This video is about the causes of
evolution. In other words, how is genetic
equilibrium disrupted. So this first
slide poses a question of how giraffes
evolved to have such long necks. 
Scientists actually believe that
giraffes evolved from an animal similar
to the modern okapi.
If you look at the okapi, it has a long
neck, but its neck is not nearly as long
as the modern giraffe. So how did that
happen? We're gonna come back to that
question, but first I want to talk about
two early evolutionary biologists. The
first is Lamarck, and Lamarck had a
theory of evolution that had two parts
to it. The first part is the theory of
use and disuse, and basically the theory
of use and disuse was the idea that if
you used a part of your body it would
get bigger or stronger, and if you didn't,
it would get weaker. So for example, if
you go to the gym and you lift weights a
lot, you might get very big muscles, and
then if you stop going to the gym, those
muscles would shrink. So sometimes the
theory of use and disuse is actually
true. However, if you stare at a computer
screen for a really long time, that
doesn't improve your eyesight. So there
are some examples where use and disuse
actually seems to have some evidence to
support it, but the second part of
Lamarck's early theory of evolution is
where problems arise. He believed that if
you acquired a characteristic within
your lifetime, you could pass it on to
your offspring. This actually doesn't
work biologically. So for example, if you
go to the gym and get very big and
strong, you're not going to pass on that
strength to your offspring. They would
also have to go to the gym. They wouldn't
be born big and muscular. So one of
the things that people have done, is they
clip ears of dogs, and when the puppies
are born in the next generation, the ears
are not in the new shape that they've
been clipped. So the theory of
inheritance of acquired characteristics
was really pretty easy to disprove
pretty quickly.
The
next early evolutionary biologist was
Charles Darwin. You've probably heard
a lot more about him, because his theory
has been supported since then. So his
idea was that parents had a variety of
offspring, and that those best suited to
their environment would survive and
reproduce in greater numbers. So if we
were to go back to the question of how
giraffes got long necks,
Lamarck would say that giraffes were
stretching to reach leaves on trees, and
the higher they had to reach, the longer
their necks would get, and that their
offspring would also inherit long necks.
Darwin would have said that those
individuals that had longest necks would
have perhaps gotten the most food, and
therefore because they could reach more
leaves on the tree, would have more food
and therefore survive longer and
reproduce in greater numbers. And
actually as it turns out, having a long
neck, in addition to being able to reach
leaves higher on a tree, actually makes
you much more able to see predators like
lions coming. So modern biologists
actually believe that having a longer
neck actually is more of an advantage for
being able to avoid predation. So now I
want to talk about the causes of
evolution, our modern understanding of
evolution. But first I want to talk about
the variation that Darwin is talking
about. Darwin's theory of natural
selection has a lot of merit to it.
The only missing piece of information was  that Darwin
did not know where genetic variation
came from. We know now that it comes from
mutation, random assortment of homologous
chromosomes, crossing over, and random
fertilization. So there are some causes
of evolution that we're going to be
talking about  and one of them is
natural selection, but it would be a
mistake to focus only on natural
selection, because there are other causes
of evolution. The first one that we're
going to talk about is called non-random
mating. Non-random mating is when the
probability of two organisms mating
depends on their phenotypes, so
therefore they
choose organisms. So for example, in Lesser
Snow Geese there are a grayish blue
variety, and white. The white geese
tend to mate with other white geese, and
the blue geese tend to mate with other
blue geese. So they are choosing certain
phenotypes, and we call that sexual
selection. One type of non-random mating
is called in-breeding, which is mating
between relatives, and it usually occurs
in isolated populations, and it's usually
disadvantageous because harmful traits
can show up if you have 
recessive alleles being inherited
together, and that increases with
inbreeding.
For example, first cousin marriage was
very common in the Hopi Indians, and the
Hopi Indian tribes had high rates of
albinism as a result. The Royal
European families had high incidence of
hemophilia  due to intermarriages
with descendants of Queen Victoria of
England.
One type of non-random mating, which I
mentioned earlier, is called sexual
selection (when mating between unrelated
individuals has to do with the phenotype).
For example, a female peahen will pick
a male peacock with the best feather
display, and many female birds actually
choose their mates based on the bright
coloration. You may be hearing the spring
peepers (little frogs) at this time of year if it's
April, which is when we usually
teach this topic. The male peepers
actually sing to attract mates in groups
of three or four, and the loudest male
gets to mate with the female. The next
cause of evolution that I want to talk
about is called genetic drift, and that's
when allele frequencies in a small
population change due to random events
or chance. So this may surprise you, but
small populations actually evolve faster
than large populations, because it is
much easier to upset genetic equilibrium
in a small population.
There are two subcategories of
genetic drift that we're gonna talk
about. Sometimes genetic drift is just
due to something random so let's say you
have a very small population, and just by
random chance, your tallest individual
dies just from some sort of crazy
accident. Then that population would be
significantly shorter in terms of the
allele frequencies, or the genes that
cause a tall  person to be tall. So
that's just one example. Another example
I want to talk about is called the
bottleneck effect, and that's when only a
small portion of the original population
serves as the sole source of the new
population. Sometimes that's because of
over-hunting. So maybe many of those
individuals get killed off, and there's
only a few left to reproduce. And
sometimes it can happen when you have a
particular species where there is one
dominant male who mates with all of the
females, and if that happens for multiple
generations in a row, that one male is
passing on his genes over and over and
over in that population. On one hand,
that is a very successful male who has
very good genes, but also it leads to a
lack of genetic variation. which is not
good at the species leve.l
Another example, is today's worldwide
cheetah population. Humans have hunted
cheetahs almost to extinction, and the
population has rebounded some in terms
of numbers, but the genetic variation is
still quite low, and if you find cheetahs,
they are so genetically closely
related to one another, that you can take
a skin graft from one cheetah to another,
and they won't reject it.
This is leading to high infant
mortality, and low birth rate, and you
know inbreeding it is happening, because
the population is pretty small too. The
next example that we're going to talk
about is called the founder effect.
That is when a few individuals 
leave a large population, and start a new
population. One example of this is
actually in the Amish
country in Lancaster Pennsylvania. 
About 200 individuals started that
population, and there's a genetic disease
that is very common there called Ellis
Van Creveld syndrome. There are
more cases of this disease in that
population, than the rest of the world
combined.
It's an autosomal recessive disorder
that leads to dwarfism, polydactyly,
and a cleft palate. Scientists have
actually traced this disease back to
Samuel King and his wife, who were part
of the original population in 1744. And
then of course, when you have an isolated
population, where outsiders are not able
to come in and join, you end up with
inbreeding. This ends up having
autosomal recessive or sex linked
recessive diseases ending up more common,
because you don't have as many alleles
that might be a dominant in the
population to mask that recessive allele.
Another factor that changes allele
frequencies is migration. Anytime
individuals move in or out of a
population, alleles are transferred from
one population, and it changes the allele
frequencies in both populations. If you
have two populations, and you have
individuals moving between them, it
increases variation in the both
populations, but it decreases variation
between the two populations. Another
cause of evolution is mutation. A
mutation is any physical change in a
gene or chromosome. For example, a "T" is
now a "G", so that would be an example of a
point mutation. Anytime you have a
mutation, you've changed the allele
frequency in that population. Now the
interesting thing about mutations, is
that they provide variation, and
variation is what drives natural
selection. So the last cause of evolution
that we're going to talk about is called
natural selection. This is
a simplified theory of natural selection,
and then we're going to go into a little
bit more detail. So first of all,
variation comes from mutation and
genetic recombination, like random
assortment of homologous chromosomes,
crossing over, and random fertilization.
Some variations are helpful, some are
harmful, and some really make no
difference at all.
Nature selects those individuals
with beneficial variations, which we call
adaptations, to survive and reproduce. An
adaptation is any inherited trait that
helps an organism survive and/or
reproduce. It's very important to
understand, that individuals don't adapt,
at least biologically speaking.
Populations adapt ,and an individual may
have an adaptation that they have
inherited, and then over many generations,
the population changes, and this is
evolution. Natural selection isn't the
only cause of evolution, it's just the
most interesting one. So anytime you have
these adaptations building up in a
population, you're gonna have allele
frequencies changing. Now let's go
into a little bit more detail about
natural selection. First of all, Darwin
noticed when he was making all of his
voyages all over the world, that most
species produce more offspring than are
necessary to maintain the population, and
in fact more than can probably survive,
due to limited resources. There is an
example of this actually in the
Galapagos Islands with blue footed
boobies. The females always lay two
eggs, and when the eggs hatch, the bigger
sibling will push the other one out of
the nest, and there are very very limited
resources in that particular location,
and the female can't really feed herself
and two offspring. In fact, they found
that any female that attempted to feed
both offspring, was less likely herself
to survive and
be able to produce offspring and future
generations, and that is sort of leading
to what I was talking about about
competition for resources. If you've got
overproduction, it's going to lead to
competition for resources. Sometimes
competition for resources might mean for
food, water, or shelter, but it also might
mean competition for mates. Now due to
mutation, there's going to be variation
in a population. New traits come only
from variation, but you can get new
combinations of traits together, due to
random assortment of homologous
chromosomes, random fertilization, and
crossing over. Due to variation, some
individuals are going to be better able
to survive and reproduce in a particular
location, so whether
something is an adaptation really
depends on the environment. So for
example, if an organism had a mutation
that made it have white fur, that might
be helpful in in a climate that's very
snowy, and may be harmful in a jungle
population. So it would be an adaptation
for an organism that lived in a northern
climate, but not near the equator. So an
adaptation is any inherited trait that
improves an organisms chance of survival
and reproduction. In order for evolution
to occur, the organism has to survive, but
it also has to reproduce and passed
down those traits. Natural selection
is when individuals with variations that
make them better adapted to their
environment, survive and reproduce in
greater numbers than those without those
traits. So in other words, mutation is the
raw material for natural selection,
because without mutation, you don't have
variation. Without variation, there can't
be certain traits that help you survive
or reproduce. Over many generations,
favourable adaptations may accumulate in
the population and unfavorable traits
disappear, and eventually, the changes are
so great, that the net result could be a
new species.
We're going to be talking about
speciation in more detail in a future
lecture. The last topic I want to
talk about is "fitness". You may hear the
term "survival of the fittest".
But Fitness is not the idea that only
the strong survive. Sometimes you'll be
watching a nature show, and some big
predator will catch prey, and the narrator says "Oh,
that's survival of the fittest",
but Fitness is a much more specific idea
than that, Fitness is reproductive
success. So just because you are a great
hunter ,doesn't mean you have great
fitness. It might lead to that if you're
able to get more food for your offspring, and therefore reproduce
in greater numbers, but fitness is
reproductive success. It's the number of
offspring and individual produces, that
survive to reproduce, and its relative
compared to others in the population. So
for example, if a human had ten offspring,
that would be relatively high fitness.
But it might not be high fitness for
bees per se, so it really has to do with
what proportion of the next generation
is related to you.
So fitness is reproductive success, not
necessarily how well adapted you are to
your environment, although there is a
link between the two.
