A question that some people have is where do complex traits come from? Because, well, when we think about natural selection, we're often using examples like things getting a little bit bigger or a little bit faster or changing color. These sort of small changes that are relatively easy to wrap your head around. But, of course, life is complex and there are some crazy complex traits out there. So where do they come from?
Well, we're going to say that complex traits can arise in different ways. And we want to note though, before we go on that these ways are still always going to be sort of a step-by-step process with little changes that add up over time. All right, but how do we add up those little changes to get to something that's very complex? Well, the first way that we're going to talk about is a process called exaptation. An exaptation is when existing traits gain a new function.
So where does a complex trait come from? Well, it used to do something else is the idea here. And the word exaptation is supposed to compare it to the idea of adaptation, where in adaptation, we sort of add on to the function a little bit every time. In exaptation, we're going to change the function. Now to be clear, this is still a form of adaptation, but we just want to make that distinction.
Now here, we want to give an example of this, feathers. Now flight feathers in birds are quite advanced, and birds can't fly without flight feathers. So how did they get flight feathers? Well, the idea is that feathers aren't an adaptation for warmth.
Right? Feathers, we believe evolved from scales. And so in some dinosaurs, they started to get these sort of fluffy scales. And this was a huge advantage because it kept them warm. They could regulate their body temperature better.
But if this dinosaur lives in a tree and it's jumping from branch to branch, and when it does that, it sticks out its arms and legs, it can maybe glide just a little bit farther. Now once it's doing that, natural selection can sort of grab onto the shape of these feathers and produce something like this, this dinosaur called Microraptor here, which very clearly could glide very well. It had these gliding feathers. So the idea of exaptation is, well, the feathers evolved for warmth, but by exaptation, now they are used for flight.
Now just to be clear, feathers are still used for warmth. Right? That's why you use, like, a down feather comforter or a down jacket to keep you warm. Alright. Well, that's exaptation, but not everything gains a new function.
Some complex traits just arise from existing structures. And when that happens, the idea that we have to keep in mind is that each intermediate structure has to have a benefit over the previous version. Alright. Now to think this through, we're going to be thinking about the eye, the vertebrate eye, the eye like you have, what's sometimes called a complex camera-like eye. Well, even Charles Darwin said that it's a challenge for natural selection to explain where the eye came from.
Now he was very clear. He believed natural selection could provide an explanation. But today, we actually think we understand how it worked. Alright. So we can't just start with an eye, but what part might you start with?
Well, an eye spot is actually a pretty good adaptation over what was there before. Right? If you have photosensitive cells and they're in a little group attached to the brain, well, that now allows you to detect light. And the ability to detect light from dark could be a huge adaptation for an early organism that couldn't detect light at all. But once you have an eye spot, just a really simple change is to make that eye spot three-dimensional.
We're going to make it an eye cup. Now an eye cup is a big advantage over an eye spot because as light enters this eye, not all light hits all cells depending on the direction it comes from. So now the eye cup can detect the direction of light. So now these organisms don't just know if it's light or dark. They know it's light over there and dark over there.
That's a huge advantage. Alright. Well, if this eye cup sort of just keeps closing up, eventually it can make a pinhole eye. And a pinhole eye can give you a simple focused image. Right?
If you have just a pinhole, that can actually work as a camera. It's called a camera obscura, and some early cameras work that way. A small enough hole as light goes through it will give you a focused image, and an image is a huge advantage over just knowing the direction of light. Right. Well, if this closes up all the way, but remains having some clear cells in front, you can imagine how you can start to get a simple lens in this eye.
Now a simple lens is going to give you a clearer focused image, and the clearer the image, well, you can imagine that's more beneficial. Right? Well, once you have a lens, you attach some muscles to it, now we can have a focusable lens. So now this isn't just a focused image. It's an image with adjustable focus.
Right now, we can see things close-up, they see things far away, and we can change that focus as we need. Now this sort of advanced camera style eye we see here, it also has an iris that controls the amount of light in it. And this is how the eye in vertebrates, like your eye, works. The interesting thing here though, what I'm showing you here are not images of vertebrate eyes. I'm showing you images of mollusk eyes.
This advanced camera style eye actually evolved twice. It evolved in vertebrates, and it evolved in mollusks. And in mollusks, every image that we have here is an example of an eye that is out there in the world still being in use. Over there on the left, that's how a limpet's eye worked. Over here on the right, that's how an octopus eye works.
Now an octopus eye is remarkably similar to yours, but there are some key small differences that we know it very clearly evolved separately. So some people say, you know, what good is half an eye? Well, half an eye is really, really good if you have the right half, and they're sort of put together in the right steps. Alright. We got more questions after this.
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