The paradox of the visibly irrelevant

This favorable set of circumstances -- rapid evolution in a predictable and presumably adaptive direction based on traits known to be higly heritable -- provides a "tight" case for well-documented (and sensible) evolution at scales well within the purview of human observation, a mere decade in this case. The headline for the news report of this paper in Science magazine (March 28, 1997) read "Predator-free Guppies Take an Evolutionary Leap Forward."

2. Lizards from the Exuma Cays, Bahama Islands. During most of my career, my fieldwork has centered on the biology and paleontology of the land snail Cerion in the Bahama Islands. During these trips, I have often encountered fellow biologists devoted to other creatures; we all, I trust, feel a shared bond of collegiality and comradeship. In one major program of research, Tom Schoener (a biology professor at the University of California, Davis) has, with numerous students and colleagues, been studying the biogeography and evolution of the ubiquitous little lizard Anolis (see "Darwin's Lizards," page 34) -- for me just a fleeting shadow running across a snail-studded ground, but for them a focus of utmost fascination (while my beloved snails must just blend into their immobile background).

In 1977 and 1981, Schoener and colleagues transplanted populations of five to ten lizards from Staniel Cay in the Exuma chain to fourteen small, neighboring islands that housed no lizards. In 1991, they found that the lizards had thrived (or at least survived and bred) on most of these islands, and they collected samples of adult males from each island with an adequate population. In addition, they gathered a larger sample of males from areas on Staniel Cay that had served as the source for original transplantations.

This study then benefits from general principles learned by extensive research on numerous Anolis species throughout the Bahamas. In particular, relatively longer limbs permit greater speed, a substantial advantage provided that preferred perching places can accommodate long legged lizards. Trees and other "thick" perching places therefore favor the evolution of long legs. Staniel Cay itself includes a predominant forest, and the local Anolis tend to be long legged. But when lizards must live on thin twigs in bushy vegetation, the agility provided by shorter legs (on such precarious perches) may outweigh the advantages in speed that longer legs would provide. Thus, lizards living on narrow twigs tend to be shorter legged. The small cays that received the fourteen transported populations tend to lack forest and to be covered with bushy vegetation (and narrow twigs).

J.B. Losos, the principal author of the new study, therefore based an obvious prediction on these generalities. The populations had been transferred from forests with wide perches to bushy islands covered with narrow twigs. "From the kind of vegetation on the new islands," Losos stated, "we predicted that the lizards would develop shorter hindlimbs." The study, published in Nature, vol. 387, 1997, validates this expected result: a clearly measurable change, in the predicted and adaptive direction, in less than twenty years.