This content explains the principles of phylogenetic trees and clades, illustrating how these evolutionary diagrams define relationships between organisms and how the definition of a group (like "fish" or "reptiles") can be inclusive or exclusive depending on the chosen common ancestor.
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well hi there this is a hagfish a
jawless slimy worm eating fish found in
the deep ocean and your nightmares and
you are basically the hagfish of
reptiles and I thought about ending this
right there but I guess I better explain
if you noticed I only called hagfish a
fish in quotes this is because it is
only arguably a fish at all heck it's
only arguably a vertebrate it is the
only vertebrate with a skull but no
actual vertebrae and it goes right here
on the vertebrate Tree of Life some of
you may be very familiar with these
diagrams but many of you probably are
not so I'd like to explain exactly how
these work this diagram called a
phylogeny depicts the proposed
relationships between different
organisms this tree shows the
relationship between different groups of
living vertebrates this tree shows the
relationships between three people Bob
Joe and Brian now as you may be able to
see Bob and Joe are more closely related
to one another than they are to Brian
this would be the case if for example
Bob and Joe are Brothers and Brian is
their cousin I would say that Bob and
Joe are more closely related because
while all three of them share ancestors
Bob and Joe share parents and not just
grandparents with one another because
they share more recent common ancestors
with one another than they do with Brian
they are more closely related even
though they do share ancestors with
Brian whichever two share ancestors more
recently with one another are more
closely related and the relationships
between them are the same if I draw the
phylogeny like this or like this or like
this and these branching points these
represent the last time that two
individuals or groups shared common
ancestors Brian shared all of these
ancestors with Bob and Joe
and this was the last time he shared
ancestors with either of them since he
shares the same ancestors with Bob and
Joe I can't say which one of them is
more closely related to Brian he is
equally related to both of them and that
isn't changed if I flip where I put Bob
and Joe on the phylogeny Bob however is
more closely related to Joe than he is
to Brian this is because he shares
ancestors more recently with Joe than he
does with Brian he shares ancestors with
both of them but he shares more reason
to ancestors with Joe and thus is more
closely related to Joe if I threw
Stewart onto the phylogeny as well and
saw that he goes here then I would be
able to see that Brian is more closely
related to Bob and Joe than he is to
Stuart I would also be able to see that
Stuart is equally related to Bob Joanne
Bryan because he shares the same
ancestors with all of them now I want to
note one final thing Bob and Joe with
their their parents form a family called
a clade
a clade also called a monophyletic group
is a common ancestor and all of its
descendants Brian Bob and Joe with their
most recent shared ancestors and
everyone in between also form a family a
clade and the same is true for Bob Joe
Bryan and Stuart with their most recent
shared ancestors and everyone in between
so it is possible to be part of multiple
families clades all at the same time and
no matter how many generations go by
their descendants will always be part of
those families as well that is why birds
can be dinosaurs reptiles tetrapods
vertebrates and animals all at the same
time and that doesn't mean that they
aren't unique or that they aren't Birds
even though Joe is in the same family as
Bob Brian and Stewart that doesn't
change the fact that Joe is unique from
Bob Brian and Stewart now let's say I
wanted to talk about a family a family
that includes both Bob and Brian I could
be talking about this family with
Stewart or this family without Stewart
which one is right
both of them so which one am I talking about
about
I might need to tell you and that brings
me to the hagfish here is a phylogeny of
the vertebrates or at least here is a
phylogeny that includes all of the
vertebrates now the question is do I
want to include the hagfish or do I not
that's up to me all of the other
vertebrates share more recent common
ancestors with all of the other
vertebrates than they do with the
hagfish I can include the hagfish and
the ancestors they share with the other
vertebrates or I cannot it's up to me
suppose now I am interested in the fish
what is a fish well I think it's fair to
say that trout are fish and sharks are
fish so I would need to look at the last
time that sharks and trout shared common
ancestors those would need to be fish as
well and everything that came from them
as I discussed in our video on whales
that group would include the whales so
while whales are mammals that doesn't
mean that they aren't part of the fish
family the fish clade as well if you
were interested in naming a clayed fish
they would have to be in it because
they're part of any family that includes
both sharks and trout but not the
hagfish they are part of some families
that include both sharks and trout but
unlike the whales they're not part of
every family that includes both sharks
and trout should they be part of my fish family
family
that's up to me and just one thing to
note here the lampreys are more closely
related to all of the other fish than
they are to the hagfish so I could even
include lampreys without including the hagfish
hagfish
poor hagfish are they just barely fish
or do they just miss the cut they don't
spend too long feeling bad for them
because you're in the same position here
are the reptiles or at least here is a
phylogeny that includes all of the
reptiles notice where the birds are
located I can't get them out of the
family without removing the crocodilians
and the turtles as well they're right in
the middle of the family I can't have a
reptile clade that includes snakes and
turtles without including the birds but
this clade that includes the lizards
Turtles birds and crocodilians called
the diopside clade does not include you
you aren't in it and yet everything
alive today that I would consider to be
a reptile is so clearly you are not a
reptile but what about this
or this
are these reptiles most would agree that
they probably are and yet these are part
of the family over here the synapse had
clayed and that Glade includes you too
you see you share more recent common
ancestors with a dimetrodon than
dimetrodon does with the rest of the
plastic reptiles in the bag of dinosaurs
or with any of the other diopsides so I
cannot create a clade that includes both
the metrodon and Gus Gus without
including you as well the synapses
including you can either be included or
excluded from this Glade because you are
basically the hagfish of reptiles and
now you know as always like And
subscribe and we hope to see you real soon
soon oh
oh
I'm so confused
so if I was in a call like create a
group called monkeys
I'd have to include the Apes including
us as well
so when people say you know you
shouldn't come from monkeys like no it's
worse I'm saying you are a monkey
okay that's if monkey is a thing so
if we just wanted to see stars in then
we're good we just have to add all the
new pterosomes all the anus first
developers all the people who speak out
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