Hello!  Welcome to our "Introduction to
Evolution".
This topic is going to be the foundation
we build the majority of our course on.
We're going to view zoology and the rise
of animals and their diversification
through the lens of
adaptations and slow change over time,
which at its heart is what evolution is.
This lecture is going to be broken into
two halves:
this first half is going to lay some of
the groundwork for you to understand the
actual
processes that drive evolutionary change,
and then after we've got those basic
principles down, we're going to start
talking about how evolution really
happens
in the real world.
All right. There's some terminology that
we need to get out of the way first- I'm
big on defining terms to make sure that
we are
all understanding what we mean when we
use some very specific language in
science.
The very first term we need to define is
the word "species".
"Species" is
formally defined as "the smallest unit of
biological classification".
What does that mean? It means when we
say the word
"species", what we're really saying is-
this is a specific unique group
of animals that have almost
all the same traits and are all each
other's closest
relatives when compared to the rest of
animal life.
What we call "biological classification"
is just an organizational system
that we humans came up with to try to
understand the huge diversity we see in
the world around us.
We like to organize things- that's how
our brains work.
Keep in mind, the classification system
I'm about to show you-
we made it up; doesn't exist in nature.
It's
a set of definitions of categories that
we created to try to better understand
the relationships between living things.
What that means is the natural world
isn't always going to fit cleanly
into these classification boxes.
Classification is messy.
Nature doesn't necessarily abide by our
rules.
There are going to be times where
this classification system, which will
come up over and over again in our class,
is going to fail us- it'll let us down.
There will be organisms that don't
necessarily
fit cleanly into one group or another,
and that's part of the beauty of the
natural world
is, it is super complex!
Here's how our
classification system works: we're going
to talk about this in a lot more detail
in our actual classification lecture, but
for now, just take a look at this diagram.
you'll see that at the
top we have
a very large category called a Domain.
Domains are the largest grouping of
organisms you can have,
in fac,t every living thing on the planet
belongs to one of three different
Domains:
Domain Eukarya, Domain Archaea, and Domain
Bacteria.
We'll talk about what those mean in the
classification lecture,
but for now, know that every living thing
on the planet- from bacteria to plants to
fungus to animals,
falls into one of those three big
categories
and each of those big categories, each of
those Domains
is broken down into smaller and smaller
subgroups,
okay, kind of like Russian nesting dolls-
those are those wooden dolls that
have smaller versions of themselves
inside.
That's how this classification system
works. We can take
Domain Eukarya and we can break it down
into
smaller and smaller groups that have
more
and more requirements to belong to
each group.
So, for example, to be in Domain Eukarya
you just have to be a eukaryote. Being a
eukaryote means, you've got a nucleus in
your cells.
That's it. That's the only requirement. If
you have a nucleus in your cells,
You're good. You're in. You're part of
Domain Eukarya.
But, we can split this domain into
smaller groups called kingdoms,
and each of these kingdoms has different
requirements if you want to belong to it.
To be in these kingdoms, you still
have to be eukaryotic
because all of the kingdoms are part of
Domain Eukarya.
If we choose Kingdom Animalia (since this
is a zoology class)-
to belong to this kingdom you have to be
eukaryotic
and your body has to be made up of more
than one type of cell (that's what
multicellular means), means you got lots
of different types of cells doing
different jobs.
You also have to be heterotrophic
"Hetero-" means "other; "-trophic" means
"to feed"- you're an "other feeder"- means you
have to feed on other organisms to get
your energy.
As animals we eat either plants or other
animals
in order to get our nutrition, and to get
our energy needs met.
if you have all three of these
characteristics: eukaryotic, multicellular,
and heterotrophic-
you belong in Kingdom Animalia. We can
keep going
using this same sort of logic- Kingdom
Animalia can be broken down into a
number of different
phyla (which is the plural of "phylum"),
Phylum Chordata, for example, are all
animals that have a backbone.
If you've got a backbone and you're
heterotrophic,
multicellular, and eukaryotic, you belong
in Phylum Chordata.
See how this works? Okay. And you can see,
as we move down in these levels in our
diagram,
animals (or I should say, organisms) keep
dropping out.
Up here, everything you see in this box
belongs in Domain Eukarya,
but when we focus in on just Kingdom
Animalia,
anything that's not an animal disappears.
okay. So, no more plants, no more mushrooms-
those guys aren't animals, so they don't
wind up in this box.
Likewise, if we focus in on Phylum
Chordata,
anything that doesn't have a backbone
disappears, which includes
our worms
and our insects. I think that's all-
yeah. And we can keep going and going-
if we drop down into Class Mammalia, you
can divide
a phylum into many different classes. If
we choose to focus on Class Mammalia
within Phylum Chordata, we're going to be
looking at animals that have sweat
glands and that produce milk for their
offspring.
Anything that's not a mammal, for example,
this reptile-
or this fish- they drop out and are no
longer considered in our diagram.
We can keep going and going, breaking
classes down into orders,
breaking that order down into families,
breaking that family down
into genera (which is the plural for this
word "genus"),
until finally we get to the bottom of
the list "species".
Species generally are defined as
"a single type of living thing".
That seems like a straightforward
definition.
We have a hard time defining
what it means to be a
single type of living thing. What I'm
going to spend the next couple slides
doing
is explaining why such a simple thing is
actually so hard.
Defining species is one of the most
challenging things that we
do in biological classification.
The example I'm going to give you to try
to clarify why this is
is Pheidole barbata. Pheidole barbata
is the genus and species name
of this group of ants, or, I should say,
of ants that are in these pictures.
So you tell me- do you think this is
Pheidole barbata?
Is this Pheidole barbata?
are those two ants totally different
species
No, not only are those two ants the same
species,
not only are both of these ants Pheidole barbata,
they're sisters. They actually share
three quarters of their genes.
These two ants come from the same queen;
the same mother. They are
full siblings, and yet look how different
they look!
This big girl here (and she is a female...
the ants that you see
out and about doing work for the colony,
foraging for food,
raiding your kitchen, they generally are
all female),
This big girl- she's a soldier-caste ant,
meaning she does a lot of defense for the
colony.
And you can see she's really built for
it- her whole body structure is built to
be strong
and aggressive. This little girl down
here,
also these, these are all workers. Workers
do the day-to-day tasks that are
necessary to keep the colony alive.
They forage for food, they do all the
cleaning, they feed all the babies.
They are small agile and quick because
it helps them get their jobs done more
efficiently.
This is one of the biggest reasons why
species defining
is so hard- looks are deceiving.
Looks lie to you. They don't necessarily
give you reliable information about
relationships.
Just because two groups of organisms
look different from each other,
it doesn't mean that they're not related.
So using just physical appearance
becomes unreliable if you're trying to
identify which groups of organisms are
each other's closest relatives,
which is what defining a species is
really doing.
There are currently 26 different "species"
definitions that are being debated in
the scientific literature.
That's how challenging this is- you don't
have to learn all 26.
Instead, we're just going to focus in on
the three that
are most prominent.
The first is the oldest definition of a
species; it's called the "morphological
definition".
"Morpho-" means physical or appearance,
so this is defining species based on how
they
look. It's the oldest definition for
species
because when we first started
categorizing organisms into groups,
it was back in Aristotle's time. all
right. So, we're talking hundreds and
hundreds of years ago,
before the microscope, before knowledge
of
molecules or genetics; the only
information that scientists
had available to them was what they
could observe- what they could see.
So, using the physical characteristics of
animals was the easiest way to try to
group them together
and determine their relationships to
each other.
So this definition, which is still
sometimes used,
it's based on physical traits that are
shared between individuals. If you share
physical traits with another individual
you're probably related to them,
according to this definition. The problem
with this is what you saw on the
previous slide with Pheidole barbata-
minor genetic changes can cause
major physical differences.
You can have identical genes
to another individual (meaning you've got
identical DNA),
but not all of your genes are
necessarily
turned on in all of your cells.
You can be genetically identical to
another individual,
but if they have some genes that are
active which are inactive in you,
those genes are going to change their
physical appearance and they're going to
change their appearance in ways that
don't match you any longer.
This is called "gene activation". Okay.
Just because you have all the same genes
as another individual doesn't mean
all the same genes are going to be
activated in both of your bodies,
and since genes control your physical
appearance, this can cause you to look
very different from each other, even
though genetically
you are very closely related.
Here's another example that I love:
Ridgeia piscesae-
these are tube worms. okay
These guys, these guys down here, it's a
nice diagram of what they look like.
These tube worms were first discovered
along
the edges of hydrothermal vents, which
are deep sea underwater volcanoes. Essentially, they
spew
caustic, boilingly hot water up
in onto the sea floor from beneath the
earth... the earth's crust.
This is an incredibly hostile
environment to live in.
Temperatures around the hydrothermal
vent are insanely high!
The water tends to be incredibly caustic
and damaging.
This is a really difficult place to live,
but
unlike the rest of the deep sea, it's
warm, so animals do tend to congregate
there and adapt to that hostile
environment.
When we first started sampling some
hydrothermal vent communities,
we would just go down and collect
examples of all the different animals we
found there.
We bring them back up to the surface,
examine them, and try to determine what
species they were.
At first, we
collected both of these types of
tube worms (they're called "tube worms"
because they're a worm that lives inside
one of these calcified hardened tubes),
we thought we had two different species
of tube worm. We thought we had a short
fat species (that's what they called it
up here),
and then we thought we had a long skinny
species
(which you see down here), because they
look different. One was short,
had a very, very thick tubular shell, and
tended to be wide and stout; the other
was more elongated and had a much
thinner and more elegant shell. Well, we
named them two different things-
identified them as two different species-
and then later, once their DNA was run
and sequenced,
we found that genetically the two groups
were almost identical to each other.
Genes were activated in the short fat
tube worms that were inactive in the
long
skinny tube worms, and what caused this
difference
depended on where the worm grew up. When
these worms disperse
as larva, wherever they land, they attach
themselves in place
and they grow there and never leave
again. So, they choose a location,
stick to it, and that's where they spend
their lives. They grow
up in that new environment.
Baby tube worms that landed near the
edge of the hydrothermal vent
had to deal with those extremely hot and
extremely caustic conditions,
so genes in their DNA activated that
allowed them to grow
really thick robust tube shells to
protect them from the environment.
The current was very strong as well, so
they didn't grow very tall; they stayed
short so they wouldn't be swept away.
Tube worms that landed further away from
the hydrothermal vent
where the water was cooler, where it was
less caustic,
the current wasn't as fast. They didn't
need to be really
tough in order to survive; their
environment was a lot gentler.
So, the genes that would have caused them
to grow that thick shell
and to grow short, those genes were never
activated. They stayed
inactive in this group of tube worms,
so they grew longer and skinnier with a
thinner shell.
And yet, the two groups genetically are
almost identical. It's part of what makes
the morphological definition
so challenging.
Now, the next definition we came up with
as biologists to try to solve the
problem with the morphological
definition, its unreliability,
was we said "Okay, Well,we know species (or I should say we know animals) that are
closely related to each other are
probably descended from the same
ancestors; it means they can make babies
with each other, they can reproduce and
make more of themselves."
So, the next definition of a species we
started to use
is called the "biological" definition. The
biological definition states that if
two animals can mate, reproduce,
and produce viable fertile offspring
(meaning the offspring can survive that's
what "viable" means, and they can also make more of
themselves-that's what "fertile" means),
then your two original parents are both
the same species.
This, though, has its own problems,
and those problems come in the form of
this guy. This is a "mule".
Mules are a hybrid,
they're the offspring of a donkey and a
horse.
Now, donkeys and horses are considered to
be two different species,
but they can reproduce. The trick is:
 mules cannot reproduce; they
can't make more of themselves, they can't
have their own babies.
They can have sex; that sex doesn't
result in fertilization or a baby being
born.
This is part of what holds up the
biological definition of a species,
but it also complicates it a bit,
This definition relies on something
called "reproductive
isolation". Reproductive isolation is a
critical concept. You've got to
understand what this means.
There's two types of reproductive
isolation you need to understand:
"physiological" and "behavioral".
Think of reproductive isolation as being
what keeps species distinct and separate
from each other.
So, what keeps them from blending their
characteristics together,
and becoming the same group of organisms?
There's two ways you could be
reproductively isolated from another
group.
The first is physiological isolation.
Physiological isolation is when you
physically
cannot create offspring together.
There's two ways this can happen:
the first
is called "prezygotic isolation".
prezygotic isolation prevents you
from fertilizing an egg.
so prezygotic isolation prevents
fertilization. I'll give you examples too.
All right. So, a couple of ways this can
happen-
if you physically
cannot have sex
your reproductive organs don't fit
together, your bodies aren't arranged in
the correct way for you to have
intercourse.
If you can't have sex your sperm and egg
can never come together and fertilize.
So right there, you're not going to be
able to reproduce.
But let's say you can have sex-
your body parts match up well enough
that you are able to
 mate with each other,
but your gametes don't recognize
each other.... (So, what are gametes?
Gametes is the formal term we use for
sperm
and egg, which are the cells that
fuse together during fertilization. Sperm
from the male, eggs from the female).
So, your gametes don't
match, let's say,
or they're not compatible.
If the sperm can't fertilize the egg,
reproduction isn't going to happen
So, prezygotic isolation are physical
factors that
prevent that egg from being fertilized;
either you can't mate, or the mating
itself
isn't successful.
Now, there's a second type of
physiological isolation,
"post-zygotic" isolation
prevents you from reproducing
successfully,
but it happens after the egg has already
been fertilized.
So, you're able to mate, your sperm
recognizes the egg,
fertilization occurs. So now you've got a
fertilized egg,
but it fails to thrive. It fails to grow
into a healthy
mature fertile offspring.
What are examples of this? Well,
spontaneous miscarriage- the egg starts
to develop,
but there are too many genetic
incompatibilities between the father's
DNA
and the mother's DNA, so it can't develop
properly.
Likewise, maybe the offspring develops,
but after it's born it has a lot of
physical malformations or physical
problems and so it doesn't survive-
it's not viable,
so it dies young.
The last one is- maybe that offspring
survives,
maybe it's healthy, it's able to grow
into an adult,
but the offspring itself is sterile,
meaning it cannot have its own babies. It
can't reproduce more of itself.
You'll notice both prezygotic and
postzygotic isolation-
the names share the same basic structure
"post-" or "pre-", and "zygote".
A "zygote" is a fertilized egg. That's our
formal name for a fertilized egg.
Now the names hopefully make more sense.
Post-zygotic
isolation is reproductive isolation
after that zygote has formed.
Pre-zygotic means isolation that
prevents the zygote from forming;
happens before.
That is all "physiological reproductive
isolation",
however, you can be reproductively
isolated
by other factors that have nothing to do
with your physiology,
and everything to do with your behavior.
Our second major category of
reproductive isolation is "behavioral
isolation".
Behavioral isolation is when your
preferences, your mating habits,
your foraging habits, your actual
behaviors,
prevent you from mating with another
individual.
Your behaviors
prevent
mating.
This is easier to explain using examples.
Let's say you forage for food (foraging
is searching for food).
Let's say you forage at different times.
You forage for food in the morning; your
possible
mate forages for food in the evening, you
guys are never going to interact with
each other.
If you don't interact, you're not going
to mate, right?
Maybe your activity time differs.
Similar to the first one,
but on a larger scale, let's say you just
in general prefer to be active at night
(so you're nocturnal)
but the animal that you might
potentially mate with
likes to be active during the day
(they're diurnal).
Again, you're just never going to
interact.
Your very mating habits themselves
might isolate you from other individuals.
In fact- usually,
mating habits are designed to do just
that.
If your mating habits differ,
you won't recognize each other as
potential mates.
A good example of this is actually on
the previous slide.
Meadowlarks are very common bird in
North America,
and they sing in order to attract a mate.
The males sing.
So, these meadow larks, you can see them
over here,
this is the Western Meadowlark, this is
the Eastern.
They are physically almost identical.
They have
vast majority of the same physical
structures: they're active at the same
times of day 
(they even overlap), and where they're
found- in the middle of
the continent- so, these guys could even
interact with each other.
The trick is: even though they look the
same and they act the same, they never
mate,
and it's because males sing to attract
females
but the song of the Western Meadowlark
is different
from the song of the Eastern Meadowlark.
Western Meadowlark females don't
recognize the eastern male's songs so
they never mate.
That's an example of a "behavioral
isolating factor".
This is... this is pretty good... this is
pretty good. I mean, this
goes a long way towards explaining how
species could stay separate from each
other,
but the problem with this definition
is that our hybrids, which
are the offspring of two different
species, they're supposed to be sterile
if your original parents are definitely
different species.
Sometimes they're not. You do sometimes
get
fertile hybrids, and if the
biological definition of a species was
totally correct,
that wouldn't happen. So this definition
also isn't perfect. Our third definition
is also imperfect but it's the best
we've come up with so far, and it's the
one we're going to use in class.
It's called the "phylogenetic" definition
Okay, notice this key word here,
"genetic"... we are now going to take
genes into account, and what genes
tell you isn't just how much you have in
common with another individual.
What genes actually tell you is your
ancestry.
All of your genes came from your parents.
All of your parents genes came from
your grandparents.
While your grandparents genes came from
your great- great- grandparents. So
ultimately, all of your genes
came from those great- great- grandparents
as well,
and you can trace that ancestry back
hundreds
or millions of generations. If you
trace how many other
individuals who are alive with you today
also share those ancestors,
it'll give you a sense of how closely
related you are to those other people.
We use that same logic when we're
looking at the relationships between
animals. We compare their genetic
compositions to each other,
determine how many genes they have in
common, and that gives us a sense
of how far back they share an ancestor.
The way we define a species using this
information
is: a species is the smallest set of
organisms
that all share an ancestor
and can be distinguished from other such
sets.
What that means is: our set of organisms
all share this ancestor, but no other
organisms do.
Right. Nobody else has this ancestor,
except for the individuals who are in
our group. Those other groups have their
own ancestors
who aren't related to us. We're basing
our groupings on
ancestry.
We use what look like family trees to
visualize these relationships,
just like you would have drawn a family
tree when you were in elementary school,
showing maybe you and your brother
and that you guys are both descended
from your mom,
and your mom's descended from your
grandma,
but your mom's also got a sister over
here...
that sort of family tree is uses the
same logic
as our phylogenetic trees that we're
going to be working with in class.
On a phylogenetic tree, we say that
species
exist at the tips of the branches on the
tree,
meaning these guys, these terminal ends.
Okay.
On this little phylogenetic tree we're
looking at three different species
of California salamander.
We're going to talk more about how to
work with these trees in the
classification lecture, but for now what
you should know
is that
you can think of an ancestor for group
"A"
existing right here on the tree. These
lines essentially
summarize all of your ancestors- they
represent all your ancestors.
So, this is an ancestor
that species "A" is descended from, but
species "B"
isn't, right? Species "B" is over here-
you have to follow this line
in order to get to species "B", and nowhere
on this line
do we connect to that ancestor; this
ancestor belongs
just to species "A". that's what makes
species "A" unique.
Occasionally you will see what's called
a "ring species"-
ring species are a single species but
they've got a huge amount of physical
variation.
Okay, this is an example of how
morphology-
physical appearance- can lie to you about
relationships.
All of these salamanders look very
different from each other,
but in fact, they're all each other's
closest relatives.
They're all the same species even though
they have a lot of physical variation.
