The Role of Predators in Evolution

Imagine a family of ducks paddling on a pond, near the shore, with six tiny ducklings clustered around their mother. A raccoon watches from behind a tree, then darts out, snatches one of the little ducklings, and runs off with it. That's a meal for the raccoon, and a quick end to the short life of the baby duck.

Why did the raccoon grab that particular duckling, and not one of its five siblings? If you glanced quickly at the ducklings, they might all appear identical. However, each duckling has its own set of traits. The duckling the raccoon grabbed was the easiest for it the raccoon reach, or it was the one that looked the most appetizing.

Imagine that in this pond, half the ducklings have large feet and half have small feet. The large-footed ducklings are able to swim slightly faster than the small-footed ones. That doesn't mean the large-footed ducklings will be completely safe from predators. Some will be caught and eaten. But on average, predators will be able to catch more of the slower small-footed ducklings than the faster large-footed ones.

Before the ducklings in the pond have a chance to grow up, many will fall victim to predators. Of those that do survive to become adults, slightly more are likely to be the fast swimmers with the large feet. If the current generation of ducklings (let's call them "G1") was born with half having small feet and half having large, by the time that generation reaches adulthood, the distribution of large feet, in the population of ducks in the pond, will have increased to something a bit larger than half.

The size of the duckling's feet is a genetic trait, something they were born with, and something they will pass on to their children. Because more adults in G1 have large feet than small, the next generation of ducklings (G2) will likely be born with more of that generation's members having large feet as well.

Now, in our example, with a new generation of ducklings born, the raccoons return. Again, the raccoons will grab and kill some, but not all, of the ducklings on the pond. And again, slightly more of the slower small-footed ducklings will be killed than their faster large-footed peers.

So we've seen that G1 started off with equal numbers of small- and large-footed ducklings, but by the time they reached adulthood, there were more with large feet. G2 started off with more large-footed than small-footed ducklings, and by the time this generation reached adulthood, the proportion of large-footed ducklings was even larger.

If the environment of the pond remains the same, and if the raccoons continue to prey on each new generation of ducklings, over time, the proportion of ducklings born with small feet will continue to decline. Over many generations, that trait may disappear completely from all the ducks in the pond, so that a population which once consisted of both large- and small-footed ducks may come to consist of large-footed ducks alone.

And that is how predators exert evolutionary pressure on a population. The outcome of many interactions between individual raccoon predators and individual duckling prey, in a situation where particular traits give some ducklings an advantage over others in escaping from predators, can after many generations lead to major changes in the traits that the duck population contains.

The same is true for any species that has predators. Some members of the species will have traits that enable them to escape from predators more often, on average, than other members of their species. Over time, those traits will be found in ever-increasing proportions of the population, until the members of the population may become quite different from their ancestors. Evolution is the name that we give to that change.