bret Weinstein replies:

A great man once told me, there are two kinds of people in the world: those who dichotomize, and those who do not. I knew instantly what sort of person I was dealing with.

Dr. Immler is correct that the human mind is obsessed with dividing and grouping entities into categories, but this tendency is not a quirk, it is an adaptation. We break the world into categories as the first step to understanding patterns, and we seek to understand patterns because it puts us ahead.

Dividing things well provides immense advantages. Earth, fire, water and air only gets us so far. Proton, neutron, electron takes us most of the rest of the way, forcing us to confront bosons and fermions in order to go further.

When we invoke “evolutionary biology,” we are describing two disciplines that relate to each other as yin and yang.

The study of phylogenetic systematics, on the one hand, seeks to understand the historical relationships between creatures. To do so, it must control for the effects of adaptation, which causes creatures to seem alike as an adaptive consequence of having faced similar challenges.

On the other hand, the study of the adaptive nature of organisms requires us to control for the effects of phylogenetic relatedness. When a form or behavior has arisen multiple times, we can often infer the forces that drove it to emerge. In order to do this well, however, we must avoid the pseudo-replication that arises when traits are seen in multiple creatures that co-inherited their condition from a shared ancestor. In short, to study adaptation, we must control for shared history.

In systematics, the term for traits that seem alike, but actually represent independent instances is homoplasy. In the adaptive camp, the term is convergence. They describe the same phenomenon, but in the first case it is a hazard, and in the second, a gift.

Homoplasy tricks systematists into grouping organisms incorrectly. Convergence grants adaptive evolutionists the ability to test hypotheses rigorously.

In order for science to uncover the evolutionary nature of organisms, and for others to benefit from these insights, systematists must seek robust, monophylogenetic clades, and taxonomists must produce a nomenclature that ultimately reflects the phylogenetic truth, the whole truth, and nothing but the truth.

Simone Immler is associate professor in the Department of Ecology and Genetics of Uppsala University, Sweden.

Bret Weinstein is a theoretical evolutionary biologist at The Evergreen State College.