New York Times,
December 28, 2004Link
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.
Evolutionary biologists generally refer to this trend toward bigger sizes as Cope's rule. Edward Drinker Cope, a 19th-century American paleontologist, claimed the fossil record showed that lineages of species got larger over time. Later scientists offered further support for Cope's rule, from mammals to corals.
Starting in the 1970's, however, many paleontologists challenged this evidence. They argued that previous researchers had simply projected a ''bigger is better'' view on the fossil record without actually testing it against alternative hypotheses. Dr. Stephen Jay Gould, the eminent Harvard paleontologist who died in 2002, dismissed Cope's rule as a ''psychological artifact.''
An apparent trend toward bigger sizes could appear in the fossil record, Dr. Gould and others pointed out, even if natural selection didn't favor bigger individuals. Small species, for example, might simply be more likely to survive mass extinctions. If they then gave rise to new species that were randomly bigger or smaller, they would still produce a trend toward larger sizes. That is because their descendants could not get much smaller before hitting a minimum size limit.
These criticisms have prompted scientists to start putting Cope's rule to a much more rigorous test. They compare the size of ancestors to their descendants, or, at the very least, to very closely related species. They then make these measurements in many lineages in a given group to see if the trend is statistically significant.
''You have to pin it to the branches of the tree of life, and then you look to see whether those branches have a tendency to get bigger,'' said Dr. Andrew Purvis of Imperial College London.
Dr. Purvis and Mr. Hone are among the co-authors of a study of Cope's rule in dinosaurs to be published in The Journal of Evolutionary Biology. The researchers compared 65 pairs of related dinosaurs separated by tens of millions of years. They found that on average, the younger dinosaurs were 25 percent larger than the older ones. The pattern spanned all the major groups of dinosaurs, from bipedal predators to long-necked grazers.
''It's one of the few really thorough studies in this area, and it finds really convincing support for Cope's rule,'' said Dr. John Alroy, a paleontologist with the National Center for Ecological Analysis and Synthesis in Santa Barbara, Calif. Dr. Alroy's own research has revealed long-term increases in the size of North American mammals over the past 80 million years.
Coincidentally, other researchers have recently found support for Cope's rule not in fossils, but in living creatures. The support comes from a database that Dr. Joel Kingsolver of the University of North Carolina and his colleagues put together of every published measurement of natural selection in the wild. Last year, Dr. Kingsolver and Dr. David Pfennig, also of U.N.C., decided to see whether natural selection tended to favor size increases or decreases, or whether it had no particular preference at all.
''We got a very clear pattern,'' Dr. Kingsolver said. ''In 80 percent of the studies, there's consistent selection favoring larger size.'' As Dr. Kingsolver and Dr. Pfennig reported in the July issue of Evolution, the pattern held up within both vertebrates and invertebrates, as well as in plants.
Dr. Kingsolver said he believed there was no single advantage to being big. ''My guess is that it's a mix of particular reasons for particular species,'' he said ''You may be able to make it through lean times better than someone who's smaller. Females that are larger are able to produce more eggs. If males are competing for females, larger size is often favored.''
Whatever the cause, Dr. Kingsolver's findings have impressed other researchers who study the evolution of body size. ''It's a beautiful study, and the results are plain to see, as far as I'm concerned,'' Mr. Hone said.
But if being big provides such a big advantage in so many species, it is puzzling that there are so many small species. What's more, some studies on fish and other animals have found no long-term trend toward bigger. ''You need some mechanism that's counterbalancing it,'' Dr. Kingsolver said.
As Dr. Purvis put it, ''Why isn't Cope's rule more of a rule?''
The laws of physics may be partly to blame. ''Insects can't get to be the size of Tyrannosaurus rex,'' Dr. Kingsolver said, pointing out that their exoskeleton cannot support heavy loads of body mass.
Once animals adapt to a certain size, they may prevent smaller ones from entering their niche. ''A small rat is probably better adapted than a slightly larger mouse,'' Mr. Hone said. ''If his offspring get bigger, they're going to suffer because they're not as good at being that size than rats are.''
But humans may now be doing their part to counter Cope's rule. Hunting by humans, Dr. Purvis pointed out, may have played a major role in the extinction of large species, from North American mastodons to the elephant bird of Madagascar.
At the same time, intense fishing and hunting also appear to tip natural selection in favor of smaller animals. As the chance of getting killed goes up, reproducing early becomes more important than growing large. Studies have shown that some populations of salmon have shrunk 25 percent in 60 years because of heavy fishing.
''We're selecting for small body size, but not everything can get small enough quick enough, and so we're extirpating animals with large body sizes,'' Dr. Purvis said. ''In a lot of lineages, we're probably putting a stop to Cope's rule right now.''
Copyright 2004 The New York Times