When you’re learning about Evolution in school what you’re usually taught is basically that over time species gain new traits, traits that make individuals more successful become more common. Through this process the species gradually changes. This is usually presented as the species becoming gradually better (smarter, stronger, more successful, you get the idea).
This idea makes sense, it’s easy to understand, but it isn’t really accurate. A common argument made against Evolution in general is that this model doesn’t hold up to scrutiny very well. Creationists like to point to things like species of insects that are known to have gone hundreds of thousands of years without changing significantly, or the Cambrian Explosion, where a huge amount of diversity appeared extremely quickly. Are Creationists right that these things disprove Evolution?
Sort of. These observations do pretty much blow a hole in the image of Evolution that I described earlier, what is normally called the gradualist view of Evolution. However, Evolutionary Scientists have known about these problems for a long time, and have an explanation for them. That explanation is: Punctuated Equilibrium.
Punctuated Equilibrium starts with the idea that if a species has been living in an environment for a few hundred generations, they have become about as well suited as they can get to that environment. Once they have reached this state, nearly all mutations that happen will be either harmful or neutral. Harmful mutations will be removed from the population by natural selection, but neutral mutations, which don’t have any effect on an individual’s success, will accumulate in the population since nothing is removing them. This state is called Equilibrium
The Punctuated part happens when the environment changes. When the environment a species is living in changes, then the selective pressures acting on the species will change. For example, suppose there was a species of beetle that only dealt with predators that hunted by smell or sound. These beetles would develop a variety of colorations since their coloration did not impact their survival. Then, suppose a new predator appeared that hunted by sight. Now, beetles with duller coloration that made them look like their background would have an advantage in avoiding predators. This would cause the duller camouflaged coloration to spread throughout the population.
When the selective pressures on a species change, mutations that used to be neutral will become either positive or negative. When this happens, the species will undergo significant, rapid change, with some traits spreading to the whole population and others disappearing entirely. This rapid change will continue for a while until the species is in Equilibrium again. This process is why species appear to change at radically different rates than each other, and why you will see a species going generations without any change at all followed by sudden, drastic change.