Listen to part of a lecture in a Biology class.

(female professor)

I’d like to talk to you today about a principle of evolutionary biology that was first proposed in the nineteenth century by an American scientist named Edward Cope. Cope made a scientific examination of fossils and thought he noticed a pattern of increasing sizes within species, and he proposed a principle, which is usually called Cope’s Rule.

What Cope said was that evolution was directed towards larger-size creatures, and that this can be seen in many, perhaps even in most, fossil lineages. Fossil lineages, uh, we talked about that before. They’re chronological charts of fossils of a specific species, um, charted over time. These lineages allowed Cope to compare fossils of different ages, some millions of years older than others. And Cope notes a tendency toward increasing size.

Now, there’re some obvious advantages to being bigger, even within the same species, like being bigger can be a deterrent to predators. So, you’re less likely to be eaten, more likely to survive if you happened to be bigger. Um, in certain species, bigger animals are more fuel-efficient. They need less food per pound to keep themselves going, although, of course, they need more food overall because they’re bigger. It’s the same thing with marine animals. In order to maintain body heat, larger whales need less food per pound than smaller whales. Also, in most species, larger females tend to produce more eggs increasing their chances of passing on their genes.

OK. So, these are all good reasons why there might be an evolutionary advantage towards larger size. And Cope’s Rule was not really questioned much. It just seemed obvious that such a thing had to be true, and fossil lineages seemed to confirm it. However, in the 1970s and 80s, there was a pretty serious challenge to Cope’s Rule mounted primarily by the famous scientist Stephen Jay Gould who promoted an alternative theory, which subsequently gained widespread acceptance by biologists.

Gould thought of evolution as a much more random process than many previous scientists thought. He was very much set against theories suggesting that evolution tends to drive in a certain direction, uh, tends to direct towards certain ends. Gould felt that such theories, like Cope’s Rule, are influenced more by the psychology of the scientist, you know, what’s on that scientist’s mind, than the actual science and facts regarding the subject.

People assumed that bigger had to be better and so they tended to look at these fossils that way, the way Cope did, but, you know, there’s a very strong danger of ignoring the evidence that’s contrary to your thesis, and often we don’t even realize we’re going it. Gould was troubled because Cope did things like, he compared the sizes of fossils with others that might be ten-million years older. Gould felt that that’s, well, it could be a misleading comparison.

And also the fossil record is not complete. There are all kinds of gaps and you can interpret the gaps in different ways. Gould, for these reasons, he believed that Cope’s Rule might not have been a rule at all, just a human invention. Cope found what he needed in order to support his theory, but he didn’t really prove it. And Gould suggested another rather interesting possibility. He claimed, and there’s a certain amount of foundation to this, that when a natural disaster wipes out large numbers of individuals in a species, they often tend to wipe out the bigger individuals more quickly.

Um, let’s say it gets really cold, and your food supply’s cut off. Well, the smaller creatures which don’t need as much food as the larger creatures; smaller creatures may find it easier to survive. So, what Gould suggested is that after a natural disaster, evolution, which is random, might not look random. It might look directed, because in effect there’s nowhere to go but up when only the smallest of a species survive. Animals were going to have to get bigger after a disaster no matter what so that growing bigger is nearly a tendency not an evolutionary advantage. It’s not evolution directing towards larger sizes.

There have been other challenges to Cope’s Rule, like, um, why then are there still so many small animals on Earth? Well, one reason is simple physics. You won’t see an insect as large as an elephant because the insect’s exoskeleton simply can’t support the weight.