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Carl Zimmer Carl Zimmer is the author of several popular science books and writes frequently for the New York Times, as well as for magazines including The New York Times Magazine, National Geographic, Science, Newsweek, Popular Science, and Discover, where he is a contributing editor. Carl's books include Soul Made Flesh,, Parasite Rex and Evolution: The Triumph of An Idea. His latest book is Smithsonian Intimate Guide to Human Origins. Please send newsworthy items or feedback to blog-at-carlzimmer.com.
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The Loom
December 27, 2004
All the News That's Too Big To PrintEmail This EntryPrint This Entry
Posted by Carl Zimmer

Size matters. At least that's the result of some recent research on long-term evolutionary trends that I'll be reporting in tomorrow's New York Times. Here are the first few paragraphs...

Bigger is better, the saying goes, and in the case of evolution, the saying is apparently right.

The notion that natural selection can create long-term trends toward large size first emerged about a century ago, but it fell out of favor in recent decades. Now researchers have taken a fresh look at the question with new methods, and some argue that these trends are real.

Biologists have recently found that in a vast majority of animals and plants, bigger individuals are more successful at reproducing than smaller ones, whether they are finches, damselflies or jimsonweed.

Nor is this edge a fleeting one. Natural selection can steadily drive lineages to bigger sizes for vast stretches of time. The giant dinosaurs that made the earth tremble, for example, were the product of the long-running advantage of being big over tens of millions of years.

"I think it holds up very well, and a lot better than a lot of people have said over the years," said David Hone, a paleontologist at the University of Bristol. Mr. Hone and others argue the push toward bigger size is so strong and persistent that there must be significant forces pushing the other way. Otherwise, we would be living on a planet of giants.

You can read the rest of the article here.

As is so often the case, I wish the article could have ended with a big fat asterisk, along with a footnote reading, "There's more to the story, but you'll have to visit The Loom for it."

This notion about size increase, known as Cope's Rule, has a long, checkered history, and this history say a lot about how the entire science of evolutionary biology has changed over the years. Cope's Rule is named after the American paleontologist Edward Drinker Cope, who made a careful study of the fossils of North America in the late 1800s. Cope belonged to the first generation of scientists who grappled with Darwin's Origin of Species, published in 1859. Its reception was decidedly mixed. On the one hand, Darwin was hugely successful in persuading scientists that life had evolved over a long period of time, thanks to the huge amount of evidence he marshalled fossils, embryos, and the distribution of living species. But Darwin didn't fare so well in his argument about what drove the evolution of life. He was trying to quash the popular ideas of Lamarck, who had offered two mechanisms for evolution. First, traits acquired over an individual's lifetime are passed on to its descendants. Second, life contains a mysterious force that continually drives it from lowly primordial slime towards higher levels. Darwin rejected both of these mechanisms almost completely, replacing them with natural selection.

This second argument did not fare as well in the late 1800s as the first. Many biologists came to see life as the product of evolution, but they saw evolution as the product of various Lamarckian, long-term forces. Cope was one of these scientists. He looked at the early mammals of North America--tiny creatures, for the most part--and saw that later they were replaced by much larger species. Here, he decided, was evidence of an evolutionary force that could operate over millions of years, a force, moreover that was separate from natural selection. Others found similar patterns in other groups, such as corals and foraminfera.

In the mid-twentieth century, evolutionary biology went through a revolution known as the Modern Synthesis. Scientists came to understand how genes mutate, and how mutations helped make natural selection possible. Leftover Lamarckism found no vindication of its own, and faded away. Biologists still accepted Cope's Rule as a genuine pattern in the fossil record, but they offered a different mechanism than Cope originally had. Instead of some mysterious long-term trend, good old natural selection was at work. Bigger individuals were favored in populations, and over millions of years, this edge produced bigger and bigger species.

In the 1970s, a group of young paleontologists challenged some aspects of the Modern Synthesis. They rejected the idea that every long-term pattern in the fossil record could be neatly explained by short-term natural selection. And Cope's Rule became one of their favorite targets. A trend towards bigger sizes could appear in the fossil record, they pointed out, even if natural selection didn't favor bigger individuals. Small species, for example, might be more likely to survive mass extinctions, and would thus have been more likely to found major new groups of species. Because they were small, their descendants couldn't get much smaller before they hit a minimum size limit. But they'd have plenty of room at the larger end of the spectrum. Even without an inherent advantage to being big, the lineage would gradually get larger.

Although a number of paleontologists were involved in this rebellion, Stephen Jay Gould was its most outspoken member. He made Cope's Rule a favorite object of his derision, a case study of how our subjective biases ("bigger is better") shape our interpretation of the natural world. And from the 1970s to the 1990s, he had pretty good reason to be scornful. The evidence for Cope's Rule turned out not to be all that strong. Or to put it more precisely, scientists who promoted Cope's Rule did not test it rigorously against other possible explanations.

My article looks at some recent research that gives it the careful look it demands. And, in something of a surprise, Cope's Rule is enjoying a renaissance. In most living populations of animals and plants studied so far, natural selection shows a strong preference for larger size. In rigorous studies of the fossil record, lineages of dinosaurs and mammals show signs of having evolved to bigger sizes over millions of years thanks to natural selection.

So was Gould wrong? Yes and no. Cope's Rule is not, as he claimed, a "psychological artefact." But there must be more to the story than natural selection favoring bigger indvidiuals. Otherwise, we'd live on a planet of giants. In my article, I mention a few possible forces that work against Cope's Rule. One that I didn't have space to mention is a force near and dear to Gould's heart: species selection. Just as individuals are favored or disfavored by natural selection, species may also undergo a selection of their own, with some species giving rise to more descendant species, while others go extinct. In the case of size, what's good for the individual may not be so good for the species.

Species selection has been kicked around for quite some time to explain why Cope's Rule hasn't made everything enormous. Recently, a nice study of fossils came out that supported the idea. Paleontologist Blaire Van Valkenburgh of UCLA and her colleauges have documented how big size may have doomed two groups of canids-the ancient relatives of today's dogs and wolves-in North America. In both cases, the canids evolved to larger sizes over millions of years, only to dwindle away to extinction.

As Van Valkenburgh and her colleagues pointed out in Science in October, small canids could have found enough energy in rabbit-sized prey and other foods such as fruits. But once the canids got above about forty pounds, they could no longer survive on this fare. They would spend more energy running after prey than they got eating them. As the canids got bigger, Van Valkenburgh argues, they shifted to hunting prey as big as themselves or bigger. Consistent with this hypothesis, Van Valkenburgh has found that as both groups of canids got large, their jaws and teeth also evolved. They lost molars, their front teeth got larger, and their jaws became stout and strong. This shift put these canids in an evolutionary trap. If their large prey became extinct, they risked starving to death. Nor could they re-evolve the versatile teeth and jaws that had allowed their ancestors to eat different sorts of food. They didn't go extinct because they were big; they went extinct because they were specialized.

Scientists I spoke to for this article were confident that Cope's Rule would figure in a lot of research in coming years. They've now got the tools they need to dissect long-term trends like never before--from databases of fossils to detailed evolutionary trees to sophisticated statistical methods. After more than a century, Cope's Rule still has plenty of life in it yet.

Category: Evolution

Chris Clarke on December 28, 2004 08:25 PM writes...

Fascinating piece, Carl, and I especially apprecate the mention of overfishing as a counterweight to Cope's (if it is a) Rule.

That's Edwin Drinker Cope, by the way. No sense making Othniel Marsh's ghost snicker any more than he needs to.

Permalink to Comment
kyan gadac on December 30, 2004 10:20 AM writes...

Surely, for some animals Cope's rule must apply, since their size seems to be limited by physiology and gravity more than any other constraint:- the blue whale, the elephant and the redwood. Presumably they all, in evolutionary terms, started out small.

While getting bigger is one option, getting faster is another competing option. As well, getting bigger also makes you more desirable as food, remembering that parasites(small and slow) are as hungry, as predators(big and fast).

As for long term trends, I await reports of giant archeobacteria.

People who think of evoluution as a single species affair are myopic and I suspect that that is the myopia that Gould et al were criticising.

Permalink to Comment
Chris Clarke on January 1, 2005 08:15 PM writes...

Um, I retract my correction. Silly and embarrassing of me.

Permalink to Comment

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