 
 
 
 
Hi. It's Mr. Andersen and this
AP Biology science practice 6. It's on scientific
explanations and theories. So remember in
science we come up with a good question. We
gather good data. We analyze the data. And
then over time we start to develop theories
which explain how the world works. And so
natural selection was developed by Charles
Darwin and it's been tested and tested over
time. What is it? It's a mechanism that explains
how evolution can occur. In other words the
finches on the Galapagos had variation in
their beaks and they each adapted to their
specific food supply over time. And so how
does a theory develop? In other words, where
do theories come from? Well they start when
a scientist comes up with an idea. They then
preform experiments and they look, does the
evidence I'm collecting fit or support this
idea? If it doesn't, then I go back. I had
a bad idea. I've got to get a good idea this
time. So I come up with a better idea. Perform
more experiments. If the evidence does support
the idea, then I can create a theory. And
that theory is used to better understand the
universe. So natural selection is a great
example of that. What happens next, then we
discover new evidence. And can we modify the
theory a little bit so it can explain this
new evidence. And so natural selection had
to be modified when we started thinking about
sexual selection. And so if we can, then we
improve upon the theory. And so this cycle
occurs over and over and over again. It's
a feedback loop to make sure that our theory
matches really what's going on in the universe.
If the theory doesn't match that, so before
natural selection we had this idea that God
had simply placed all of the species on the
planet, perfectly evolved to where they are.
And so if that theory doesn't fit with the
data we're collecting, that leads to a revolution.
And so over time science gets better and better
and better. Explanations get better and better.
So again the College Board want you to be
able to look at theories and apply theories
in each of the four big ideas. So in evolution,
not only should you understand how natural
selection works, but how antibiotic resistance
or changes in bacteria as a resistance to
our antibiotics, how that fits into natural
selection. Or maybe you understand how feedback
loops work. How we regulate the amount of
blood glucose. But what if we have changes
to the insulin receptor? How would that effect
our explanation? Or we all know that the genetic
information sits in the DNA. But how do we
get changes that aren't related to changes
in the DNA? How can the environment change
what genes we're going to express and that
has to do with remember the histones and methylation
of the histones are opening up those histones
so we can get to the DNA. Of if we're looking
at systems, how is that the order by which
molecules are added to macromolecules? For
example the amino acids are added to proteins,
how is that going to effect the overall structure
of the protein itself? And so I'm going to
go through the five ways they want you to
deal with theories and explanations. And the
first one they would like you to be able to
justify claims with evidence. And so which
of the following statements most directly
supports the claim that different species
or organisms use different metabolic strategies
to meet their energy requirements? And so
these are examples of questions you might
find on the test. And so you may want to pause
the video and take a stab at each of these.
So you may want to read through A, B, C and
D. As I worked through this, they all were
good, but the only one that really got to
the root of changing metabolic strategies
was D. So the right answer is D. They also
want you to construct explanations based on
evidence. And so let me set this up. We've
got different plates of agar. On these ones
there's no ampicillin in the agar. Ampicillin
remember is an antibiotic. On these we do
have ampicillin. We have wild type e. coli.
Just regular e. coli. And then e. coli that
we've transformed with an ampicillin resistant
plasmid. And so looking at this data, which
of the following plates have only ampicillin
resistant bacteria growing? The right answer
we would say is C or 4. Because we've transformed
some of these bacteria. Now we probably transformed
them here, but they only show ampicillin resistance
in plate 4. Or let's go to the next one. State
the conclusions reached my Mendel and his
work on the inheritance of characteristics.
And explain how these following deviate from
this conclusion. So here we're trying to articulate
the reasons that explanations and theories
are refined or replaced. And so again if I
were to do Mendel's Laws I would first say,
the law of segregation. That alleles are going
to segregate as we form gametes. And then
the second one is the idea of independent
assortment. That genes are not going to effect
other genes. And then I could go through and
look at autosomal linkage. Do genes ever assort
non-independently. They do if they're on the
same chromosome. Or we could look at sex-linkage
or polygenic inheritance isn't going to follow
straight Mendelian inheritance. And so again,
it's not that Mendel was wrong. It's that
we had to refine his theory over time. So
this would be like an essay type question.
They also want you to be able to make claims
and predictions about natural phenomena. So
this would be another essay question. Explain
two of the following three processes using
an appropriate example for each. And then
for each process discuss its impact on the
genetic make-up of the population. So not
only are we explaining the phenomena, but
we're saying you know for each of these how
is that going to impact the genetic make-up?
Genetic drift remember is when we decrease
the population size. An example can be a bottleneck
of the northern elephant seals or the cheetahs.
Migration would be movement into or out of
an area. Artificial selection, like on our
fast plants, is when we're actually doing
the choosing who mates with whom. And then
if we go to the last one, this is a pretty
tough genetics question you may want to take
a stab at it. So here they're presenting you
with a lot of data and then you come up with
alternative explanations. So you could pause
the video and take a stab at this one. They
are telling you that we've got a dominant
and a recessive allele. They aren't giving
the parental numbers here. It's a cross. They
give you the f1 and the f2. And you have to
determine the genotypes of the original parents.
So they're giving you the data and then you
have to work backwards to kind of figure out
the inheritance pattern that's going on. This
is kind how I work through this. We looked
at the parental cross. And so they gave us
a parental of a wild type male and white eyed
female. I then kind of gave my two opportunities.
Number one, since we're getting differences
in the females and the males we know it's
a sex linked trait. And now I have to figure
out, is it sex linked recessive or sex linked
dominant? And so I said here is my wild type
male. Maybe the wild type is going to be dominant
and the white eye is going to be recessive.
And then here's my other one where the white
eyed females, maybe that's dominant as well.
And so then I did all the possible f1s. And
so if we did these two parents, you could
do this just as a punnett square. I could
end up having a male that is different than
the male in the p generation. And same thing
here. So that fits. So we could have a white
eyed male and a wild eye female being created.
But then in the other alternative, it just
doesn't match up. And so again you're looking
at data. You're applying that. Seeing how
it fits in with a theory or we have to refine
that theory. And so theories get refined over
time. So, the germ theory didn't come about
until we could actually see germs and we could
test them using the scientific method. So
before that, we thought you got sick using
Miasma Theory. There was something like bad
air that moved around and caused us to get
sick. And until Pasteur developed his Pasteur
flask and did experimentation on that, you
know the germ theory shouldn't have been accepted.
Until we knew this, that air could move into
the Pasteur flask, but the bacteria would
start to settle out here, did we know that
it was germs rather than bad air that was
being passed on. And so again, theories get
better and better over time. And on this test
they want you to be able to apply those theories
to new data. And I hope that's helpful.
