Think it takes thousands or even millions of years for animals to evolve significantly new traits? Think again. New research lends just a touch of credibility to the idea behind the popular sci-fi TV series Heroes, which portrays certain humans as having quickly evolved new astounding traits in response to increasingly tumultuous environmental pressures. In 1971 biologists moved 5 adult pairs of Italian wall lizards from their island home of Pod Kopiste, in the South Adriatic Sea, and introduced them to the neighboring island of Pod Mrcaru. An international team of researchers discovered that introducing these small, green-backed lizards, Podarcis sicula, to a new environment caused them to undergo shockingly fast and large-scale evolutionary changes. Researchers returned to the islands twice a year for three years, in the spring and summer of 2004, 2005 and 2006. Captured lizards were transported to a field laboratory and measured for snout-vent length, head dimensions and body mass. Tail clips taken for DNA analysis confirmed that the Pod Mrcaru lizards were genetically identical to the source population on Pod Kopiste. In other words, there is no doubt that these lizards are the offspring of the 1971 transplant.
The results of the study were recently published in Proceedings of the National Academy of Sciences. The lizards evolved entirely new digestive system features to cope with dietary changes, evolved bigger heads and also ceased to defend territories an instinct once very integral to the species behavior back on their original home territory. “Striking differences in head size and shape, increased bite strength and the development of new structures in the lizard’s digestive tracts were noted after only 36 years, which is an extremely short time scale,” remarks Duncan Irschick, a professor of biology at the University of Massachusetts Amherst. Observed changes in head morphology were caused by adaptation to a different food source explains Irschick. The lizards on the barren island of Pod Kopiste were well-suited to catching mobile prey, feasting mainly on insects. Life on Pod Mrcaru, where they had never lived before, offered them an abundant supply of plant foods, including the leaves and stems from native shrubs. Analysis of the stomach contents of lizards on Pod Mrcaru showed that their diet included up to two-thirds plants, depending on the season, a large increase over the population of Pod Kopiste. “As a result, individuals on Pod Mrcaru have heads that are longer, wider and taller than those on Pod Kopiste, which translates into a big increase in bite force,” says Irschick. “Because plants are tough and fibrous, high bite forces allow the lizards to crop smaller pieces from plants, which can help them break down the indigestible cell walls.”
Examination of the lizard’s digestive tracts revealed something even more surprising. Eating more plants caused the development of new structures called cecal valves, designed to slow the passage of food by creating fermentation chambers in the gut, where microbes can break down the difficult to digest portion of plants. Cecal valves, which were found in hatchlings, juveniles and adults on Pod Mrcaru, have never been reported for this species, including the source population on Pod Kopiste.“These structures actually occur in less than 1 percent of all known species of scaled reptiles,” says Irschick. “Our data shows that evolution of novel structures can occur on extremely short time scales. Cecal valve evolution probably went hand-in-hand with a novel association between the lizards on Pod Mrcaru and microorganisms called nematodes that break down cellulose, which were found in their hindguts.”Change in diet also affected the population density and social structure of the Pod Mrcaru population. Because plants provide a larger and more predictable food supply, there were more lizards in a given area on Pod Mrcaru. Food was obtained through browsing rather than the active pursuit of prey, and the lizards had given up defending territories.“What is unique about this finding is that rapid evolution can affect not only the structure and function of a species, but also influence behavioral ecology and natural history,” says Irschick.
By Rebecca Sato.