Classifying life

How do you go about organizing organisms?

Last week we made the assertion that yeast was (probably) the oldest domesticated plant.  It has since been brought to our attention that yeast is not actually classified as a plant.  Of the five or six kingdoms of living organisms, yeast is placed among the fungi.  Of course we’d rather not think of our bread as being a product of a fungus, since some other members of that kingdom are unattractive (though edible mushrooms have their place on our table).  Still, biology in general is not among the subjects in which we claim expertise, so we’ll defer to more learned opinion.

The question is, why did we think yeast was a plant in the first place?  Mostly because of out-of-date and oversimplified lessons in biology in our early schooling.  Then, everything living was either a plant or an animal.  We were later vaguely aware of strange things like Archaea and Prokaryotes that complicated the system, but they didn’t seem to affect familiar organisms.  And an in-depth book on bread and yeast cooking from 1973 (admittedly not by an expert biologist), names it a plant.  (We find that a more recent one, from 2021, places it among the fungi.  But we hadn’t read that one cover-to-cover as of last week.)

That got us thinking about classification systems in general.  Science has great masses of data, and in order to do anything with them there must be some kind of organization.  The system of kingdom, phylum, class, order, etc. was originally produced by considering what things looked like and how they functioned.  When evolution was accepted, the idea that everything in one branch of the classification tree shared a common ancestor took hold.  This was, of course, impossible to demonstrate directly (pending the invention of a time machine).

However, once DNA sequencing was developed, actual family relationships could be determined.  We understand that some reshuffling of the tree of life has resulted from this (and continues, as more work is done).  Also, the neat boundaries we learned in the old days are permeable.  A species is defined by the ability to interbreed; but there are organisms on their way to being separate species, not quite there yet.  And genes can jump between unrelated species.

It reminds us of the teaching of Chemistry.  There are rules that are sort of good in general, useful for organizing ideas in an introductory course, but that need to be modified later.  And all the rules have exceptions.  If we were to add genetics to our fields of expertise, no doubt we’d have to start with Gregor Mendel and his peas, then add the complications.  And at the end, we’d still be faced with working out whether to include viruses as living organisms.

Yeast may not have been the first domesticated plant.  But at least it was the second domesticated living thing.