The Tempo of Evolution
We know that the speed of evolution depends a lot on generation time. Bacteria and small insects, which can have super fast generation times—we're talking minutes to days—are going to change and evolve faster than redwood trees, which have generation times of hundreds to thousands of years. That seems reasonable. If we want to make generalizations about the evolution of all life, we might ask if speciation occurs at a constant speed, or if it's at faster some times and slower at others. There are two hypotheses we should know regarding the tempo of speciation: punctuated equilibrium and phyletic gradualism.
Punctuated EquilibriumIf we're interested in how fast things evolve, it makes sense to look at the fossil record for evidence of speciation and change. Does the fossil record show a slow, gradual, even pace of evolution for most species? Slow and gradual, yes, but not always even. The predominant pattern suggested in the fossil record is that there are long periods in which species don't change much (called stasis), and then relatively short periods where there's lots of change and cladogenesis. Yup, we just used that word in a sentence.
Phyletic GradualismThis is in direct contrast to the theory of phyletic gradualism. This claims that the vast majority of species are slowly changing via anagenesis without much splitting. Species change gradually from one form to another, but speciation is rare. Does anyone actually buy this theory? Not really. It's just an alternative to punctuated equilibrium, but it's not really taken seriously in scientific circles. Try saying that 10 times fast.
What can we conclude about the tempo of evolution then? We know that it's usually slow and gradual, but there are times when speciation rates are much higher. Furthermore, the timing of rapid speciation in one group of organisms doesn't necessarily mean that other organisms are diverging rapidly at the same time, though there are periods when global speciation rates seem to be booming. One of the best examples is known as the Cambrian Explosion. We also know that speciation happens much more slowly than humans can experience. Or does it?
There are cases where observable changes in populations could lead to speciation. Any millennium now, they'll probably be distinct. We call this incipient speciation. Maybe we see evidence of host plant shifts, or reproductive isolation, or something else that suggests we are witnessing a brief moment in the speciation process. We don't know what the future holds for those populations. They may not diverge after all, so we can't say with certainty that they'll be distinct species. A biologist can dream, can't they? A couple of Harvard dreamers are currently researching possible incipient speciation among tropical Heliconius butterflies and have found some really interesting results—check out their video for a great example of speciation in action.