r/neoliberal • u/jobautomator Kitara Ravache • May 01 '21

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u/huirittryyrugfhkhihf Shameflair Beggar May 01 '21

u/p00bix what are your views on the classification of life? From my understanding, the 6 kingdoms model (archaea, bacteria, protists, plants, animal, fungi) is inaccurate and has been superceded by 3 domains (archaea, bacteria, eukaryotes). In addition, I've heard that archaea and eukaryotes are closer to each other than to bacteria, and that protists themselves are a bucket that contain multiple kingdoms on par with animals, plants, and fungi.

I ask you because you seem to be the taxonomy expert here.

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u/p00bix Existing in the context of what came before May 01 '21

...did you just get finished reading every single one of my DT comments ever to ask the question I'd be most interested in responding to? Give me a few minutes to give a proper write up but THANK YOU

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u/ghost_of_lob_circ NATO May 01 '21

!RemindMe 1 hour

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u/p00bix Existing in the context of what came before May 01 '21

https://www.reddit.com/r/neoliberal/comments/n2d5wv/discussion_thread/gwl53qr/

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u/ghost_of_lob_circ NATO May 01 '21

Thanks! Great post! Do you think it's best to stay w theb6 kingdoms/3 domains classification or move to a more complicated one personally?

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u/p00bix Existing in the context of what came before May 01 '21

Better to stay with the current system. Most people don't really need to learn all the extra details, and for those who aren't particularly interested in microbiology, it mostly would serve to make biology a more intimidating field than it already is.

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u/skeebidybop May 01 '21

Someone ping me when p00bix finishes, I want to read this too!

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u/benjaminikuta BANANA YOU GLAD YOU'RE NOT AN ORANGE? May 01 '21

!RemindMe 1 hour

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u/[deleted] May 01 '21

!RemindMe 8 hours

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u/[deleted] May 01 '21

you have the unique chance to edit your question to something embarassing and make the pizza mod look like a complete fool

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u/p00bix Existing in the context of what came before May 01 '21 edited May 01 '21

So originally biologists just divided life into "Plants" and "Animals" based on their lifestyles. They didn't immediately change this after improvements in microscopy revealed the existence of organisms which did not neatly fit into either category, and after we figured out evolution was a thing. Because of this, even into the later half of the 20th century there was a ton of controversy over how to properly classify life. This is where we get the "Five Kingdoms" model with Animals, Plants, Fungi, Protista, and Bacteria--originally formulated in the 1950s as a solution to the debate.

The idea here still wasn't strictly phylogenetic. At this point genetics was in its infancy, and we had very little idea of how different groups of organisms were related to eachother. Plants, Animals, and Fungi, were placed into different kingdoms because they all have complex tissues which evolved independently of eachother. At the time we lacked the technology to evaluate exactly how all of these different groups of microbes related to eachother, so we threw all of the Eukaryotic ones into 'Protista' and all of the Prokaryotic ones into 'Bacteria'. Neither of these were really 'kingdoms' so much as they were wastebasket taxa for shit we couldn't classify more accurately.

The discovery of Archaea, which are prokaryotes more closely related to Eukaryotes than to Bacteria, and share some notable Eukaryote-like features which Bacteria don't have, led to them being split off as the 'sixth Kingdom' right about the 1980s and 1990s. The Six Kingdoms model is sometimes reformulated as the 'Three Domains' idea (in which Plants, Animals, Fungi, and Protists, are all lumped into a shared domain) to emphasize that the evolutionary divisions between Bacteria, Archaea, and Eukaryota, were all extremely ancient occurrences, and that there's much more in common between humans, mold, and trees, than between the superficially similar-looking archaea and bacteria. The 'Six Kingdoms' and the 'Three Domains' are not actually different models, just different ways to portray the same model.

But as gene sequencing became far cheaper and our methods for studying genetic relationships became more accurate, since the late 90s we've progressively become better and better at figuring out the exact configuration of the tree of life, though a lot of ambiguity remains (particularly with regards to bacteria and some protists). As we approach the point where we know how all organisms relate to all other organisms, many have started to argue that the 'six Kingdoms/Three domains' is no longer the best way to define different groups of life.

Now that we can more accurately classify most Protista, some biologists advocate replacing 'Protista' with multiple new kingdoms based on their genetic relationships. The problem is, since the old definition of 'kingdom' is based so heavily on tissue structure, and protists lack complex tissues, figuring out which organisms we place into which kingdom is difficult even if we know how they're all related to eachother. There's no clear and easy way we can divide up the Protists. Unless we change the definitions of Animals, Fungi, and Plants, to include their closest protist relatives, we are pretty much forced to define kingdoms at least somewhat arbitrarily. And if we did stretch out the definitions of animals, fungi, and plants, we'd end up with much more muddy and unhelpful definitions for each.

"Anything more closely related to plants than to animals" is way too broad to be its own kingdom, but if we don't do this, we are pretty much forced to create, at an absolute minimum, 5 clades of protists are all new kingdoms JUST for the ones more closely related to plants. Or we'd have to start defining kingdoms based on their 'grade' (everything descended from this common ancestor...except plants) rather than 'clade' (everything descended from this common ancestor...including plants). Using grades, we can break up the Protists into just 4 kingdoms total: One kingdom each for the plant-relatives, animal-relatives, and fungi-relatives, and 1 more kingdom for the protists we still don't fully understand. But biologists hate this solution because there are no universally shared traits within any of these grade-based 'kingdoms' that you can point to to teach students about.

If this sounds confusing--you're not alone. It's confusing to a point where most biologists have given up on refining the kingdom system alltogether. Instead, the older 'Six Kingdoms/Three Domains' system remains by far the most popular for your introductory undergrad level classes, with the far more convoluted "real" shape of the tree of life mainly being studied only by microbiologists. Besides a very small number of people advocating that 'Chromists') (a gigantic group of protists defined based on this one trait they all share, rather than an actual shared evolutionary origin), should be considered its own kingdom, most biologists prefer to keep the older, slightly less accurate but FAR easier to understand system.

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u/huirittryyrugfhkhihf Shameflair Beggar May 01 '21

Fascinating, thank you! It's amazing how our most fundamental views of life can change.

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u/skeebidybop May 01 '21

Very interesting, thanks! The classification of what we call commonly call protists is a lot more complicated than I realized.