Adaptation and environmental change | Mechanisms of evolution | High school biology | Khan Academy

Hi everybody, Dr. Sammy here, your friendly neighborhood entomologist. Here to talk to you about how adaptation, which is dependent on the environment, responds in the context of environmental change. Natural selection promotes adaptation in populations. It encourages populations to develop traits that better allow individuals to survive and reproduce. Those adaptations are thus linked to the environment in which they were forged.

You can look at a lot of organisms, and based on their physical traits and/or their behavior, you can tell what sort of environment their population likely evolved in. So adaptations are products of, and are inextricably connected to, the environment. But what happens when the environment, which forges a specific set of adaptations, no longer exists in that form? How do populations respond? Well, this happens to actually be a pretty common occurrence on Earth. I wouldn't be surprised if the saying "things change" was first coined by a paleontologist studying the history of life on Earth.

You'd be well acquainted with the dogged persistence of change. Well, the good news is the forces that drive natural selection don't just disappear when an environment changes. Because of the persistence of heritable diversity and differential survivorship and reproduction, natural selection is still there to promote new adaptations suited to the new environmental conditions. This means that when the environment changes, the distribution of traits in the population will often change as well.

What those changes will ultimately be is based on the adaptations promoted by the previous environment and the pressures presented by the new one. So let's look at some examples, so I can make sure you're picking up what I'm putting down.

Okay, so in evolutionary biology, we like to discuss three primary shift patterns that can be observed as a result of environmental change: directional selection, disruptive selection, and stabilizing selection. For each of these, I'll show you a graph and tell you a story of the stubbornness and resilience of life, because every graph has a story.

The first of these stories is that of directional selection. The snow vole is a mouse-like mammal that, even I, as an admittedly bug-biased scientist, have to admit is pretty cute. These rodents are born at high elevations in the Swiss Alps, and researchers have observed something that may seem obvious on its face: large adult snow voles have better survivorship than smaller adult snow voles. They're more difficult for predators to capture, overpower, and consume. They have greater temperature tolerance and typically more fat stores for a long winter.

So historically, that distribution of snow voles has been shifted toward the higher end of the adult weight spectrum. However, climate change has led to changes in the snow voles' environment. Snow has been falling earlier and earlier in the Alps, giving the voles less time to actually develop. If the voles haven't reached maturity by the time the winter begins, they typically can't continue their life cycle. Because larger body sizes require more time to develop, there's a strong selective pressure against the genes underlying larger body sizes as the environment shifts towards earlier winter onset.

This also means that the genes underlying smaller body sizes are favored. So the smallest individuals in the original population likely had the most offspring, and the variation in those offspring likely included some that were even smaller than in the original population. This means that as the smallest individuals in each generation continue to be favored and have the most offspring, the distribution of the population will likely continue to march to the left until it reaches a point where the individuals are too small to do well given the environmental conditions.

This shift can be represented by a graph that shows directional selection. Now, let's go over the graph one more time to make sure that everything is clear. Here on the x-axis, you'll see the spectrum of average adult body size, ranging from the smallest to the largest individuals. Traits are quite frequently on a spectrum, such that some individuals possess a greater or lesser expression and are thus differentially impacted by environmental change.

On the y-axis is the percent of individuals in the population, which allows you to see proportionally how individuals fare by comparison to those of greater or lesser size. The red arrow represents selection pressure, which in this case is imposed by the abiotic factors of weather. When looking at these graphs, you want to consider who is least able to handle the pressure imposed. It's typically this area of the distribution where you'll see the greatest difference between the population before and after the environmental change.

The white arrow then shows which direction the population as a whole is then shifting. Directional selection events are those which favor one extreme while excluding another. However, change itself can be as diverse as the organisms it changes, and thus you can also get circumstances where selection favors both extremes. This is called disruptive selection.

For this example, we look to the pungent world of dung beetles. Some populations of the beetle Ontophagus acuminatus have a bimodal size distribution, where you'll see a large number of large males in addition to a large number of small males, with very few representatives of the sizes in between. The large males use their superior size, strength, and horns to fight other males for the opportunity to mate.

A male having selected a mate will spend most of his time guarding the entrance to the burrow where she dwells, grappling with males of similar size who challenge him for the chance to depose him and mate with her. Medium-sized males don't do well in this context; they can't overcome the larger males. But why then are there small males, and why are they so abundant in the population? Well, they've actually got a trick up their sleeves.

Instead of trying to fight, they forego the battle entirely and simply use their diminutive size to sneak past unnoticed. These small males typically lack horns, which leaves them similar in appearance to females, which makes male dung beetles less inclined to engage them in battle. The little guys further avoid trouble by deftly digging a backdoor entrance into the female beetles' subterranean nesting chamber, such that they rarely have to encounter the large male guarding the front door. This allows them plenty of time to mate before making their exit, ensuring the representation of their sneaky genetics in the gene pool.

Disruptive selection selects for both extremes on the spectrum to the exclusion of the moderate trait in the distribution. But there's also a form of selection that is the opposite of disruptive selection; it's called stabilizing selection. Stabilizing selection is observed under circumstances where selection is favoring the moderate set of traits in a distribution to the exclusion of the extremes.

For this story, we once again turn to the wondrous class Insecta, where insects have learned to make a home in some of the most ingenious ways possible. In the world of gall flies, life begins when a female gall fly lays an egg in the stem of a suitable host plant. The developing larva induces the plant to form a spherical growth around it called a gall, which protects the developing fly's squishy body and provides it with all the food it needs to reach adulthood—unless a parasitic wasp shows up and attempts to lay eggs on the developing gall fly.

If this happens, the developing wasp kills the fly and takes over its living space. This is easiest to do with small galls because the wasp has to stick its thin egg-laying tube through the gall to access the developing fly. A large gall forms enough of a barrier between the gall fly and the wasp's egg-laying tube that it can't quite make it, so this selects for larger galls and would promote directional selection, all other factors in the environment being equal.

But what happens when keen-eyed birds are introduced? Well, large galls become an easy meal for them because they're conspicuous in the environment. This creates a situation where small galls are selected against, as are large ones, ensuring that the medium-sized galls are favored. To remember what this distribution looks like, I tell myself that the trait distribution is being stabilized by pressure on both sides.

Environmental change can be pretty diverse. Without the incredible adaptive powers of natural selection, our dynamic world would be unsuitable for the unpredictability that is life as we know it.