Rapid evolution in early trilobites fueled by high variation

Trilobites were even more ubiquitous on the Cambrian seafloor than they are now in museum gift shops. Their abundance in the fossil record — partly a function of their hard, easily preserved exoskeletons — bore witness to the Cambrian Explosion, Earth's most dramatic boom of multi-celled animal life, says paleobiologist Mark Webster of the University of Chicago. Now a new study is teasing another great story out of the ancient creatures.

Webster compiled morphological data for nearly 1,000 of the 17,000 different species of trilobites, a class of marine arthropods that died out by 250 million years ago, from 49 previously published sources. By tracking different morphological features — the number of body segments, for example — Webster found that trilobite species exhibited more variation during the Cambrian than in later periods, he reported in Science July 27. "Once you go beyond the Cambrian, the diversity of forms within any one species drops off," he says.

Early and Middle Cambrian trilobite species, especially, exhibited greater morphological variations than their descendants. This high within-species variation provided more raw material upon which natural selection could operate, Webster says, potentially accounting for the high rates of evolution in Cambrian trilobites. Such findings may have implications for our understanding of the nature of evolutionary processes, he says.

Why the early trilobites were so morphologically diverse is a whole different mystery. The variation could be caused primarily by genetics, or ecology, or both, according to Gene Hunt, a paleobiologist at the National Museum of Natural History in Washington, D.C., who wrote an accompanying comment in Science.

University of Chicago

Paleobiologist Mark Webster compiled morphological data on nearly 1,000 species of trilobites to learn more about their evolution.

In the Early and Middle Cambrian, trilobites likely faced relatively low pressure to conform, as fewer species were fighting for resources, Webster says. "Organisms perhaps did not have to be as specialized to beat out competitors and avoid predators,” he says. “They got away with being more variable, in the evolutionary sense." However, as the number and types of organisms on Earth rose with time, trilobites found that they couldn't make do with such variation any longer. "Strong evolutionary pressures resulted in the fine-tuning of trilobite morphology” by the late Cambrian through the Permian, Webster says.

Webster says that this study provides empirical support for a rather obscure hypothesis developed in the late 19th century known as "Rosa's Rule." The hypothesis postulated that the level of variation within species should decline through time, which might in turn lead to a decrease in evolutionary rate. As high within-species variation might have played a major role in the dramatic Cambrian diversification of trilobites, this leads to the possibility that the extent of within-species variation might be a general predictor of evolutionary diversification rates, he adds.
 
“I think it is valid to say that reduced levels of competition eventually lead to higher within-species variability,” says Bruce Lieberman, a paleontologist at the University of Kansas in Lawrence. “When competition is less severe,” he says, “a much greater range of individuals within a species can survive because fewer are selected against.” However, he says, it is a very different matter to invoke those same processes of competition, variation and evolution to explain patterns between different species, such as why one group might have produced more species over millions of years than another group.  “Most evidence gathered suggests that greater variation within species does not lead to higher rates of speciation, but rather typically leads to lower rates of speciation, and I do not think this study … will tip that balance,” Lieberman says.

That genetic variation is necessary for evolution is “uncontroversial,” Hunt wrote, “but scientists have long wondered if abundant variation might play a more active role in facilitating or channeling evolutionary change.” That is a question that cannot yet be answered.

This research does support the idea that the link between morphological variation and evolutionary change was especially strong in the Early Cambrian, Hunt said. Furthermore, he wrote, the study “implies that evolutionary processes in the distant past may have acted different, or in a different balance than in more recent periods of time.”

Indeed, the study supports that within-species variation and evolution were rampant in the immediate aftermath of the Cambrian explosion, but not again to that extent, Lieberman says. “My suspicion is that there is something unique about the Cambrian period, probably related to greater flexibility in the developmental systems of organisms, as Stephen Jay Gould and others posited,” he says. “For this reason, and given that this pattern of more variation is likely to largely be a Cambrian phenomenon, I think that it is unlikely that this result has much significance for evolution in general, and for dinosaur or human evolution in particular.”

Still, Webster's study is the first to test this hypothesis on such a large scale, though it certainly will not be not the last. Further study is needed to determine whether the findings are unique to trilobites or this time interval, Webster says.

Ari Hartmann
Geotimes contributing writer

Links:
"Reworking the Cambrian Explosion," Geotimes, June 2004

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