In this video we are going to explore two
different
ways that biologist explain the living world.
We are going to think about proximate explanations
and ultimate explanations.
What do I mean by these two
different types of explanations?
What are these two different approaches to
biology?
I want to emphasize that these different types
of explanations are complementary.
Each informs the other.
Proximate questions are
"how does it work?" questions.
Proximate explanations are concerned
with how living things work.
These can also be called mechanistic questions,
they explain the mechanisms
that underlie a biological system.
Ultimate questions deal with why living
systems work in the way they do.
These are questions about how
living systems evolved,
and how we can explain why organisms
are the way they are
based on our understanding
of evolutionary processes.
Proximate questions explain
how living systems work.
They deal with mechanisms.
These mechanisms can be internal,
such as how genes are turned on or off,
or how nerve cells work,
or how cells communicate.
These mechanisms can also rely
on external signals,
such as how an organism responds
to environmental changes.
Obviously internal mechanisms are used
to respond to external signals.
For example,
you automatically pull your hand
away from a hot stove,
a behavior based on the
external environment,
but depends on how your nervous system is designed,
an internal mechanism.
I find that these sorts of questions are
difficult to think about in the abstract.
So we are going to think about a specific
example,
control of the lac operon in bacteria,
which you may have learned about before.
But don't worry if you haven't -
this is just an example,
we won't be using this example in the case
study.
The lac operon is a group of genes in a bacteria
that allows the bacteria to use the sugar
lactose as an energy source.
There are several genes and a control region
associated with the operon.
For simplicity we are only going to talk about
one of the control regions and three of the genes.
Lac I encodes a repressor of the lac operon
and this gene is controlled separately
from the other genes.
Lac Y encodes a transporter that allows
lactose into the cell where it can be used
and Lac Z encodes an enzyme
that cleaves the disaccharide lactose
into monosaccharides and can
be used for energy production.
Lac Z and Lac Y are controlled by cis-regulatory
regions upstream of the operon.
This region contains a promoter,
where RNA polymerase can bind,
and a control region to which
the lac repressor can bind.
So let's start with what happens when
the bacterial cell finds itself in an environment
where there isn't any lactose around.
The lac repressor gene is transcribed
and translated to make repressor protein.
The repressor protein binds to
the operator control region.
When RNA polymerase binds to the promoter
and tries to move down the DNA
it runs into the repressor and falls off the
DNA.
So lac Z and lac Y are
not transcribed or translated.
Okay, but what happens when a bacterial cell
is in the presence of lactose
and needs to use lactose as an energy source?
When lactose is in the environment,
there is a bit of a derivative of lactose in the cell,
called allolactose, another disaccharide.
Again, the lac I gene is transcribed and translated.
But it binds to the allolactose.
So how does this affect the behavior
of the repressor protein?
The repressor protein with allolactose bound
to it
does not bind to the control region of the
operon.
So now, after RNA polymerase has
bound to the promoter
it is able to move down the DNA and the lac Z
and lac Y are transcribed and translated.
Lactose can now be brought into the cell
and used as an energy source.
Okay, so that is HOW the lac operon works.
Now we might want to understand
why it works that way.
To understand this we need to look at
the adaptive value of only expressing genes
needed for lactose metabolism
when lactose is around.
What effect does the ability to express the
lac operon only when needed
have on survival and reproduction
of the bacterial cell?
Any differences will
result in natural selection.
And selection on genetic variation
among bacterial cells
in how well they control their
lac operon results in evolution.
So we can speculate on why the lac operon
is so tightly controlled
and why bacterial cells
have mechanisms to only express
these genes when they are needed.
It takes precious energy (ATP) to produce
enzymes
and it's wasteful to produce enzymes
that aren't needed.
Imagine two bacterial cells.
One genetic variant, the yellow cell,
has alleles that suppressed the production of enzymes
until they are required.
(In this case, only producing the enzymes
to
transport and metabolize lactose if lactose
is available.)
This genotype is less wasteful and had more
energy to produce more offspring.
This less wasteful cell might be able to divide
3 times between time 1 and time 3,
while the more wasteful cell might
only be able to divide twice.
That extra division means there are
twice as many efficient YELLOW cells
as there are wasteful GREY cells.
Think about how rapidly bacterial cells divide.
It wouldn't take very long for the more efficient
genotype to swamp out the wasteful genotype.
Thus, these alleles would be present in higher
proportion
in the next population of bacterial cells.
Here's another example.
Golden jellies migrate across the lake each
day -
They are found in the west
in the mornings and evenings,
and in the east side during the day.
Again, we can consider the proximate explanation
-
photosensitive neurons and the nervous system
allow swimming behavior to be coordinated
with light.
The ultimate explanation is that
symbiotic algae provide nutrition to the
jellyfish that is dependent on photosynthesis.
Jellyfish that contained alleles that allowed
them
to maximize exposure to sunlight maximized
photosynthesis and energy production
and this enhanced their reproductive fitness
relative to jellyfish without those alleles.
Okay, we are now ready to talk about how and
why
giant pandas are able to eat such a unique
diet.
