One of the most common objections to evolution is that it can't be seen in action, so it doesn't meet the scientific requirements of an observable experiment. Joshua Feuerstein, whose popular video launched Starbucks's red cup controversy, once said in a video with more than 200,000 shares, "Evolution is not a science — never has and never will be. Why? Because it cannot fit within the parameters and parentheses of science for one simple reason: It was never observed."

But this is wrong. Evolution has been observed. In the video above, by PBS's It's Okay to Be Smart, biologist and science writer Joe Hanson gave an example of scientists seeing evolution in nature.

As Hanson tells it, some crickets in the Hawaiian island of Kauai survived by evolving to not make their well-known chirping sounds. Crickets normally make these noises to attract mates. But several years ago, the crickets' chirps attracted a more dangerous listener — parasitic flies. These flies would sense the crickets' song, land on the crickets, and plant eggs, and the parasites would devour the crickets from within. Over time, this wiped out the chirping crickets. But some crickets developed a genetic mutation that stopped them from chirping, allowing them to survive and flourish in Kauai.

This is the basic framework of how evolution works: A creature has a trait that exposes it to a threat or at least makes it less fit than others of the same species. So over time, the trait is phased out, since the weaker animal is less likely to survive.

With the Hawaiian crickets, however, this played out quite rapidly, since the parasitic flies quickly wiped out chirping crickets — in fewer than 20 generations. So it let various researchers observe this evolution right as it was happening.

The same scenario played out on another Hawaiian island, where crickets developed a different genetic mutation with the same result — no chirping sound.

"Nature arrived at the same destination by two completely different paths," Hanson said. "While evolution is often a long, gradual process, rest assured we witnessed plenty of fine-tuning right before our eyes."

Watch: NASA's 3-billion-mile journey to Pluto, explained