Introduction to Genetics and Evolution is a college-level class being offered simultaneously to new students at Duke University. The course gives interested people a very basic overview of some principles behind these very fundamental areas of biology. We often hear about new "genome sequences," commercial kits that can tell you about your ancestry (including pre-human) from your DNA or disease predispositions, debates about the truth of evolution, why animals behave the way they do, and how people found "genetic evidence for natural selection." This course provides the basic biology you need to understand all of these issues better, tries to clarify some misconceptions, and tries to prepare students for future, more advanced coursework in Biology (and especially evolutionary genetics). No prior coursework is assumed.
Optimality and Adaptive Feeding (G)
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Adaptive Behaviors and Sexual Selection
This module changes gears a bit to look at the exciting field of animal behavior-- specifically, how particular behaviors are or may be adaptive, and why individuals choose particular others as mates.
與講師見面
Dr. Mohamed NoorEarl D. McLean Professor and Chair,
Biology
Hello, and welcome back to introduction to genetics and evolution.
In today's lectures, I'd like to take a little
foray into a very exciting subarea of evolutionary biology.
And that's the intersection between evolutionary biology and
the study of animal behavior.
In this first lecture, we'll look at optimality and adaptive feeding behavior.
But first, let's talk a little bit more broadly about animal behavior.
Animal behavior is a very, very active research area, and
particularly studying animal behavior in an evolutionary context.
You can study animal behavior from many different approaches,
you can look at hormonal basis for animal behavior, you can look at neural basis for
animal behavior.
What we want to look at now, rather than proximate cause, is we want to look at
the ultimate reason for why animals behave the way that they do.
Now behavior, as I mentioned, integrates these many different aspects of biology,
such as physiology or hormones and
nerves, genetics, ecology, and fundamentally evolution.
Now some areas where evolutionary thinking is applied to the study of animal behavior
is in the context of survival or avoiding predation.
Obviously, very important for passing on your genes,
is that you don't get killed early.
Feeding or foraging behavior,
it's obvious this is what's happening at the other end of this.
[LAUGH] Choosing where to live, communicating, reproductive behavior,
sexual selection.
This one we'll talk about in a different context much later on,
but not here in these videos.
Parental care and social behavior.
So we'll dabble into a couple of these in the videos that you have here.
The ones in this particular video that we'll talk about are looking at optimality
theory, or specifically, how you can get the maximal effect for the minimal cost.
Which is what we would infer,
that natural selection would push a species to ultimately be able to do.
And we'll look at applying optimality theory to the study of feeding behavior,
and why feeding behavior is often adaptive.
Now as you know, the evidence for natural selection abounds across species.
So let's look at the example of the Eurasian oystercatcher.
Now this species feeds on mussels.
So, not muscles like this, but
mussels like the small clams that you have over here.
Now, there's an optimal size of mussel for this bird to pursue.
If the mussel is too small, they're not gonna get enough nourishment.
It's not worth it for
them to spend the energy to open something that's gonna give them almost no energy.
If it's too large, it becomes extremely hard to open, at which point they'll
expend tons of energy and may never get any nutrients out of it as well.
So there should be some optimal intermediate size mussel,
these oystercatchers would be selecting that would be, in principle,
selected by natural selection.
Now, people have ran a bunch of models and
they suggested that oystercatchers should pick 50 millimeter size mussel.
That was the prediction.
From these initial models they looked for this, and
in fact there was not a good fit of the models to the data.
In fact, these oystercatchers were picking smaller mussels.
So they refined the mussels, and they predicted with this refinement that
the oystercatchers should pick 30 to 45 millimeter mussels because the larger ones
are covered with barnacles and that may make them harder to open.
Now in fact, the birds do prefer 30 to 45 millimeter mussels,
so with this revision of the model, the model fits the data.
This should be causing you a little bit of concern when somebody just
manipulates a model to make it fit the data, right?
Well, there is this view that selection is everywhere, right?
That structures and
behaviors are optimally designed by natural selection for their function.
And whenever we see something that's not optimal,
we infer a trade-off or some compromise among competing demand.
Now this is a little bit of a risky thing because basically you're making it so
every possible observation will eventually fit your model.
Now Gould and Lewontin criticized this, and
referred to this as the Adaptationist Program.
They suggested that, many researchers were doing this,
they were interpreting data by assuming the near omnipotence of natural selection
in forging organic design and fashioning the best among possible worlds.
This is often referred to as adaptive storytelling by people who don't like it.
That, we see something and just make up an adaptive story until it seems to fit and
then say oh, that must have been it.
Yeah, yeah, that's it.
[LAUGH] That's not good science.
Unfortunately, this kind of thing has happened for a very long time.
Overinterpretation goes back over a hundred years.
In fact, if you look back before 1909,
Spencer asked Galton to look at his fingerprints.
But Galton said he didn't know the functions of these patterns, and
many people had studied this, dissecting the fingers of unborn children trying to
study their prints, and things like that.
Now Spencer suggested, well the ridges obviously, this is an important word here,
obviously functions to protect the sweat glands in the valleys.
Sounds like that makes sense.
Problem is, the glands are actually in the ridges themselves, so
actually doesn't make sense at all.
[LAUGH] Science has been referred to by Thomas Henry Huxley as the organized
common sense, where many a beautiful theory was killed by an ugly fact.
In fact, despite the use of optimality theory, and
optimality theory is actually a good thing when applied properly,
there are many traits and behaviors out there that are not perfectly adapted.
Now there are many reasons why traits or
behaviors may not be perfectly adapted as we see them now in the wild.
Well one possibility is that there's been a failure for
the appropriate mutations to occur.
That, if the mutation for making it just that much better never happened,
then it's stuck where it is, right?
You can have single genes that cause multiple phenotypic effects.
This referred to as pleitropy, where for
example, an allele may be good for one trait but bad for a different trait.
So it's that there's this sort of trade-off going on.
There is a possibility that ultimately the trade will be perfectly adapted, but
there hasn't been sufficient time for selection to operate.
Or the environment keeps changing and it's this moving target that
makes it very hard for a trait to be optimally adapted for any particular.
There are many, many other reasons.
This is just a subset of those.
But very importantly, optimality predictions should be tested and judged,
not just presumed.
Optimality, when I use this term,
is the assumption that by knowing exactly how natural selection's acting on
this trait, we should be able to predict exactly what the trait should look like.
Now let me give you an example where there was some misunderstanding again,
in terms of understanding a trait.
This is the example of the oxpecker.
This is a bird that you often see on top of an ox, for example.
They live on large mammals,
they're thought to feed somewhat on ticks at least.
And the dogma that was repeated for many, many years is it was mutually beneficial
for the oxpecker bird and the ox to be associated because the bird is picking off
these ticks that are bad for the animal, and so the mammal just leaves him there.
Unfortunately, there were no good tests of this until around 2000.
And what happened then is people started exploring whether the cattle that had
oxpeckers had more or less ticks than cattle without oxpeckers.
Now if the oxpeckers are picking off the ticks, then you would assume
the cattle that have the oxpecker bird should have fewer ticks, right?
Well in fact, tests showed there is no significant reduction in tick
load when you had oxpecker.
So they tested this multiple times and there was no significant reduction.
In fact, further tests showed that these oxpeckers were actually
enlarging open wounds and drinking the blood from the ox.
That's not good.
That's not mutualistic at all.
Something very much to the benefit of the bird,
but to the detriment of the large mammal.
Now the question you might wanna ask is, why do the cattle
tolerate these birds hanging on them that are opening their wounds?
Well I asked my class this back in 2012, but here's a set of some of the responses.
On the negative side, somebody suggested that the hosts don't
actually tolerate the oxpeckers, but they just are not able to get them off.
That is a possibility.
There's a neutral side that maybe it's not actually that big a deal to the cattle.
So it's kind of like when a fly's buzzing in my face and after awhile,
I just let it pester me cuz it won't go away.
This suggests that maybe the oxpeckers affect on the cattle was fairly small,
even though they're getting potentially nutrients themselves.
There's the mutual advantage side that somehow they're strengthening
the cattle's immunity [COUGH] possibly by increasing antibodies to fight antigens.
That would definitely be an adaptationist's explanation.
And there were some crazy out there explanations people suggested,
like maybe they like the company, or the oxpeckers generate mini force fields.
That's probably not it.
[COUGH] When we look at the research results for why this was the case, so
looking at the research results for why the hosts tolerate these, it turns out
in fact that some of these hosts try to get them off, but are unable to do so.
Just like that one student had suggested the previous slide.
That, in the case of rhinos they tried quite hard,
but were only able to get them off half the time when they were at wounds.
So this figure down here shows attempts to remove the oxpecker from at wounds.
So successful versus unsuccessful, so
only about half of them were successful at getting them off the wounds.
In contrast, when they are on the ears, they are able to flick them off fairly
easily, or other places on their body they're able to flick them off easily.
So it is not a mutualistic thing as was previously assumed,
even though that was the adaptationist's explanation.
So let's look at applying optimality theory to studying adaptive
feeding behavior.
But let's try to be careful not to be overly adaptationist.
So what considerations go into optimal feeding?
Well if you want to feed, and
again you wanna do it as efficiently as possible, what you wanna do is you want to
maximize energy while minimizing consumption of energy and time.
So high energy per low unit of time is the best.
So on the plus side, you're getting calories from food, that's your energy.
On the minus side, you're expending energy to get the food, so
energy may be involved in searching, handling, eating or digesting.
And on the minus, there's also time involved in getting food,
which also involves, of course, burning more energy.
But there's time involved in searching, handling, and digesting.
So if we were to put together a very simple formula for
optimal foraging, we want to maximize this quantity.
We want to maximize calories obtained from food minus calories expended getting food,
divided by time to get it.
Again, we want the prey with the highest caloric content, we want to spend
the minimal energy and time getting this prey, so this is the idea behind it.
So let's look at this in the context of whelk-eating crows,
another set of birds eating shelled animals.
[LAUGH] In this case, these are crows in British Columbia that pick up whelks on
average about 4 centimeters long rarely less than that.
They fly up into the air with these whelks and they drop them.
They fly almost exactly 5 meters and then drop them try to shatter the shell and
get the snail meat outside.
Now sometimes it takes a couple of flights to do it.
Is this adaptive?
There's a couple of different things these could do.
They could pick different sized whelks, they could fly different heights,
they could fly more times.
So let's look and see how this might be adaptive.
So break it up into parameters.
First, let's look at the height of drop.
Obviously, it takes energy to fly high, so
you wanna minimize the energy you're taking for flying.
But you want a high probability the shell will break.
You don't wanna have to fly 40 times.
You wanna fly like once or twice and be able to break it.
So people looked this in the context of the underlying physics, and it turns out,
in fact, 5 meters height is optimal for that sized shell to shatter.
So you can look here on the x-axis, the height in meters.
So this is from 0 to 15 meters in the air.
On the y-axis is number of drops required to break the whelk shell.
And we note when you're at 5 meters,
you're pretty much at a level point there where it doesn't seem like if you go much
higher you have a much higher probability of breaking the whelk shell.
So that's at a very optimal point.
There's very little increase in probability in shell
breaking if you go any higher.
So that does seem to be adaptive.
What about the size of whelk?
Couldn't they have picked maybe a smaller whelk?
Why didn't they pick the smaller whelk?
Well they experimentally tried three sizes.
In this case, the one they picked is referred to as large.
These are the 4 centimeter whelks.
And they found that the bird would need to fly higher and/or
drop more times to break the smaller shell.
So again, on the x-axis here we show the height of the drop,
on the y-axis we show the number of drops needed per whelk.
For the large one, again, this is what they ended up choosing.
If they chose a smaller whelk, a medium-sized one, they would have to
drop it twice that many times, and even more if they chose a very small whelk.
So again, it seems like they did choose adaptive.
Now a question a lot of people like to ask is, do we see optimal feeding in humans?
Well there is the case of spices.
And spices have been suggested as a place where humans may actually choose some sort
of optimal.
Now the caloric content in them is typically very low, so
why do we like spicy food, why do you like garlic?
Maybe you don't like garlic.
One possibility is it's just random or some correlated response to smell or
taste that's unrelated to the actual garlic itself.
Other possibility is there's an antimicrobial property
associated with some of the spices we use.
In fact, many spices are known to have very strong antimicrobial properties.
Cinnamon, cloves, and mustard have strong antimicrobial effectiveness.
Some of the others have medium antimicrobial effectiveness,
like allspice, cumin, oregano, etc.
And some inhibit very specific things.
Like garlic, for example,
as I mentioned inhibits salmonella, E.coli, staph, bacillus.
Cloves inhibit Aspergillus, things like that.
So let me ask you to generate a prediction in video quiz here.
[COUGH] If spices are used because they're antimicrobial, and given what you might
guess about what countries have particular environments that are warm or
moist, and thus more likely to have a lot of microbes.
If you surveyed the following countries, India, Hungary,
and Norway, which would you expect to use the most spices in their cuisine,
specifically the most spices with antimicrobial properties?
Well that was a pretty easy question, I think.
The expectation is that more antimicrobials are needed in
environments that will favor the growth of microbes.
So places where its warm and moist will favor the growth of microbes.
So if we look at these three countries, India, Hungary, and Norway,
it's a no-brainer that the wettest place and the hottest place is India.
So this is a place where more antimicrobials will be needed,
Hungary is kind of intermediate, and Norway really is not particularly wet.
Well, it's somewhat wet but very much not hot overall.
If we look at the spices used, what we see here is, this is a list of spices on
the x-axis, on y-axis, percent of recipes that have any of those spices.
Tons of spices are used in India, somewhat in Hungary, and not so much in Norway.
Way more spices are used in India than the other two, and importantly with the red
arrows, I designated the use of garlic and onions in particular.
Remember I mentioned that garlic had very specific antimicrobial properties.
Let me show you the antimicrobial effectiveness of a suite of spices.
Here are garlic and onions.
They show almost complete inhibition of bacteria, so these are very,
very effective antimicrobials.
In contrast, something like celery seed is not particularly effective at all.
So you see here garlic and onion are almost completely effective, allspice and
oregano also very highly effective, thyme,
etc., starting to show slightly weaker inhibition to bacterial growth.
So that's pretty cool.
Now, in this case, we didn't really consider alternatives.
So what are some of the alternative explanations that should be considered?
One is that there actually are micronutrients being provided by
these spices.
It's a possibility.
I know that spices are used to disguise the smell of spoiled food.
And last is maybe they're just used where they grow.
That maybe all these things grow in India.
They don't grow in Norway or Hungary so much.
Well these have been investigated.
Now in terms of the micronutrients that doesn't really explain the correlation
with temperature, why is that people in this particular temperature need these
micronutrients more than others.
In terms of disguising the smell of spoiled food, that actually doesn't make
sense evolutionarily because it's not to your advantage to consume spoiled food.
If anything, it could be to your disadvantage.
And in terms of being used where they grow, there is actually a very weak
correlation between where they grow and where they're used.
But that doesn't really explain the overall pattern.
For example, pepper is one of the most widely used and it grows in a very,
very small fraction of the places so it doesn't seem like that's explainable.
Now, just extrapolating from this into the importance of communication.
Communication is critical for the context of finding food for a lot of species.
Not just for making food available to us.
A classic case for this is that of honeybee dances.
This was elegantly described by Nobel laureate Karl von Frisch.
He showed that scout bees, actually, will come back to the hive and
can actually give hive mates detailed information
about the location of food by away of how they do their little dances.
Now, I won't try to explain this dance to you, but I'll refer you to a YouTube link.
This is also available online.
But you should definitely watch this.
It's very cool, this means of communicating where food is.
So we've been looking at this all in the context of a adaptive feeding behavior but
as you seen with the examples I'm talking about here in the context of bees
communication, sometimes is very important as well
We'll come back to that in the next video
Thank you
