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  • In school, you likely learned about the ranks in the tree of life with

  • some kind of clever mnemonic, like Dear King Philip Came Over For Great Soup.

  • This probably gave you the impression that those ranks were pretty important.

  • Fixed, unchanging steps on the path from general to specific.

  • And of course, one phylum or order must be treated the same as any other.

  • Unfortunately, that's not true. At all.

  • That method for naming living things

  • and sorting them into groups is called taxonomy.

  • And it's a foundational part of studying life on Earth.

  • We use these names and ranks

  • to understand evolutionary relationships and preserve biodiversity.

  • But it's also more complicated than it seems on the surface.

  • Trying to shoehorn the messy, complicated web of interrelationships that is biology

  • into neat boxes has resulted in a pretty messy tree of life.

  • And straightening it out is going to take time and effort.

  • Time and effort we're going to need to put in,

  • because we might be running out of time

  • to name organisms before they disappear forever.

  • If you remember that King Phillip thing, you might also remember that taxonomy

  • got its start in the 1700s due to the work of a Swedish botanist named Linnaeus.

  • He came up with a scheme to name and categorize life on Earth,

  • with ranked categories and binomial, two-part names,

  • like Homo sapiens or Felis catus.

  • That system, now updated to reflect our understanding of evolution,

  • forms the basis of what's called rank-based taxonomy.

  • It runs from the very high-level domain down through

  • kingdom, phylum, class, order, family, genus, and species,

  • with each describing more narrowly defined groups of related organisms.

  • The names and naming process are regulated by international codes,

  • like the ICZN, or International Code of Zoological Nomenclature.

  • And these groups are generally organized

  • by describing characteristics shared within the group,

  • which can be anything from having a cell wall to laying eggs with shells.

  • In general, this works fairly well,

  • but there are some weird quirks to doing things this way.

  • Part of the problem arises from using a system and groups invented before Darwin,

  • and trying to paste in evolutionary relationships over top of them.

  • One of the ways scientists have updated the tree of life to reflect evolution

  • is to try to keep groups like orders or families monophyletic.

  • That means everything in the group is descended from a single common ancestor,

  • and the group contains all the descendants of that ancestor.

  • The advantage is that this system

  • can easily and quickly tell you about evolutionary relationships.

  • Like, giant pandas are in the same family as bears, but not red pandas.

  • So giant pandas are more closely related to a black bear than a red panda. Easy.

  • But not all groups are monophyletic.

  • Consider the class of vertebrates called Reptilia.

  • It includes animals like alligators, turtles, and snakes;

  • pretty much all of what we'd traditionally consider reptiles.

  • In day-to-day life, it's usually not a problem to lump all of these critters together.

  • They're all scaly, generally cold-blooded, and generally do similar things.

  • But when we start to think in terms of evolution, things get a little trickier.

  • Because reptiles as a group are what's known as paraphyletic.

  • In a paraphyletic group, all members of that group descend from a common ancestor,

  • but the group doesn't contain all descendants from that common ancestor.

  • Someone really important is missing when we talk about reptiles: Birds.

  • Birds, which are descended from dinosaurs,

  • are actually an off-shoot of the reptile family tree.

  • And a relatively recent one.

  • In fact, crocodiles, which we think of as reptiles, are more closely related to birds

  • than they are to turtles, which we also think of as reptiles.

  • And yet birds have their own class: Aves.

  • You can't have a class within a class, so birds can't be included under Reptilia.

  • This makes Reptilia a paraphyletic group.

  • So if you were simply comparing one class to another, you might not realize

  • how close birds and crocodiles are, nor how ancient and unique turtles are.

  • Taxonomy should ideally help us understand how life evolved,

  • but in cases like this, it can be misleading.

  • Incidentally, we've mostly phased outfishas a group,

  • since in order for it to be monophyletic, we'd have to include humans.

  • Because we're also descended from the same ancestor.

  • Over time, we've focused more and more on these systematic relationships,

  • and we've retired some old groups when they stop making sense.

  • But it's an ongoing process, and we still have wonky groups like Reptilia.

  • There are also some weird problems with the idea of the ranks themselves.

  • At the most basic level, there are tricky problems with how we define species.

  • But there are also questions about how some of those

  • higher-order groups are organized as well, like orders or phyla.

  • That's because there isn't an objective test

  • for whether something should count as a class or phylum.

  • It's not like 99% DNA match is a genus, 95% is a family, 90% is a class, et cetera.

  • This can create situations where groups of the same rank

  • aren't comparable to one another; they're not equivalent.

  • Take Cnidaria, a group that includes critters like jellyfish and coral.

  • Cnidaria is a phylum, a high-level rank just below kingdom.

  • As vertebrates, our phylum is Chordata.

  • And since they're both phyla,

  • you might be tempted to think of them as roughlyequal”.

  • Like, maybe the groups are roughly as old as each other,

  • or contain about the same amount of genetic diversity,

  • or the same number of species.

  • But depending on how you look at it, Cnidaria is actually a much bigger group.

  • In fact, the genetic difference between true jellyfish and box jellyfish

  • is about the same as between a human and a sea urchin;

  • a group that belongs to another phylum altogether.

  • So, again, for a system that's supposed to help us understand the world, it's got some serious flaws

  • Now, don't get me wrong: Taxonomy is really useful.

  • Life is a continuum, and classification is a tool that we use to understand it.

  • Without it, we'd struggle to describe the organisms around us

  • or understand evolutionary relationships.

  • But we've also seen some of the quirks of our current system.

  • And those quirks can have consequences.

  • For one, we need the insights taxonomy gives us

  • to be able to protect the natural world.

  • For countries or governments to make policy decisions

  • about animal and plant species, they need to know what species are out there.

  • Like, if a national park is trying to understand the diversity of animals in their borders,

  • it's important to know whether two populations of fish are related.

  • Or if crops are being attacked by a new invasive species of insect,

  • knowing exactly what species it is can

  • help farmers, scientists, and policymakers protect the food supply.

  • So if our systems and ranks aren't clear,

  • it can become harder to understand these kinds of problems.

  • And of course, taxonomy can really affect researchers, too.

  • For example, a paper published in 2016 in the journal Current Biology

  • claimed that it's not uncommon for researchers to treat these ranks

  • as actual biological patterns rather than human-made aids.

  • They're there as a tool to aid our understanding, not as an absolute rule.

  • Yet as long as they exist, the paper suggests,

  • people will be tempted to focus on them.

  • To fix things, some researchers have proposed standardizing taxonomic ranks,

  • like by using when different groups diverged in Earth's history

  • as a yardstick for assigning ranks.

  • But it's worth noting that re-organizing things can sometimes be cumbersome,

  • due to the way those rank names work.

  • While many of the classic groups like Reptilia or Mammalia

  • have names that wouldn't change, taxonomic convention dictates that

  • certain categories, like family or subfamily, have precise suffixes.

  • This means that, by convention, if you upgrade or downgrade a group,

  • it might need to have its name changed.

  • And that can cause headaches for future researchers.

  • So why not just ditch the idea of trying to classify life by characteristics,

  • like those body plans or egg shells?

  • Some researchers have proposed and backed a whole new system called PhyloCode.

  • First published in 2000, it's based on evolutionary relationships from the get-go.

  • So a group like Reptilia could be described as

  • the group that contains crocodiles and turtles, but not frogs.”

  • Traditional orders and phyla would still exist as part of these groupings,

  • but wouldn't necessarily be assigned any special rank.

  • There's been pushback around things like

  • how stable and useful these new categories would be long term,

  • like how often they'd need to be rearranged or reclassified,

  • as well as whether or not people could still use Latin names.

  • Since the first draft in 2000, the idea has been updated to make things easier,

  • but it's a big undertaking.

  • In the end, this problem is complicated,

  • because we've had the old system for literally centuries.

  • So revamping it would, well, take a bit of work.

  • And to make things more complicated, it's work we may not be able to afford.

  • Because the biggest problem facing taxonomy today is probably a lack of resources.

  • There aren't enough taxonomists, and they're facing serious funding shortages.

  • A 2015 report published by the Royal Society of New Zealand, for instance,

  • suggests that New Zealand was losing its taxonomists

  • to a lack of investment and staff not being replaced.

  • This is a problem, because although we've described about 1.5 million species so far,

  • and though about 20,000 new animal species alone are described each year,

  • it's still only a fraction of the tree of life.

  • By one estimate, there might be at least 10 million species on the planet.

  • And that's only considering eukaryotic species, those whose cells have a nucleus.

  • Another study estimated that nearly 90% of those eukaryotes have yet to be described.

  • And there are some habitats, like the deep sea,

  • that are so remote that scientists find new species every time they visit.

  • Many fields do rely on amateur collectors.

  • Entomology, which is the study of insects, is a major example.

  • And those folks are great to have.

  • But to describe a species, there are a lot of things people need to keep in mind,

  • like providing enough descriptions, potentially including illustrations,

  • as well as making everything fit within the existing ranks.

  • That takes time, and we're kind of on the clock.

  • Climate change and other human activity puts us on a timer as species disappear.

  • So it's a messy problem.

  • To get around the lack of taxonomists,

  • some researchers have proposed basing taxonomy

  • not on painstaking analysis of body parts or ranges, but on DNA barcodes.

  • For example, a 2019 paper proposed that

  • for hyper-diverse groups of animals, like parasitoid wasps,

  • that new species be described by comparing specific sequences of DNA,

  • rather than by analyzing their bodies or homes.

  • However, not everyone wants to do things this way.

  • For one thing, it'd mean that you might need to have a DNA lab

  • to investigate whether something you found is a new species,

  • which could put taxonomic work out of the reach of amateurs

  • or people working in less developed areas.

  • So yeah, it's really complicated.

  • And there's probably not one easy solution, either.

  • In the end, taxonomy is a powerful tool,

  • and one that modern biology would not be able to function without.

  • But some scientists have argued that it needs to change with the times,

  • the way biology as a whole has done.

  • Linnaeus didn't know what a gene was,

  • and he also predated Darwin by more than a century.

  • So while his binomial system has gotten us this far,

  • it doesn't always reflect what we now know.

  • Ultimately, it might be time to prune a few branches on the tree of life.

  • Thanks for watching this deep dive, and thanks to our patrons

  • who make it possible for us to make episodes just like this one.

  • You guys are awesome.

  • If you want to help us keep making SciShow,

  • head over to patreon.com/scishow to get started.

  • [♪ OUTRO]

[♪ INTRO]

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B1 group ancestor rank tree evolutionary life

The Tree of Life Is Messed Up

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    林宜悉 posted on 2020/03/30
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