We said that speciation is kind of a link between microevolution and macroevolution, but so far we've really been talking about it more as a microevolutionary process. Right? How do you have this one population that could split into two? Well, here we want to ask the question, what does speciation look like as a macroevolutionary process? Right?
We want to take this sort of big deep time look at things. And when you do that, we're going to come up with two models. We're going to have gradual evolution and punctuated equilibrium. So let's first talk about gradual evolution. Now this is how I sort of, in my mind's eye, just sort of naturally think about evolution and speciation.
You get this slow, relatively, I'm going to say constant change. Just, you know, every generation just a little bit different from the last, and it might in a way that you can't even tell, but over a million years, that adds up to this big change. Well, if that's the way it works, when you go back in the fossil record, you should be able to easily find many types of, I'm going to say here, intermediate forms. Right. So to illustrate this, we have this model of whale evolution.
Right? We're going from our terrestrial ancestor to modern whales and we see some intermediate forms there, but we have this very smooth line transitioning from one to the other. Well, the idea here is if I picked a time on this line, say, maybe half exactly halfway between these two organisms in the middle, I should be able to find a fossil that represents exactly halfway between that transition. You know, maybe a little bit longer tail, a little bit shorter legs. If I go three quarters of the way between these two organisms, I should be able to find a fossil that is three quarters of the way transition from one to the other.
Now that definitely does happen sometimes, but a lot of times when you look at the fossil record, it actually looks more like there are jumps from one to the other, and that gives us this idea of punctuated equilibrium. This says that you actually have periods of stasis or long periods of no or very little change interrupted by this very large rapid change. Now if that's the model we're going with, when you look back at the fossil record, what you would expect to see is fewer types of intermediate forms. Now to be really clear, all those intermediate forms had to have existed. They just weren't around long enough for a lot of specimens to be preserved in the fossil record for us to find.
So when we go digging, we just don't see them. Right? So our model now, instead of that smooth gradual change, well, we have each one of these species living for very long times with very little change, you know, mostly stabilizing selection going on. And then linking them are these incredibly rapid periods of really strong directional selection that introduces a lot of change rapidly. Now just to be clear, when we say rapid change, we're still talking tens of thousands, maybe a hundred thousand years.
But in the fossil record, that is a very short amount of time, and it's hard to find fossils from any, you know, specific time period that short a time. And so a lot of them, you just don't. Alright. So again, gradual evolution, this sort of slow constant change, we see that in the fossil record. But we also see these sort of jumps.
And that's what we call punctuated equilibrium, where we think there are these long periods where things are mostly having stabilizing selection, no change, linked by rapid change and rapid speciation. Alright. We'll practice more coming up, and I'll see you there.
