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.
Intro to Sexual Selection (G)
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Introduction to Genetics and Evolution
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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.
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Dr. Mohamed NoorEarl D. McLean Professor and Chair,
Biology
Hello, and welcome back to Introduction to Genetics and Evolution.
In this video, we'll be talking about a new process.
We'll be looking at sexual selection.
We've talked extensively before about natural selection.
Now, we wanna focus on that facet of natural selection
devoted to how individuals attract mates.
Now we talked briefly before about the advantages of sexual reproduction.
But we didn't do is we didn't address some of the striking outcomes in terms of both
behavior and appearance.
So let's dive right in.
First, what is sexual selection?
Well again natural selection and sexual selection are intrinsically related.
Natural selection again is when you have heritable traits that aid the survival or
reproduction, and those traits then spread in natural populations, okay?
Now with sexual selection,
we're gonna be looking at just a subset of those features.
We're gonna be looking at those related to the ability to secure and
reproduce with mates.
Now obviously you need to reproduce to pass on your genes, and
doing so better passes on more of your genes.
So there's more copies of it in the population.
So, if you can have traits that make you mate more, that's probably good.
So if you have a trait that attractive, that's good right?
Well, some traits that seem to be tied to one sex appear to
be maladapted to survival.
They appear to be bad for survival.
So this creates a bit of a conundrum.
How do these maladaptive traits spread?
And why do you intend to see these, it's not always, but
we tend to see these exaggerated traits in males.
Let's look at a few examples here.
So we have over here in the left, top block the extinct Irish Elk.
This had huge antlers.
And they're estimated maybe to weigh on the order of 80 pounds or
perhaps 35 kilograms.
That's huge.
Why on Earth was it good to carry this heavy thing on your head?
Well we have a couple of other things that are probably familiar to you like
the peacock with its elaborate tail.
In this case, it has it down.
Here's a rhinoceros beetle, look at that big horn that it has there.
And, we have here the red-collared widow bird that has this
very long extended tail.
Well, Darwin saw all these things, and he figured something must be up.
So, what he suggested and has since been confirmed many,
many times, is that these traits are exaggerated through sexual selection.
So here's a direct quote from Darwin.
The habitual or even occasional preference by the female of the more
attractive males would almost certainly lead to their modification.
That because these are attractive,
they are enhancing fitness overall and they are spreading.
Now, what Coyne actually clarifies this in his book associated with his class,
sexually selected traits will evolve if they more than offset
the male's diminished survival with an increase in his reproduction.
So yes, although maybe he's more likely to be caught by a predator.
Since he mates so much more, it more than offsets that survival disadvantage.
Well, there's a key for this to work.
The key is you have to have a lot of mates, okay?
So how many mates do most organisms have?
Well, when you look across the animal kingdom, multiple mates for males or
females are quiet common.
Then you're maybe familiar with this idea of social monogamy.
That's when you have the social bond of one male and one female at a time.
People tend to think of ducks, so this is actually a poor example.
People tend to think of ducks because they see the male duck and
the female ducks swimming around together.
Now, separate from social monogamy, there's also sexual monogamy.
That's when partners are sexually exclusive with each other.
Now with the growth of DNA based assays for paternity.
What people have found is a lot of these species that appear to be
socially monogamous, are actually not sexually monogamous.
In fact, extra-pair copulations are quite common.
Now the extra-pair copulations are when you're essentially breeding with somebody
outside of your social pair.
This is quite common across much of the animal kingdom.
And again with more genetic data we see this much better.
Now if we assume there are multiple mates, then the increase in number of matings, or
increased number of partners, will often increase your potential reproduction.
Again, this assumes non-monogamy, right.
So this is an important facet, to this.
We are assuming non-sexual monogamy.
Now what's interesting is the difference between sexes
in potential reproductive rate, is very important.
Like basically, how many offspring could you have?
Well, let's look at the world records for humans,
though I don't usually like using humans for these sorts of examples.
The world record that's know for a human female is about 69 children.
That is obviously huge.
Now, what would you think the world record for a human male would be?
Now, potentially you can think, well, how many kids could a male sire over
the course of a year versus a female sire over the course of a year?
The female, you're looking at one to two,
maybe a little bit more if they have multiple kids at a time.
But with males, the potential reproductive rate is much higher.
And, in fact, the world record for a human male is somewhere between 867 and
1,000 [LAUGH] over their lifetime.
Again, the potential reproductive rate for
males is much higher than the potential reproductive rate for females.
Now why this difference?
Well, part of the reason is probably that females often invest more in resources and
in care, and that limits their reproductive opportunities.
All right, so for example, the human female.
If they're gonna give birth to a child, they have this long period of labor
through which they cannot be reproducing anymore.
They are stuck with that period.
And then even after that,
there is this care post-reproduction that also slows them down.
Now, an estimate from one study suggested that, on average,
females devote 100 to 1000 times more resources to egg production
than males devote to ejaculate production.
That's a pretty big difference.
Just as one extreme example many of you are probably familiar with
the bird, the kiwi.
Well it's egg is 15% of its overall weight.
So just that physical investment in creating this one offspring is huge.
Now a lot of these ideas fall from an interesting principle.
This is referred to as Bateman's principle.
So let's look at this first in terms of the females, and
then in terms of the males.
With the females there is typically low variation in female mating success.
And, by that, I mean that most females will tend to mate, and
they will tend to produce offspring, okay?
Now, interestingly, more matings by female does not necessarily
lead to more offspring, because again, they're sort of capped,
that they can copulate multiple times, but that's not going to
increase the number of times they actually produce offspring, for most animals.
And they are limited by resources then, again there's a high investment for
producing these offspring.
With males a lot of that isn't true,
now there is typically very high variation in male mating success.
There's some males who will produce many, many,
many offspring with many different females.
And there are a lot of males who will just produce no offspring.
So there's quite a bit of this variation in male mating success.
I'll show you an example of this in just a couple of minutes.
Typically with males more matings, in this case meaning more matings with
different females, will typically lead to far more offspring.
And finally they are limited by access to females.
So there's a lot of this competition for access to females.
Now, what is the relationship between mate number and offspring number.
Let me show you one example.
Here's a difference of outcome from having more mates.
These are from studies of the Rough-skinned
newt as you can see over here.
Now I'd like you to look at how the x axis here we have different numbers of mates.
And the y axis here numbers of offspring.
The orange dots are for females the blue dots are for males.
Now you look there at the correlation for males, as you have more mates,
you typically have a lot more offspring.
With females, interestingly, as you have more mates,
you don't typically have a lot more offspring.
So how does that work?
One question people always ask is how is this possible since
everybody has a mother and a father?
How can you get this difference?
Well, let me give you some made-up, but representative, data to illustrate.
And I'll show you some real data in a couple of slides.
Here's a likely scenario, what you might see within a species,
when you look at variation in mating success within a mating season.
With females, the vast majority will mate once.
Some may mate zero, some may mate a little bit more, but on average,
they're just getting mated and that's it.
Mostly the reason for this is a lot of females, actually once they have mated in
different species will not mate again with another male for awhile.
Okay? Now what you'll see here for males,
you see this much bigger spread that a lot of males have no mates.
Some have one, some have two, some have three, but
you have a couple of the super males who have many many offspring.
If you look at these two things, they have the same average number of mating for
males and females because, again, every copulation has to involve one of each.
But with females, most of them succeed once.
With males, many, many fail, and they're very, and the subset are very,
very successful.
Now this spread here, leads to very intense competition among males.
All the males wanna be here to the right end of it.
They don't want to be at the far left end of this.
Let me show you some real data with elephant seals to illustrate.
With elephant seals, and
this has been studied quite extensively as one of the extremes examples,
you have this super variance in male mating success.
So in one study they showed that 28% of males were observed mating at least
once in a two year breeding season.
So what does that mean?
Is that probably about 72% were never observed to mate.
So a large number of males were never successful.
Among those that did mate, the average number of mates was quite high.
They actually mated about 20 different times.
Now the distribution is highly correlated with the distribution of paternities.
So those that did mate a lot did sire a lot of offspring.
So this big spread for males.
Here's an example from one group with 14 males.
The 11 bulls sired no pups.
Two bulls sired one pup each and one bull sired 24 pups.
So again, there's this intense competition.
There's a couple of big winners, and there's a lot of loser with males.
All right.
So again, this is very, very strong sexual selection.
You don't tend to see this sort of spread with females.
Because again the potential reproductive rate is much lower.
Now what is the outcome of this?
Well one outcome which is quite interesting is that
we tend to see male-female dimorphism correlating with sexual selection.
What do I mean by dimorphism?
By dimorphism I mean, some difference between males and females.
Often in physical size or
weight that you know if you are, in this case on the right,
we have a dimorphic couple where the male's huge and the female's little.
This case here we have a mono morphic couple where they're about the same size.
Typically speaking,
when you have few males monopolizing most of the females, and
then you tend to see this big difference in size between the males and the females.
So in the case of the Southern elephant seals I showed you earlier, the males can
be as large as 8,000 pounds, whereas the females maybe about 2,000 pounds.
So big, big difference in size.
Now with humans you can see that yes, there's an average difference, but
we're a lot closer.
There's probably not quite the same extreme variance in
humans as you see in elephant seals.
Now, when you have less variation in male-mating success, the females and
males tend to be more similar in size.
So this is a decent indicator.
In this pattern, you can't even see it when you look across primate species, so
even including Chimps, gorillas, etc.
Now it's not perfect, but at least gives you some idea.
So how might we leverage this?
How might we leverage association?
Well, here's a couple of choices for you.
This is a question for you to answer.
Could we use this to assess sexual selection in extinct species?
Could we use this to assess sexual selection in newly discovered species?
Or could we use it to study which characters are influenced by
sexual selection?
Or maybe all of the above?
Think about that.
Well I hope that wasn't too difficult.
The answer was indeed, all of the above.
We can use this to assess sexual selection in extinct species using fossils,
because if we see this extreme dimorphism, and that tends to be associated
with extreme variance in mating success, so strong sexual selection going on.
So that's pretty cool.
We can do the same thing with a newly discovered species which we haven't had
a chance to do extensive tests on.
And we can look at the specific characters that are likely to be influenced by
sexual selection,
because those are the ones that are likely to show the greatest dimorphism.
Okay?
So, how is this competition occur?
I've just been describing in a context of competing for access to mates, but
I haven't really gone into it any more than that.
Well, there's one sex,
typically the males, that is subject to the strong sexual selection.
In this case, there should be competition,
there should be competition among the males for the females.
So it's competitive, there might be fighting,
there might be something to keep other males away from the females.
Now, the other sex, typically the females, is subject to much weaker
sexual selection because they all tend to get mated to and produce offspring.
That sex,
the one which is under weaker sexual selection, should tend to be choosy.
They can be very picky.
They can afford to be picky because they know they're going to mate.
I'm anthropomorphizing, they don't actually think this, but there's a high
probability that they will mate, so they can afford to be quite picky.
Now Darwin recognized these two types of sexual selection that broadly referred to
as male competition and female choice.
And that will be the subject of our next video.
Thank you, I hope you enjoyed this.
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