Taxonomy: Life’s Filing System – Crash Course Biology #19


Taxonomy! It’s the science
of classifying living things. That sounds exciting. Today we’ll basically be learning
the Dewey Decimel System of evolution! It’s like filing! You must be on the
edge of your seat. OK, shut up. When it comes down to it,
this science doesn’t just categorize organisms, when you look a little deeper,
you realize it’s telling the story of all life on earth. And it’s a pretty good story. Every living thing on this planet
is related to every other living thing. If you go far enough back,
we all have a common ancestor. An organism that both you
and I are descended from. Or something that a star fish and
a blue whale are descended from. Or, even weirder, that an
oak tree and a salmon are both descended from. That organism lived. It lived very long ago. But it was here. And I dig that. The trick of taxonomy,
is basically figuring out where all those branches
of the evolutionary tree are, and finding some convenient labels
to help us understand all of these remarkable interrelationships. Let’s be clear though,
taxonomy isn’t about describing life in all of it’s
ridiculous detail, it’s mostly about helping
humans understand it, because it’s way too
complicated without structure. To get that structure biologists
use the taxonomic system to classify all the
organisms on the Earth. It’s sometimes called
the Phylogenetic Tree, or the Tree of Life, and it illustrates the
evolutionary relationships between all living species. There are about 2 million
known species, but there could be anywhere from 5 million
to 100 million species scientists really have
no freaking idea. New species keep getting
discovered all the time, and the more organisms we
have to keep track of, the more complex the
Phylogenetic Tree becomes. So, there’s not always a consensus
about how to classify this stuff. There’s a lot of gray
area in the Natural World. Actually, let me rephrase that: the Natural World is
one giant Gray Area. Sometimes it’s just hard
to know where to put a certain group of organisms, and eventually the
group gets so big, the classification system has to
be messed with to make room for it. So, the system isn’t perfect, but it’s good enough that we’ve
been using it for around 250 years. [Sniffing] What’s that? Do you smell a
Bio-lography coming on? Carl Linnaeus was a
Swede born in 1707. And early in his career as a
botanist he realized that the botanical nomenclature of
18th century Europe was.. well, just crap. For instance, in his day, the
“formal” name of a tomato plant was Solanum caule inerme herbaceo,
foliis pinnatis incisis, racemis simplicibus. Linnaeus actually said once,
“I shudder at the sight of most botanical names
given by modern authorities.” Not only did this sloppiness
bother him, he saw a whole sugarstorm blowing in: New plants were still being
discovered in Europe, but that was nothing compared
to the crazy stuff that was coming from the New World. Linnaeus saw that pretty soon,
naming conventions were just going to collapse under
all these new things to name. And THEN what? Linnaeus famously started
off by naming himself. He came from a peasant family,
and at the time, surnames were just for
rich people, so when Carl went to college, they asked him
for his surname and he just made one up: Linneaus, after
the Linden trees that grew on his family’s homestead. Linnaeus got a medical degree
and became a professor at Uppsala University where he
devoted himself to the study of nomenclature. He had his students go places and
bring back specimens for him to study and categorize. The method he eventually adopted
was based on morphology, or physical form and structure. This wasn’t necessarily a new idea. Back then, people grouped organisms
by analogous or homoplasic traits, structures that appear similar
but actually come from completely independent origins. By this definition, birds would be
more closely related to butterflies than to reptiles because
birds and butterflies can both fly. But Linnaeus had a good mind for
this stuff and turned out to have a real knack for choosing actual
homologous traits for his classification system traits that stem from a
common evolutionary ancestor. Linnaeus didn’t know
jack about evolution Darwin wouldn’t come around
for another 100 plus years but he just intuited that some
traits were more important than others. For instance, he was struck by the
fact that reproductive apparatus seemed to be a good way
of classifying plants. He also caused a scandal by
classifying the Class Mammalia based on the female’s ability to
produce milk from their nipples. Because apparently that was
pretty racy stuff back then. In his lifetime Linnaeus catalogued
roughly 7,700 plants and 4,400 animals,
and he published his classifications in a catalog
called Systema Naturae, which by the time he wrote the
12th edition, was 2,300 pages long. In the meantime, Linneaus actually
adopted a personal motto: “God created, Linnaeus organized.” Although taxonomy has come a long
way since Linnaeus, we still use a bunch of the conventions
that he invented. For instance, we still arrange
things into taxa, or groups of organisms, and we still us the
same Taxa as Linnaeus: kingdom, phylum, class, order,
family, genus and species. We also still use Linnaeus’
convention of binomial nomenclature using a unique, two-part
name for every species the genus and species name,
in Latin or Latin-ish. This practice actually started back
in the Middle Ages when educated people were expected to know Latin. We know a lot less latin now, but we know a lot more about
evolution which Linnaeus didn’t. And we have technologies like
genetic testing to classify relationships between organisms. And yet we still use Linnaeus’s
morphology-based system because genetic evidence generally agrees
with classifications that are made based on structure and form. However, because there was a lot
of life that Linnaeus had no idea about, we had to stick a new
taxa above Linnaeus’ Kingdom. We call it Domain. And it’s as broad as you can get. The Domains are Bacteria,
Archaea and Eukarya. The bacteria and archae are
prokaryotes, meaning their genetic material goes commando
with no nucleus to enclose it. While the Eukarya make up all the
life forms with a nucleus and include pretty much all the
life that you think of as life, and quite a lot of the life
that you don’t think about at all. It might seem like, since all
macroscopic life only gets one domain, it’s kinda silly
to give prokaryotes two and for a long time, we didn’t. We didn’t divide them up
into different domains. They hung out together in
a single domain called Monera. But it later became clear that
Bacteria, which live pretty much everywhere on earth, including
inside of you and deep in the Earth’s crust, and Archaea
which are even more hardy than bacteria, have distinct
evolutionary histories. Archaea being more closely
related to eukaryotes and, yes, thus me and you. They have totally different cell
membranes and the enzymes they use to make RNA, their RNA polymerase,
is much more like ours. Under the domain Eukarya,
which is by far the most interesting and even occasionally
adorable domain, we have Kingdoms: Protista, Fungi,
Plantae and Animalia. Now, scientists have settled on
these four. For now. But these are categories that
are a human creation, but there are good reasons for
that human creation. The unscientific truth is that we
looked at life and divided it up based on what we saw.
So we were like, “Well, protists are single-celled
organisms, so, they’re very different from the
rest of the domain. Plants get their energy from the
sun and fungi look and act very different from plants and animals,
and we already know what animals are, so they have
to get their own kingdom.” And though scientists are loathe
to admit it, that system of just looking and dividing things up
actually worked pretty well for us. Not perfectly, but pretty well. But there’s a reason why
this worked so well. Evolutionarily, there are
actual categories. Each of these kingdoms is a
huge branch in the tree of life. At each branch, an evolutionary
change occurred that was so massively helpful that
it spawned a vast diversity of descendents. Plants or Plantae are the
autotrophs of the Domain Eukarya. Autotrophs meaning that
they can feed themselves, through photosynthesis of course. Their cellulose-based cell walls
and chloroplasts giving them a distinct difference from
all other multi-cellular life. There are two other
sorts of -trophs. The heterotrophs, which get their
energy by eating other organisms. And Chemotrophs, which are weird
and crazy and only show up in bacteria and achaea, and they
get their energy from chemicals. Now the kingdom Protista is weird
because it contains both autotrophs and heterotrophs. Some protists can photosynthesize,
while others eat living things. Protists are basically a bunch of
weird, eukaryotic single-celled organisms that may or may not
be evolutionarily related to each other scientists are still
trying to figure it out. Some are plant-like,
like algae, some are more animal-like,
like amoebas, and some are fungus-like,
like slime molds. Protists are one of those gray
areas I was telling you about. So don’t be surprised if,
by the time you’re teaching this to your biology students,
there are more than four kingdoms in Eukarya. Fungi, which are,
you know—the funguses. They include mushrooms,
smuts, puffballs, truffles, molds, and yeasts and they’re pretty cool because
they have cell walls like plants, but instead of being
made of cellulose, they’re made of another
carbohydrate called chitin, which is also what the beak of a
giant squid is made out of, or the exoskeleton of a beetle. Because fungi are heterotrophs
like animals, they have these sort of digestive enzymes that
break down their food and get reabsorbed.
But they can’t move, they don’t require a
stomach for digestion they just grow on top
of whatever it is they’re digesting and digest it
right where it is. Which is super convenient! And finally, we have
Kingdom Animalia. Which is the lovely kingdom that
we find ourselves and 100% of adorable organisms in. Animals are multicellular, always. We’re heterotrophic, so we spend a
lot of our time hunting down food because we can’t make it ourselves. Almost all of us can move,
at least during some stage of our life cycle. And most of us develop either
two or three germ layers during embryonic development, wait for it… …unless you’re a sponge. So like I said, we use this
taxonomic system to describe the common ancestry and
evolutionary history of an organism. Looking at the phylogenetic tree,
you can tell that humans are more closely related to mice
than we are to fish, and more closely related to fish
than we are to fruit flies. So how about we pick an organism
and follow it all the way through the taxa, from kingdom to species,
just to see how it works. I know! Let’s pick this kitty. Because I know she’d like it. Right, cat? So, kitties have cells that have
nuclei and membrane surrounded organelles. And they’re multicellular
and heterotrophic and have three germ layers of cells
when they’re embryos, so they’re in the
kingdom Animalia. And they have a spinal cord
running down their backs, protected by vertebrae,
and disks in between them. And they have a tail that doesn’t
have a butthole at the end of it like a worm, which
I’m really glad about. And that puts her in
the phylum Chordata. Kitty clearly does not like this,
so I’m going to put her down now. And the kitty lactates and gives
birth to young like a cow, instead of laying eggs
like a chicken, and they have fur and three
special tiny bones in their ears that only mammals have,
so they’re in the class Mammalia. So, she is more closely related
to a cow than a chicken. Good to know! And like a bunch of other
placental mammals that eat meat like weasels (the mustelids),
and dogs, (the canines), kitties are in the order Carnivora. And they’re in the cat family,
Felidae, whose members have lithe bodies and roundish heads and,
except for cheetahs, retractable claws. And they’re littler than tigers
and panthers, which puts them in the genus Felis. And then, at the level of the
species, the descriptions get pretty dang detailed, so let’s
just say that, you know what a cat is right?
So the species name is catus. And look at that: Felis catus! Aw. Kitty. I could have that whole
thing cross-stitched onto a pillow for you to sleep on! And it would be cute! Thank you for watching our taxider- I mean, our taxonomy episode
of Crash Course Biology. We hope that you learned something. Thanks to everybody who helped
put this episode together. If you have any questions for us,
please leave them on Facebook or Twitter or in
the comments below. And we will get to them. Hopefully very quickly. I will see you next time!

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