ma == million years before present varieties and their ability to withstand environmental pressures eventually lead to populations that are so profoundly different that they become reproductively incompatible with populations other than themselves. And
voil�, a new species is born.
Darwin termed the process
gradualism, a theory that invokes the slow accumulation of small evolutionary changes over a large period of time, as a result of the pressures of natural selection. What Arnold and Parker found is almost a textbook example of gradualism at work.
We've literally seen hundreds of speciation events," syas Arnold. "This allows us to check for patterns, to determine what exactly is going on. We can quickly tell whether something is a recurring phenomenon--a pattern--or whether it's just an anomaly. This way, we cannot only look for the same things that have been observed in living organisms, but we can see just how often these things really happen in the environment over an enormous period of time.
Such a revelation flies in the face of latter-day rethinking of Darwinian evolution, which during the past 20 years has tended to gravitate toward a new theory called
punctuated equilibrium. First postulated in the early 1970s by paleontologists Niles Eldredge and Stephen J. Gould, this idea refutes the central premise that great amounts of time are necessary to create new species.
Punctuated equilibrium holds that new species may arise fairly quickly (over thousands instead of millions of years) from small animal populations that somehow become isolated. Intermediate stages are too fleeting to become fixed in the fossil record--thus the conspicuous gaps or so-called missing links. (Darwin blamed the "imperfection in the geological record" for the gaps in the fossil record.)
But in the near-perfect record exhibited by the forams studied at FSU, the highly touted Eldredge-Gould theory of punctuated equilibrium apparently doesn't work. The record reveals a robust, highly branched evolutionary tree, complete with Darwin's predicted "dead ends"--varieties that lead nowhere--and a profusion of variability in sizes and body shapes. Transitional forms between species are readily apparent, making it relatively easy to track ancestor species to their descendants. In short, the finding upholds Darwin's lifelong conviction that "nature does not proceed in leaps," but rather is a system prepetually unfolding in extreme slow motion.
In the hands of less scrupulous observers, the foram record may have been construed to
support Gould's hypothesis about the suddeness of speciation. Darwin would have been shocked to find out just how fast the great family of forams churns out new species, says Parker. Through dating analysis, he and his colleague showed that the forams could produce a whole new species in as little as 200,000 years--speedy by Darwinian standards. "But as fast as this is, it's still far too slow to be classed as punctuational," says Arnold.
Other curiosities are beginning to emerge from the probe into the forams' past. One finding is being described--perhaps too hastily--as disproving Cope's Rule, named for its synthesis by American paleontologist Edward Drinker Cope. His time-honored evolutionary principle holds that, within a group, animals teend to start out small and increase in size over time.
"We've found out that apparently, lineages don't exactly work that way," says Arnold. "Many of the forams start out small, and essentially stay that way until extinction. Others do manage to wander into dramatically larger sizes, but they're the rare ones."
This find doesn't necessarily contradict what Cope said, only what many scientists
think he said, says Parker. "Cope's observation was simply that there are a few extremely large examples (of individuals) in any given lineage, and these examples always occur at the later stages of the organism's development. And that's apparently true. But our findings show that the vast majority of forams start small and end small, even though the mean size increases somewhat due to a few very large specimens. As you get more and more species evolving, some of them eventually manage to get moderately to very large, but most of them don't increase in size at all."
It may be in what the foram record suggests about how life copes with mass annihilation that eventually draws the most attention to the FSU paleontologists' work. The geologic record has been prominently scarred by a series of global cataclysms of unknown, yet hotly debated, origin. Each event, whether rapid or slow, wreaked wholesale carnage on Earth's ecology, wiping out countless species that had taken millions of years to produce. Biologists have always wondered how life bounces back after such sweeping devastation.
One of the last great extinctions occurred roughly 66 million years ago and, according to one popular theory, it resulted from Earth's receiving a direct hit from a large asteroid. Whatever the cause, the event proved to be the dinosaurs'
coup de grace, and so wiped out a good portion of the marine life--including almost all species of planktonic forams.
This period of massive death, which ended the Cretaceous Period, ushered in the modern chapter of biological development. Earth entered the new era, the Cenozoic, with a wide range of ecosystems virtually devoid of life (and thus competition between species), yet quite fertile and primed for repopulation.
Some observers, perhaps following Darwin's lead, have envisioned a sedate repopulation sequence, with speciation occurring at an immensely slow rate. None of the species die off until their numbers begin to saturate the environment, exhausting its capacity to sustain such proliferation of life.
Other scientists have theorized, but never been able to demonstrate, that in the absence of competition, an explosion of life takes place. The evolution of new species greatly accelerates, and a profusion of body shapes and sizes bursts across the horizon, filling up vacant spaces like weeds overtaking a pristine lawn. An array of new forms fans out into these limited niches, where crowding soon forces most of the new forms to spin out into oblivion similar to sparks from a bonfire.
The ancient record of foram evolution reveals that the story of recovery after extinction is indeed busy and colorful. "What we've found suggests that the rate of speciation increases dramatically in a biological vacuum," says Parker. "After the Cretaceous extinction, the few surviving foram species rapidly evolved into new species, and for the first time we're able to see just how this happens, and how fast."
As the available niches fill up with these new creatures, the speciation rates slow down, and the pressure from competition between species appears to bear down in earnest. The extinction rate then rises accordingly. This scenario, says Arnold, suggests that the speciation process is sensitive to how fully packed the biosphere is with other
species, not the number of individuals. Ecologists, in referring to a given environment's ability to sustain life as its
carrying capacity, generally mean the natural limit, in shear numbers, of
individual organisms that any environment can support, as opposed to the number of different
kinds of organisms or species. "This is an intriguing concept--a
species carrying capacity, so to speak," says Arnold. "This implies that the speciation process is sensitive to how many spesies are already out there."
Perhaps if life were any less strange, its fundamental processes any simpler to fathom, scientists would not be so sensitive about their inability to write the definitive book on evolution. It may well be in the abyssal depths of the mystery itself that scientists find their innate compulsion to explain things magnified.
Exactly what new light the findings of Arnold and Parker shed on the evolutionary riddle as a whole is still unclear. Punctuated equilibrium can be a real process impinging on the evolution of other groups of organisms. Critics may argue that, while the FSU findings are interesting, they apply only to a rather peculiar organism and therefore do little to unmask the grander, biological scheme of things.
Arnold and Parker concede that evolution may in fact be little more than a collection of developmental options, all tailored along the same lines, presented in haphazard fashion before a sea of struggling life. One option may work splendidly for this organism, and fail miserably for that one.
"It's very likely that there are going to be some differences between species in the way evolution works," says Arnold. "There are particular guiding principles, however, that we believe should work for
all spesies."
No doubt the relentless search for a better understanding of how life came to be will lead scientists into many ecosystems not yet known or fully explored. Scientists might do well to follow the fossil trail, no matter how hard to track, to its timeless beginnings in the sea.
