Very Old Eggs Reveal A Fast, Changing Path Through Evolution
Copyright (c) 2005, Dow Jones & Company Inc. Reproduced with permission of copyright owner. Further reproduction or distribution is prohibited without permission.
BIOLOGISTS STUDYING how species change over the eons have always been hampered by the little problem of previous generations of a species being, well, dead. Sure, you can infer something about what a creature was like from fossils, but fossils generally fail to preserve much except bone. As a result, some of an animal's most interesting features vanish into the dust of time.
But these days, not even death is forever. A few years ago, biologist W. Charles Kerfoot was examining "cores" -- basically, muck deposited decades earlier -- in a Michigan lake. Lo and behold, he and his colleagues discovered eggs, and not just any eggs. They had been laid long ago by tiny creatures (mostly insects and crustaceans) that no longer lived in the lake. Even better, there was still life in the eggs. Under the right conditions, they would hatch.
"We knew right away that we were founding a whole new field," says Prof. Kerfoot of Michigan Technological University, Houghton. "I call it 'resurrection ecology.' " By hatching the eggs one muddy layer at a time, he realized, he could compare one generation with another to investigate evolutionary change.
It has always struck me as odd that evolutionary biology is caricatured by opponents as being static, a tower of unchanging (and unchangeable) dogma dating from Darwin. In fact, it is full of competing ideas, new discoveries and bickering scientists.
IN HIS RESURRECTION work, Prof. Kerfoot focuses on eggs of a tiny water flea, Daphnia retrocurva, from Portage Lake. He sieves them out of the deep muck, pops them into an incubator, and is a proud papa a few days later. "We've resurrected eggs from 300 years ago," he says. "That's 3,000 generations, equivalent to 120,000 years of evolution for humans."
And evolve is just what the little guys did. Daphnia share Portage Lake with creatures great and small, including predators, such as the shrimplike Leptodora. Prof. Kerfoot wondered whether the daphnia were doing something that biologists had hypothesized, but had struggled to prove -- namely, that the Red Queen in "Alice in Wonderland" was describing evolution when she told Alice, "It takes all the running you can do, to keep in the same place."
In evolutionary biology, the Red Queen Hypothesis means predators and prey must evolve like heck just to keep from falling behind (and to remain able to hunt or elude capture).
Sure enough, daphnia eggs taken from muck with a high population of predators hatched into veritable warriors: They had long spikes on their tails and an impressive helmet, the better to make themselves too prickly to eat. "But as predators became less abundant, spine length and helmets became smaller," says Prof. Kerfoot. "Evidence for the Red Queen is very strong here. It looks like these populations really are changing just to stay in place."
He isn't the only scientist tinkering with classic Darwinism. The reigning theory of the molecular basis of evolution is that whether a mutation takes hold depends solely on natural selection: beneficial mutations last, detrimental ones disappear. But something else may be at work.
IF A SLEW of mutations show up at once, more of them endure, scientists led by Bruce Lahn of the University of Chicago report in the July issue of Trends in Genetics. In my world, that's like an editor flooding you with dozens of suggestions for changes in your column. You're unable to fend them off, so more survive than if the requests come one-by-one over time.
Thousands of scientific papers presume that the fraction of retained mutations depends solely on how beneficial they are. "This theory has been the workhorse of molecular evolution," says Prof. Lahn. His discovery that a gene accepts more mutations when many hit at once is counterintuitive and controversial; a handful of journals actually rejected his paper. But if he is right, the molecular underpinning of evolutionary biology is itself in need of mutation.
Another pillar of evolution is that natural selection sculpts species toward some ideal fitness. In fact, what's "fit" is a matter of opinion. Consider the males of a little reptile called the side- blotched lizard, which come in three kinds. Orange-throated giants beat up on their diminutive blue-throated rivals, which in turn lord it over tiny yellow-throated guys. You'd think the yellows would eventually die out.
But natural selection is more forgiving than that. The yellows are so beneath the contempt of the oranges that they are able to steal assignations with females attracted to the oranges' territory. As a result, the yellows reproduce and survive.
Just as the game rock-paper-scissors has no single winning strategy -- it depends what your opponents choose -- so in lizard-dom there is more than one route to evolutionary fitness.
Critics contend that evolutionary biology is a haughty club that forces members "to circle the wagons against any and all would-be challengers, and to achieve consensus on the most contentious issues," Michael Shermer, director of the Skeptics Society, has written. "This conclusion is so wrong that it cannot have been made by anyone who has ever attended a scientific conference," or dipped so much as a toe into the roiling waters of evolutionary research.
All which we don't know needn't be attributed to God.
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