| The Magazine of the CALIFORNIA ACADEMY OF SCIENCES |
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| Horizons Ready...Set...Evolve!
Evolution takes time, and patience is a necessary virtue for biologists who want to catch wild animals in the act. But as two recent studies from the Caribbean seem to show, only a few years need elapse before natural selection plays its hand: species can change, sometimes dramatically, within a geologic blink of an eye. In one study, led by evolutionary biologist David Reznick, of the University of California at Riverside, groups of guppies from two streams in Trinidad were transplanted to test how predators would impact their life history. The guppies in question, Poecilia reticulata, are related to the common pet store variety but live in tributaries of the Aripo and El Cedro rivers. Downstream, the guppies contend with other fish, mainly cichlids and characins, who eat the big adults. Other guppies lucky enough to live in upstream pools isolated by rapids and waterfalls have only one predator to worry about, a killifish that occasionally eats young fish. The waterfalls provided a perfect barrier to control the experiment, so guppies were moved from high-predation pools to a less stressful home upstream and then left to fend for themselves. Eleven years after the switch--or 18 guppy generations later--the upstream fish in the Aripo River matured at a later age and larger size than those from the other side of the falls. They also produced fewer, bigger offspring. In short, the guppies changed their life history in response to less frequent attacks by predators. Subsequent lab studies confirmed these were genetic changes. Offspring inherited the new, evolved strategy. Similar changes occurred in the El Cedro guppies as soon as four years after the move upstream for male fish. Females followed suit after a few more years. "The first thing that our data show is that rapid change is no big deal," says Reznick, whose report, co-authored with colleagues Frank and Ruth Shaw and F. Helen Rodd appeared in the March 28 issue of Science. Males reached a mature age nearly twice as fast as females and they seem to possess more genetic variability for this trait. But females grow to a bigger size more quickly and continue to grow after reaching maturity. The scientists could then use their data about differing selective pressure to calculate how quickly the guppy population changed. The estimated rate of evolution ranged from 3,700 to 45,000 darwins, which rivals rates estimated for cases of artificial selection such as those generated by crop growers or dog breeders. In 1949, British biologist and essayist J. B. S. Haldane coined the term "darwin" as a unit of measure for evolutionary change, say, an increase or decrease in size of some feature over a million-year period. Haldane conceived a formula to make such measurements, and while he expected high rates of change under artificial selection, he found that evolution, as captured in the fossil record, occurred at the agonizingly slow pace of a fraction of a darwin. Yet, given the rapid rates detected in wild guppies, says Reznick, "It would take 400 years for guppies to get as big as trout, but it's not going to happen" in the Trinidad streams, since the transplanted fish apparently attained an optimum age and size at maturity and have essentially stopped evolving for now. But, as they often do in nature, things could change. The second study analyzed anole lizards and took place on a series of small islands in the Bahamas. Anoles are an evolutionary success story; in the past 30 million years or so, they have spread throughout the Caribbean and Central and South America, and as far north as the southern United States. With as many as 400 species alive today, among vertebrates Anolis is second only to the frog genus Eleutherodactylus in number of species. In 1977 and 1981, biologist Tom Schoener of the University of California at Davis introduced small numbers of Anolis sagrei from Staniel Cay onto 14 nearby tiny and less lush islands where recurrent hurricanes had probably kept lizards from persisting in the past. He intended the experiment to reveal how quickly small populations go extinct and what role small island area might play. While a few populations did perish, others thrived, in one case growing from a handful or two of lizards to more than 700. That set the stage for an entirely different study to see how the lizards adapted to their new environment. Enter herpetologist Jonathan Losos, of Washington University in St. Louis, who had an idea that the unwitting castaways might be diverging from the Staniel Cay lizards. So he went to the islands and to Staniel in 1991 to catch a bunch of male anoles (which are larger and easier to spot than the females). Losos measured the length of their hind legs, and, as he and colleagues Ken Warheit and Schoener report in the May 1 issue of Nature, a consistent pattern emerged. Because the experimental islands had only sparse, scrubby vegetation, compared to the denser shrubs and trees of Staniel Cay, the lizards had only narrow branches on which to perch. In other Anolis species, lizards that cling to narrower perches have shorter legs. Losos predicted, correctly, that the colonists would likewise develop shorter legs and wider toe pads than the Staniel anoles. The degree to which the lizards differed depended on how far each island's plants departed from those on Staniel. "We know that species and populations can adapt very quickly," says Losos. Although the lizards changed quickly, the extent of the change after only 15 generations was not dramatic. "If I showed you pictures of the lizards you probably couldn't tell any difference," says Losos. "In fact, I didn't even see any difference when I was in the field." Only careful statistical analysis revealed the change. But, Losos suggests, such minute anatomical adjustments may be the first baby steps that an anole population would take on its way to becoming a new species. "Given enough time and diverse enough habitats, good ol' natural selection can drive them to very big differences," he says. "If you can get a little difference in just 15 years, then you ought to get a lot of difference in 10,000 years." A lingering loose end in the anole work concerns whether or not the shrinking legs are a genetically driven change. If they are, then the study is indeed catching natural selection in action. An alternative explanation would be that young lizards forced to use narrow perches simply grow up with shorter legs. Losos is currently seeking the answer with anoles of the same species from Florida, which are being raised at the St. Louis Zoo on perches of different diameter. Even if the changes turn out to be driven by growth rather than genes, Losos surmises that populations may go through a period of molding to a new environment before a chance genetic mutation somehow fixes a new trait, such as shorter legs, and provides a hereditary engine that could drive the emergence of a new species. Reznick's guppies appear to have evolved at least 10,000 times faster than rates of change inferred from fossils. Even Losos's lizards, which changed at rates up to 2,117 darwins(d), far exceed the rates estimated from the fossil record. In a 1983 Science paper, University of Michigan paleontologist Philip Gingerich found that fossil organisms yielded, for a given million-year period, an average rate of change of only 0.21 d for invertebrates and 0.12 d for vertebrates. With longer stretches of time the changes become less perceptible and the rates get even smaller. That indicates, Reznick says, that "The fossil record is biased in a way that makes changes seem slower than they really could be." These guppy and anole results come in the midst of a debate about whether a single process presides over the day-to-day changes that populations of individuals undergo, microevolution, and macroevolution, the big-picture view from fossils of the comings, goings, and splitting of species through time. As Reznick says, "The question is whether or not we can explain the larger scale events of evolution in terms of what we can see and study." Fossil evidence points to three recurring trends in macroevolution: stasis, or long periods with little or no change, punctuations of rapid change, and speciation. Twenty-five years ago, paleontologists Stephen Jay Gould, of Harvard University, and Niles Eldredge, of the American Museum of Natural History, proposed their theory of punctuated equilibrium to explain the abrupt appearance of species and long gaps between conspicuous changes in the fossil record. As part of this idea, they have emphasized the need to seek separate mechanisms behind macro- and microevolution, arguing that while natural selection influences the survival of individuals, it cannot account for large-scale change in the history of life. Instead, they hypothesized for macroevolution a distinct process called species selection, whereby successful species rapidly replaced their ancestors. But Reznick and his co-authors believe the guppies challenge such an assertion. "Our work lends support to the argument that neither punctuations nor stasis requires special explanations beyond natural selection," says Reznick. And while their paper does not directly address this issue, Losos and colleagues conclude from the anoles that "macroevolution may just be microevolution writ large." Gingerich also suspected as much in his earlier study. Though signs of microevolution may vanish within the wide swath of the fossil record, he nonetheless wrote that "A microevolutionary rate of 400d is sufficient to change a mouse into an elephant in 10,000 years." More importantly than how they figure in such heady theoretical debates, however, is the plain fact that the guppy study, and perhaps the anole one, too, reveal natural selection to be a powerful force. Both studies show that, under the right circumstances, experiments about evolution can succeed outside the contrived setting of a laboratory, right in the rough-and-tumble wild. Blake Edgar is an associate editor of California Wild and co-author with Donald Johanson of From Lucy to Language, recently published by Simon & Schuster Editions. |
Fall 1997
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