Natural Selection

Now that you’re signed up for the course, I suppose I should tell you what behavioral ecology is. The easiest way to explain it is with an example.

Why do birds sing?

If we ask why do birds sing, we might have lots of different answers to that. Your aunt might say because they’re happy. Somebody else might say because another bird stimulated them, or it could be that their hormone levels are high. Those are valid answers. So are answers that physiologists might give, or psychologists might give, or somebody just birdwatching might give.

But the answer that a behavioral ecologist would give has to do with the function of the behavior and why it’s an adaptive behavior: birds sing to defend territories and attract mates. In other words, they ask what good does the behaviour do for the animal? What’s its adaptive function?

In this course, we’ll look at a whole bunch of ecological problems that are thrown at animals and see behavior as the solutions to those problems. Animals have to find food, so we’ll look at how they choose their diet, how they search for food. Animals have to avoid predators, so we’ll talk about how they group to do that, and how they hide from predators, and watch out for predators. They have to find a mate, so we’ll talk about behavioral solutions to that, too.

We’ll be asking questions like, what good is a particular behavior? Why do the behavior, and why is the behavior performed in this particular way? Why is it good for solving a particular problem? Why does a birds sing this song rather than another song? And how has that behavior evolved, especially through natural selection? How is it been shaped to to serve some sort of function for the animal? So when we ask why do birds sing, we’re interested in the bottom line: what good does it do?

We’re leaving aside all sorts of other, equally valid answers. Maybe birds sing when they’re happy, or when they’re stimulated by another bird, or because their hormones levels are high. Maybe birds are thinking it’d be a good idea. But all these possibilities, we set aside.

By the same token, we’re not we’re not saying the bird itself thinks it’s a good idea to increase its reproductive success. The birds suddenly decides, “well, I think I should sing because I’ve got to get a mate that way.” People, when they first start behavioral ecology, often get confused because behavioral ecologists say, why do birds sing? Well, they sing to defend territories and attract mates. And beginning people in behavioral ecology often think, well, that means that we’re implying that, that the bird is actually thinking that. But no, we’re not. Actually, what we’re doing is using a shorthand explanation of behavior that it’s been shaped by a history of natural selection.

So in this lecture, what I’d like to do is give you an idea of how to think about adaptive decisions as behavioral ecologists use that term; decisions like how should I sing in order to attract the most mates? It’s not something that the animal is thinking, but it’s something that the animal is wired to do through a history of evolution by natural selection.

A second thing I want to do in this lecture is, is impress on you how not to think about evolution by natural selection. In particular. I want to shoot down this sort of for the good of the species argument that many students still give in the final exam. That’s not how behavioral ecologists, at least, think about evolution by natural selection.

So to do all this, we’re going to start with a review of evolution by natural selection.

Evolution by Natural Selection

This will go fairly quickly because you’ve had all this in your previous courses.

You all know the story of evolution by natural selection. Here’s a cartoon example of it. You’ve got a bunch of beetles. Some of them have spots. Some of them don’t have spots through successive generations. More of the ones without spots survive and reproduce than the ones with spots. For whatever reason, we might, given this cartoon example, could be the other way around. It’s just a cartoon until at the end you get a the trait goes to fixation, the trait of not having spots.

So, yeah, that’s how evolution by natural selection works. And it’s actually very simple. You just need certain things for evolution by natural selection to work:

1. First of all, you need individuals to vary. Yeah. Easy enough. Different things are different. So that’s easy to get.

2. But some of that variation has to be heritable. The main way that happens, of course, is through the genes. But it can be heritable in other ways, too, to which we’ll talk about late in the course. For instance, it could be culturally transmitted, so transmitted through learning and so on. But for most of this course, we’ll be talking about heritable variation through genetics. Now, we’re not going to worry in this course too much about how that’s all done. We’re going to treat that as a black box and not worry about it. But it’s implied in a lot of what we say.

3. Finally, some of the individuals survive and reproduce better than others. It’s just bound to happen, right? But this survival and reproduction is nonrandom. When that’s the case, then inevitably individuals with more suitable traits are going to do better. The ones that survive and reproduce better, there’s going to be more of them in successive generations. And that’s natural selection right there

4. And as a result, what you end up with is that the organisms that are left have adaptations.

That’s really an amazing thing. It’s a counterintuitive thing when we follow all those steps, we say, yeah, yeah, of course that all makes sense.

But it’s really pretty miraculous that, through this whole process, what we can do at the end of this process, say for these beetles, is ask ourselves: “why do these beetles black spots?” And we can actually analyze why they survived and reproduced better than the spotted beetles. And we might do observations, experiments that show that the spotted beetles were more likely to be, I don’t know, confused as raisins, like toddlers thought they were raisins because they’re so dark, so toddlers eat them. And that’s why the the the spotless beetles reproduced and survived better. Or it could be that that the spotted beetles overheat in the sun and just kind of explode in the sun, whereas the spotted beetles don’t.

If that’s the case, if our experiments and observations show that (and we’ll talk about many such observations and experiments throughout this course, when we’re talking about various behaviors that animals do), then we can say that the beetles are unspotted so that they won’t explode in the sun or so that they avoid predation by toddlers.

And that’s pretty cool. What we’re not saying is we’re not saying that they’re that they’re doing anything, that the beetles are doing anything per se in relation to the toddlers and so on. But we’re what we’re saying is that there’s been this history of evolution by natural selection where the selective forces of toddlers and the sun have have changed the beetle in this way.

That’s a hard thing to get your head around, so I’m going to spend a little bit time doing that. Here we have some other examples of adaptations. On the left are is the familiar peppered moth that you’ve learned about in another courses. And you could say that the peppered moth is camouflage in order to blend into the appropriate background so it won’t get eaten by predators, just like we were talking about the spots and spot the beetles.

Here’s some more exquisite adaptations: a stick insect here, and this kind of leaf insect here. We have no problem imagining that these insects are shaped through natural selection, much like the one that I showed you in that cartoon that I just showed you. But when we get to behavior, things get interesting.

So here’s one of those leaf insects, a phantom insect, that not only looks like a leaf, but it kind of moves around, like when I sort of wave a it like it’s just a piece of green vegetation and clearly it’s doing that. So it looks more like a leaf. But when we say it’s giving that to look more like a leaf, as a behavioral ecologist, what we’re doing is we’re we’re not implying anything intentional on the insect’s part, although it may have that intention, but we’re using a shorthand for this history of evolution by natural selection.

And that goes for all the ecological all the behaviors that we’ll be talking about throughout the course. We’ll often view behaviors as decisions, what to eat, where to look for food, shall live alone or live in a group, what song to sing, which mate is best. When we talk about these decisions, we don’t mean a colloquial sense like a conscious or intentional decision, but just wild animals do one thing rather than the things one way rather than another way.

Why is it adaptive in an evolutionary sense? A corollary of this is that how the choice is made, the mechanics behind the choice, what’s going on inside the animal, how the animal came to make that that optimal decision is less important to us, to us behavioral ecologists, than whether it’s adaptive. We’re not saying that’s not interesting. It just tends not to be the focus of behavioral ecology.

Right. So what we’ve got is selection on traits that somehow must be conveyed to the offspring. And then the offspring do their thing. And somehow in that package of genetic material (usually genetic material, that’s the usual way that traits are heritable in this course), all these complex decisions have to be made. There’s lots of ways that that can be wired up, and we’re not going to worry about them in this course. But I’ll just show you a few of these so that you can kind of have them in your head, because I think it makes it easier to just kind of say, yeah, that might be going on, but that’s not what we’re talking about here.

Decisions

In some cases, the decision is made in a straightforward way by the genes. In fruit flies, for example, there are rovers and there are sitters. I’ll just whip through this example, but basically, rovers move around more than sitters do when they find a patch of food, and that’s more adaptive in certain circumstances. And in other circumstances, it’s more adaptive to be a sitter. And this “decision” of which one to be is basically a mutation. It’s at one one locus of the gene tat codes for a particular enzyme. So that’s a case of a very simple behavioral decision that’s just like being spotted or not spotted. But it’s a behavior.

Often there’s decisions that are made during the course of development, and that’s known as phenotypic plasticity. So you might share a gene type with another another individual, but because of the circumstances in which you find yourself, because of the environment that you’re in, you develop in a completely different way. Here’s an example. Here’s two sibling caterpillars, brothers or sisters or sister and brother or whatever. And they’ve developed completely differently because of the food that they feed on. One feeds on this kind of food that looks like this and ends up looking just like that food, the other eats food like this and ends up like this. The details are really fascinating, but I’ll leave them aside for now. The thing is that there’s a developmental switch that is dependent on the food and depending on that switch, they go one way or another. So there’s another way to do that, a way that adaptive decisions can be made.

Adaptive decisions can be made on the fly by, so to speak (a little pun there). See the fly there in this nice example. Here’s a insect that lays its eggs on the eggs of other insects. It lays the eggs inside it, inside the eggs of other insects. So it’s kind of like a cuckoo. And when it lays eggs in there, its larvae eventually hatch out and gobble up the egg of the host. And the larger the egg of the host, the more eggs the this insect, Ken Lay, because there is going to be more food for its offspring.

How does it calculate how many eggs to lay? Well, it has to calculate how big the host egg is. And it does this with a very simple mechanism where there is a particular angle that’s made between its scapes. This part of it here and its head and that angle changes depending on how big the host aid is, in a very predictable way. All you need is to connect that, some kind of neuronal signal of that angle to the ovipositor, the egg laying device. And then you’ve got yourself an insect that can lay the optimum number of eggs. That’s a very simple example of how a decision, behavioral decision can be wired up in such a way, wired up by evolution by natural selection, not by any sort of clever thought process in the insect to make a very fine grained adaptive decision.

Here’s another, more spectacular example of a very complex adaptive series of adaptive decisions made by an animal that’s beautifully wired to be perfectly camouflaged in almost whatever surface it finds itself in. And also to, quote unquote, “know” when to flee. So here it’s going to blend into this completely different environment now. And that’s a behavioral decision made by a very complex neuronal and sensory system that we won’t get into not only neuronal, but also it signals it. The octopus changes the texture of its integument, its its outer layer, to match things pretty perfectly.

That’s a complex song of a zebra finch that just saying there. And here is a graph sort of that song you can see that’s complex. It’s not just a bunch of noise. It’s a very finely crafted song that’s shaped through a process of learning and development over the course of the finch’s lifetime. And it’s adaptive in various ways. Females like particular songs and songs that are shared in particular ways with other zebra finches and not shared with certain other zebra finches. So they’re very picky about which song male sings. And so the male has to learn kind of, you know, as best a song as he could. There’s a complex series of events in the brain and the physiology and so on and the zebra finch that we never get into. But we will talk about the adaptive form of the song that’s ultimately there and why that form of song is adaptive.

So that’s a whole host of ways that we can get from a question like why do birds sing to the answer: Here’s how they should sing. Here’s the decision they should make. And there’s all sorts of stuff that might be going on in the animal that we’re just going to simply ignore.

So that’s a lot to throw at you. But I like to throw it at you at the beginning of the course that you’re starting to think like a behavioral ecologist. And if you take just one message out of it, it’s that adaptive decisions that we talk about in the course don’t imply that the animal itself is intending to behave adaptively. Instead, they’re shorthand for a history of evolution by natural selection.

OK. Another thing I want to get clear at the beginning of this course is or try to is to try to get you to avoid it for the good of the species arguments. A lot of times you’ll see a wildlife show that presents behavior like this. And I hope this isn’t too loud for a recording. Here we go.

Males chase each other to establish their dominance. Serious physical harm is nonexistent; the display is ritualized to ensure the strongest males win to promote propogation of the species.

So all the males are holding back in doing these contests, because it’s going to improve the stock of the wildebeest,so that the species survives better. We’ll talk about a lot of behaviors in this course where we’re suggesting that a particular way of behaving is more adaptive than another way of behaving. But we’re not suggesting it by saying that that that this helps the species survive. And it would be really good if you avoided that kind of language and in this course. It’s a it’s a group selection argument for the good of the species argument.

And the problem with it is basically this. If you’re arguing that a whole bunch of lemmings run off a cliff so that there’s more food for the remaining lemmings that survive, inevitably there is going to be cheaters. So you do going to have one lemming that’s that’s wearing a lifesaver. There’s gonna be one lemming that that that cheats on this and they’re the ones that are going to survive and reproduce. This one’s going to going to survive and reproduce. And these other ones are gonna be the suckers.

What behavioral ecologists study and how they talk about it and why behavioral ecologists avoid group selection arguments in the next lecture.