Monophyletic Girlfriend

[jack]

Biology[1]

A "taxon" is a taxonomical unit.

A "monophyletic taxon" is "all the species descended from a common ancestor". E.g. Mammals. E.g. Birds. E.g. insects. For many purposes, this is the obvious grouping to use. A monophyletic taxon is also called a "clade"

A "paraphyletic taxon" is "all the species descended from a common ancestor, except some". E.g. dinosaurs are "everything generally dinosaur-like, but not birds". E.g. invertebrates are "all animals, except the vertebrates". A "polyphletic taxon" is one including several different groups but not their common ancestor. E.g. "everything vaguely creepy-crawly" or "everything bipedal" or "everything flying" etc.

There are definitely reasons to classify animals in ways other than their genetic similarity, but it's not normally biologically helpful, so polyphletic taxa are normally a plausible but embarrassing hold-over, as to a lesser extent are paraphyletic taxa.[2]

Pedants will have noticed that any non-monophyletic group on a planet where all life has a common ancestor could probably described either way with judicious inclusions/exclusions. Congratulations, have a cookie! However, fortunately, these terms tend only to be used when people are talking about comparatively similar groups, so it's normally obvious if the group is "all, except" or "all, plus"

Mathematicians

When someone first mentioned monophyleticity to me, something when click. As a typical mathematician, I think my first thought was "Thank you! Now, knowing the best way of classifying things, I can immediately forget all the others," and all the terminology immediately fell out of my head, except that the obviously right way was to group together common descendants, and there were a bunch of other ways that were still used out of tradition[3].

Mathematicians expect to spend thousands of years coming up with a proof, and then decades seized with a perverse urge to hone and polish it until it looks easy. That's just how it works.

But it reacts badly when mathematicians meet another discipline that expects you to, for instance, learn thousands of years of mistakes (made by very very very intelligent and learned people) and properly appreciate how hard it is, before telling you, "OK, but now forget all that bullshit you learned to respect the journey, the right answer is [Predestination is true and free will can't be defined well enough to be determined true or false without being trivial]" or whatever.

Language

I realised I was doing exactly the same thing with monophyletic with other words. I instinctively treated "wife" as a subset of "girlfriend", not an exception to it. E.g. I think the right answer to "do you have a girlfriend" would be "yes, I have a wife" not "no, I have a wife". Even though I know that's not what the word usually means.

Footnotes

[1] With thanks to Liv, Ptc, etc who explained this to me. Apologies for mistakes.

[2] Or do some people use these words in a completely different way?

[3] I assume it's more complicated than that, since biology usually has messy edge cases the neat categories only partially fit? With bacteria, maybe?

Comments

[azurelunatic]

"Actually" has an admittedly deserved reputation for being used by pedants for making corrections that are doubtless very significant within the field, but that people outside of the field might not care about in the slightest.

I think the right answer for "Does jack have a girlfriend?" is neither "yes" nor "no", but "actually...".

[andrewducker]

The fact that "invertebrates" includes "vertebrates" tells you that paraphyletic taxonomy is frequently very useful...

[ptc24]

I think the reason we have the word "invertebrate", and don't have "non-arthropod" or "non-mollusc" might have something to do with what sort of creature we are. Sentient octopodes, for example, might feel differently about which possible taxa are worth specifically naming. For that matter, they might also have a sense of the word "animal" that includes humans and cuttlefish but excludes sentient octopodes.

I now have thoughts of how one octopus could insult another by calling it a squid, an Architeuthis, a Vampyroteuthis, a cuttlefish, a nautilus, a snail, a mussel etc.; all insults with different connotations.

[redbird]

Sometimes it's because terms got into non-technical language: "reptile" is paraphyletic whether or not you include birds, but non-specialists seem to want a word that groups snakes with turtles and lizards.

"Sponge" is worse: a grouping that makes a good deal of sense in terms of body plan and ways of living, but consists of a few related taxa and almost none of their descendants. To make the group monophyletic you'd have to include (almost?) all living animals and most of the extinct ones, from worms to butterflies to us, and we already have a word for animal.

[andrewducker]

Yup, there are many way of differentiating groups of animals, and "by their distant ancestry" isn't necessarily the best one for every situation.

A little gentle ribbing between paleontologists

[hairyears]

Bacteria are a particularly difficult case because of horizontal gene transfers: in the absence of a common definition of a 'core' or subset of the genome that reliably denotes descent, the edge cases in bacterial taxonomy are a mixture of statistical measures of 'relatedness' and arbitrary decisions based on 'marker'.

Gene sequencing transformed taxonomy and, while it is gratifying to see that the 'anatomical' phylogeny published in previous centuries is *mostly* correct, there were some major changes; and minor ones are still emerging, even in relatively well-known genera.

[ptc24]

Words: there's the confusingly named "hypernym" and "hyponym" for cases where term A (the hypernym) is a superset of term B (the hyponym) - however, I don't think there's a single word for "sister term". One thing that seems to come up quite a bit is confusion over whether two words are hyper/hyponyms or sister terms (or possibly synonyms, or possibly even partially-overlapping terms).

Of course, when it comes to philosophy, you can come up with some nice pithy statement, but soon you learn to ask the question "OK, so what do all of those words mean?", and learning the history might help you to understand all that. And since when did philosophy have established results?

[forestofglory]

My training and inclinations are in ecology. As such I'm not very interested in molecular biology, and DNA sequencing though I do have some interest in evolutionary history. I think modern taxonomy is a bit too focused on clades. The focus on clades right now is very much a focus on DNA sequencing. Some times it is useful to have other types of grouping, based on ecological function are more useful. (e.g the Shredders, Collectors and Grazers used in the River Continuum Concept ) Also I don't want to be a fish :)

Also plants are quite good at confounding monophyletic groupings, as they are busy doing strange thing with lots of chromosomes.

[naath]

I think it's good to have a word that means "life descended from this common ancestor" 'cos maybe you want to think about how genetics works an' stuff.

Then again you might want to talk about how some particular feature is useful; or how similarly shaped life can occupy different niches. And then the fact that two creatures look the same might be more relevant then whether they are closely related, or at least orthogonally relevant.