Natural selection and adaptation | Mechanisms of evolution | High school biology | Khan Academy

Hi everybody, Dr. Sammy here, your friendly neighborhood entomologist, and I was hoping that we could take a few minutes to talk about adaptation.

What comes to mind when you think about adaptation? You might think of cryptic morphology that helps organisms hide from predators, scaly armor or spines to protect organisms in a scuffle, or increased melanin pigmentation in the skin to shield an organism from the damaging impacts of harsh sunlight in really sunny areas. And if this is the sort of thing that you thought of, you're right! These are all adaptations.

But what people often miss about adaptations is that they all help organisms survive and reproduce in a particular environment. The word adaptation can refer to a trait that makes an organism more suited to its environment, but it can also mean the process by which a population becomes dominated by organisms that are suited to their environment.

The point is that adaptation happens in a population as its organisms accumulate adaptations. And this all happens through the process of natural selection, which you may have heard about. With natural selection, we see organisms with beneficial heritable traits increasing their share of the gene pool in the population because they're more likely to mate and produce offspring.

This means these well-adapted organisms have higher fitness, a measure of an organism's ability to survive and reproduce in its specific environment. These traits or adaptations make the organisms better suited to survive and reproduce because they're better suited to the specific environmental context that they live in.

That is, our green caterpillars blend in and thus survive better on the green leaves than the orange caterpillars on the orange leaves. As you can see, natural selection is intimately intertwined with the environment in which it takes place, and this is why natural selection can lead to adaptation.

Even the concept of adaptation is diverse. Adaptations can be anatomical changes to physical structures or physiological changes in the function of these structures, or adaptations can be changes to behavior. We see this with honeybees adapted to life in Africa, sometimes called by the suboptimal names "Africanized honeybees" or "killer bees."

They're the same species as and look identical to populations of honeybees adapted to North America, but their behaviors diverge dramatically because the African populations are adapted to life in a hot tropical environment. Sometimes they construct nests right out in the open air. This is extremely rare for populations adapted to the temperate climate of North America, where the temperature range is far too broad between winter and summer for open-air nests to be successful.

So, North American honeybee populations much prefer to live inside of protected structures, like inside the trunks of hollow trees or the boxes that beekeepers provide. Nests in protected structures! And in Africa, there are also more large, fearsome predators interested in getting into the bees' nest, so the frequency of traits related to hyper defensiveness is much more common than in populations adapted to North America, where there are fewer aggressive predators—heightened defensiveness, low to moderate defensiveness.

The success of honeybees adapted to life in Africa left a lot of people worried that they would be similarly successful in North America and replace the honeybees there. But those concerns ignored the connection between adaptation and the environment. Traits that helped those bees survive in Africa and even help them in similar environments found in many regions of tropical South and Central America—well, those same exact traits were liabilities in much of North America.

Okay, so this is all fascinating, but you may be wondering: if natural selection is passing out adaptations, why doesn't it choose better stuff? Fish tend to get eaten by birds and bears a lot, and that's gone on for literally millions of years. So why haven't fish populations developed something like laser eyes or teleportation instead?

When a population of fish does evolve defense, it's typically something like a few bones that it already has getting longer and sharper and protruding out of their fins, like in the case of the three-spined stickleback fish. These spines protect the stickleback from birds and other predators, but only if the spines are long enough to make it difficult for that specific predator to swallow them. Birds with wider throats are still able to eat even fully grown fish with fully elongated spines.

Laser eyes really do seem like they would be more effective. It's almost like natural selection is just tinkering with whatever it has lying around instead of going out and buying something new and perfectly suited to the task. And that is exactly what's happening. Populations don't adapt by accessing a bottomless well of awesome problem-solving options; quite the contrary!

With natural selection as the driver, adaptations can be pretty limited. You might already be tracking with the idea that if an increase in fitness is what natural selection is after, you don't need an adaptation that allows you to incinerate all of your enemies; you just need one good enough to solve whatever problem is keeping others in your population from surviving and mating.

If that's a kingfisher bird eating the other fish, the resolution might be just a spine that's only half a centimeter longer than the other fishes. That's the reason why we see adaptations that look like natural selection just grabbed whatever happened to be lying around and modified it: because it kinda did! A turtle's remarkably protective shell may look fancy and novel, but it's literally the turtle's overgrown ribs fused with its spine. A narwhal's face spear? That is just a tooth that grew all the way out of its face.

Natural selection only has the options currently in the genes of a population, with the occasional addition of new genetic sequences through mutation to work with when a problem arises. And you might be thinking, oh great, mutations! Those will help; that's how my favorite superhero got her powers. Well, unlike what we see in movies, most mutations either have no real effect or result in problematic gene expression or non-functional proteins, which is pretty bad news for living things.

So adaptations are not likely to be super weapons that ensure survival in all circumstances; instead, they're useful traits that help increase an organism's fitness as the environmental conditions apply pressure. So adaptation is a remarkable system, but one with some pretty pronounced limitations. Natural selection has to work with what an organism has.

But consider for a moment what it has given us despite those limitations. Pistol shrimp have the ability to slam their claw shut so quickly that the water around them collapses in on itself—it implodes, knocking nearby fish unconscious. Some honeybees can bundle together and vibrate their flight muscles so rapidly that they can create a biological convection oven which can cook predators to death.

And there are parasitic wasps that have evolved to live inside of parasitic wasps that live inside of parasitic wasps that live inside of caterpillars—like tiny strange little Russian nesting dolls. And don't even get me started on the hornets that have yellow patches on their abdomen that absorb solar energy to supplement the energy provided by their diet. They have solar panels! I'd say natural selection has done pretty well for itself.