Scientists ponder 'the Lilliput effect'

In Jonathan Swift's "Gulliver's Travels," Lilliput was a fictional island nation inhabited by people "somewhat under six inches high" – the Lilliputians. Modern paleontologists have adopted the term, "the Lilliput effect," to mean something slightly different. Organisms that survive mass extinctions tend to be much smaller than those that came before.

A variety of presentations on the Lilliput effect at the recent annual meeting of the Geological Society of America (GSA) in Philadelphia explored the many reasons organisms can shrink. They include the knockout punches of volcanic activity and an asteroid strike thought to have killed off the dinosaurs 65 million years ago, and the peculiar evolutionary pressures exerted by islands. (Many of the papers from the GSA meeting won't be published until 2007.)

Of intense interest to all is how humans, often said to be causing Earth's "sixth mass extinction," will affect evolution on the planet. As with naturally occurring global catastrophes of the past, human activity has suddenly changed the definition of evolutionary fitness. Some evidence indicates that the human "footprint" is leading not only to the selection of smaller species over larger ones and generalists over specialists, but also the dwarfing of individuals within a species. The ecological disturbance implied by such miniaturization has implications not only for the species themselves, which may disappear, but for humankind's well-being.

Conventional wisdom says that as lineages move through time, they tend to get bigger, not smaller. Small protohorses, for example, evolved in North America, crossed the Bering Strait when it was dry land and evolved into the larger modern horse in central Asia, even as they died out in the Americas.

This "bigger is better" process is known as Cope's rule, named after 19th-century paleontologist Edward Drinker Cope. This rule is based on what would seem to be the obvious advantages of largeness: the ability to fight off predators and take advantage of a wider variety of resources, larger and potentially more numerous offspring, and better metabolic efficiency. But heftiness is advantageous only when the going is easy.

After a catastrophe, the opposite tendency – the Lilliput effect – prevails. "It's not that the things that survived became small," says Peter Harries, professor of geology at the University of South Florida. "It's that the small survived." Smaller species often have shorter reproductive cycles, enabling them to quickly recover from population losses. If they suffer a rapid decline, like the one implied by the huge asteroid impact that extinguished an estimated 50 percent of all species, they recover faster. They also need less food, which is probably in short supply after a cataclysm, than big animals do.

As with many "rules," exceptions to the Lilliput effect abound. While the overall size of land animals shrank after the dinosaurs died off, some mollusks stayed as large as ever, says David Jablonski, a professor of geophysical sciences at the University of Chicago: "It happens except for when it doesn't happen."

When times are tough, many organisms compensate by reaching sexual maturity faster than their progenitors did, says Gerta Keller, professor of geology and paleontology at Princeton University. But with limited resources, accelerated reproduction comes at a cost: Reaching sexual maturity in half the time, the new generation may be half the size, too.

"It seems to be a universal way of adapting," Professor Keller says, "getting smaller and reproducing faster." By having more offspring in a shorter period of time, organisms accelerate evolution and improve their lineage's survival chances.

But overly specialized species like the California condor – or, by extension, the dinosaurs – cannot "Lilliputify" fast enough. They become extinct, even as generalists such as rats and some of our mammalian ancestors (the "disaster opportunists") thrive.

A dramatic shift in body size also occurs when species end up on islands, both literal and figurative. The "island rule" says that when isolated from founding populations, small species tend to get large, and large species, small.

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