Thursday, September 8, 2016

No Such Thing as a Protist

So I care about cladistics, which is a way of classifying living things based on traits that are inherited from a common ancestor.  I might care about cladistics too much.  I've been trying for a while to understand why I care so much.  I think that it's because without an understanding of evolution and cladistics you're not really understanding the biological world.  I think the fact that birds are theropod dinosaurs just makes them a whole lot cooler and the fact that octopuses' closest relatives outside the other cephalopods are clams and snails is simply amazing.

I've also been after a life goal of not being such a pedant.  Sometimes my desire to revel in the true nature of a living thing comes into conflict with this.  (Like when I refer birds as "reptiles" without missing a beat and not really explaining myself.)  But I think that's where I've actually found the most fascinating discoveries.  There are moments when our words for the natural world start to break down because...well hundreds of years ago some people who really didn't understand evolution or cladistics came up with them and they don't actually work all that well a lot of the time.

So while I really am trying to be less of a pedant:  I have a real problem with Kingdoms.  Specifically Protista.  I'll show you why...protists don't exist.  Here's how you probably learned about the tree of life:

Here's a (probably fairly up to date) image of the actual tree of life, simplified to mainly show the eukaryotes which are living things that are like us...that is their cells are quite a bit bigger than a bacteria or archaea and their cells have a nucleus:

Vojtech.dostal CC BY.0-SA 3 click to embiggen
See the kingdoms?  Yeah you've got Bacteria and Archaea down at the bottom.  Fungi's right there just above bacteria inside opisthokonta.  Metazoa, that's animals, inside the same group as fungi.  Plants are also pretty obvious, inside archaeplastida next to green algae.  So...the $64K question: where is protista?
click to embiggen modification of the above illustration CC BY 4.0

OK I'm calm...I've been trying to reflect and figure out exactly why this bothers me so much.  I think I've got a decent analogy.  It has to do with monophyly which is when you group organisms into what are called clades (clades, cladistics same root word) which contain a set of living things and all common ancestors.  This is basically the idea that "kinds" of living things are ones that evolved from one another, you don't combine things that don't have a direct evolutionary relationship, you only combine things that are descendants.  So if you go back and look at the tree above it seems very obvious to me that there are six groups of eukaryotes plus bacteria and archaea.  You can call fungi a "kingdom" if you want but that means you also have to call malawimonas a kingdom...and well all of these things.  

So here's my analogy using a subset of the so called "protists" for my comparison.  Here are algae:
click to embiggen modification of the above illustration CC BY 4.0
(well actually there's things we call algae in several other groups in Eukaryota and also we sometimes call cyanobacteria [which if you couldn't tell from the a type of bacteria] algae but whatever, I'm just going with full clades of things we call algae for now).  We call these things algae, from what I understand (and full disclosure here, I am not a taxonomist, I am a science educator and I could have this wrong) because they are photosynthetic and they are not plants.  And also from my understanding, and I believe this is the rub here, photosynthesis appearing in at least many of these different groups is due to convergence, that is it has evolved multiple times.  So the different groups having photosynthesis is not because they are the "same thing" but because photosynthesis is useful and has evolved several times.

Here's my analogy: this is a (very, very) simplified cladogram of things with powered flight:
by me CC BY 4.0
As we all know, bats, insects and birds all have powered flight.  So, we could call all of them the same thing.  We could have a word like "flappydoo" to describe all of these things.  

Flappydoo are algae.  Algae are called that simply because they are tiny things most humans don't really understand that can photosynthesize.  Flappydoo are slightly bigger things that most humans reasonably understand that have powered flight.  I'm just lumping things in a pretty big clade (animalia) together due to a trait they evolved because its useful.  But that trait is convergent.  

I was talking about a similar thing just today and I think I reached another level of understanding of when I'm supposed to be pedantic and when it doesn't matter.  In my work I spend some amount of cognitive energy telling people that horseshoe crabs aren't crabs (they're related to spiders) and that starfish aren't fish (right, they're inverts in echinodermata) but *technically* hermit crabs are not "true crabs," that is they are in a different clade in a very closely related group.  But I don't think that matters.  Not one bit.  I also don't think it matters if someone calls a lobster a crab.  Why?  Convergence.  Most of decopoda, the group that contains lobsters, crabs, squat lobsters, shrimp, mantis shrimp, etc. probably had ten legs and probably some of those legs had claws.  That describes a crab and a lobster and a hermit crab.  They're the same thing.  Probably none of those traits evolved convergently.

I brought up falcons during my discussion today because they are not hawks and again convergence was the important thing.  It was agreed that it's important to distinguish hawks and falcons because they're convergent and ultimately, that's cool.  That's interesting.  The fact that hermit crabs and "true" crabs are not *technically* the same clade isn't interesting.[actually it is but I think for most people this is a level of detail that's over the line of pedantic].  They didn't really (probably) evolve anything really interesting through convergence. 

I think there's a lot of grey area here, clearly.  But my main goals are two: I hope that you care a little less about what things are called and spend more energy thinking about what is interesting about a particular group of living things.  Two?  I do hope you stop using the term "protist" and you stop using kingdoms and start only using domains if you haven't yet.  I think it's really silly that students in most schools are still learning about kingdoms.  

Then again I learned about the Bohr model of the atom in school.  Waste of time...

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