Mechanisms of Evolution
Topics in Depth
The Theme of Natural Selection in Mechanisms of Evolution
Survival of the fittest
Natural selection is determined by an organism's fitness. But by fitness, we don't mean winning a strong man's competition. In fact, if natural selection depended on fighting for your life like The Running Man and The Hunger Games, humans would have evolved into a species with Austrian accents and gladiator skills. Nature doesn't use a strong man competition to select for traits
If it's not doing 100 pushups, what makes an organism fit? It simply comes down to whatever helps an organism live and reproduce. You're only as fit as your hand-me-down genes.
There are three types of natural selection:
To understand the differences between them, it's easier to first think about the range of a phenotype. In these types of natural selection, you need to see both ends of a phenotypic spectrum.
Take a population of not-so-cute, completely fictional, wiggly earthworms and give them two extreme and opposite phenotypes.
Some earthworms in this population are super wiggly, due to thinner skin. This allows them the flexibility to slither their way through mazes of grass and rocks. Other earthworms in this population have thick skin. Not only can they survive a range of insults, these worms are more likely to survive being stepped on by pedestrians. They might not be very flexible, but surviving the assault of a high heel is obviously a desirable trait if you're an earthworm.
Very thick and very thin. These are the two extremes of skin width in our earthworms. However, most earthworms in the population have skin that is somewhere in between thick and thin. If we were to measure their skin width and plot it on a diagram, it would resemble a bell curve. The majority of skin width, as well as the mean, would be in the middle of the two extremes. We could also skin the earthworms to measure the thickness, but we're not sure how to do this. There's probably more than one way.
Now carefully pick up a few of these wigglers and test their fitness in different environments.
How can the environment of an earthworm affect its natural selection?
Ew. Now let's go wash our hands before we continue.
In stabilizing selection, the mean of the phenotype is favored.
Imagine we put the earthworms in an environment where they would need to be very mobile and avoid being killed by a pair of spikes at the same time, like a baseball field. The worms that are best adapted for this type of environment are the ones in between the two extremes.
When this population goes through reproduction, the second generation would look a lot like the first. There would still be a bell curve, where the majority of the population is not too thick, not too thin, but just right—the Goldilocks of earthworms. The important difference, however, would be less variety in the population. The two extremes wouldn't be as extreme anymore.
If directional selection is occurring, then one extreme of the phenotype is favored. In this case, our earthworms might be in an environment where there is a lot of foot traffic but no real need to slither through grass, like a sidewalk outside the baseball stadium. Those earthworms are going to need some pretty thick skin to survive the rush of fans leaving the game, especially angry ones from the losing team.
These thicker-skinned earthworms survive more stampedes and are able to go through more reproduction. The result is a second generation where the average earthworm also has thicker skin. The mean of the phenotype has changed. There is also less variety.
Disruptive selection really disrupts the allelic frequency. In this example, we'll pick up these slimy guys and put them in an area where both phenotype extremes are favored. This could be a place where there were also two extreme environments.
Assume this is an area outside the baseball stadium, with a few concrete paths and a few grassy sections. Also assume that most people stay off the well-manicured lawn. Although there is only one population of earthworms here, there are two environments for them. The thicker skin is favored near the pavement while thinner skin is favored on the green. There really is no place for Goldilocks worms here; the porridge is just too hot or too cold.
The result? A bimodal pattern where there's greater variety at both extremes of the phenotype.
Directional, stabilizing, and disruptive selection change the mean and variety of a phenotype within a population.
Okay. Earthworm experiment finished. Feel free to throw them in your bait and tackle box and try them out as fish food.
Natural selection can do the craziest things. The South American Fulgoroidea, an insect, has evolved a head that looks like a scary multi-toothed alligator to avoid being eaten. See here—we're not kidding.
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