Monophyletic Girlfriend
Oct. 12th, 2013 11:28 pmBiology[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?
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?