Following a hurricane that cleared small islands near Great Abaco, scientists released pairs of brown anole lizards in May 2005. Four years later, measurements revealed a 13-fold population increase and a distinct decrease in hindlimb length across all studied colonies.
In May 2005, a team of five biologists from the University of Rhode Island and three other American institutions executed a field experiment on seven small Bahamian islets. The researchers placed one male and one female brown anole on each island. The opportunity arose after Hurricane Frances struck in September 2004, sending a storm surge over the islands near Great Abaco and wiping out any resident lizards.
Hurricane Frances Creates an Experimental Lab Near Great Abaco
Before the introductions, researchers searched the selected islets repeatedly and confirmed they were free of the study species. These locations were not barren. They supported functioning ecosystems with plants, insects, and other organisms across vegetated areas ranging from roughly 380 to 1,880 square feet.
The scientists captured the founder animals—scientifically designated as Anolis sagrei—from Iron Cay, a nearby forested island featuring wide tree trunks. They selected the breeding pairs at random and followed the resulting populations through 2009, tracking them alongside 12 reference islands in the vicinity.
Researchers Measure a 6.5 Percent Drop in Hindlimb Length
The introduced lizards multiplied rapidly. Their populations expanded by an average of 13-fold during the first two years before stabilizing. By 2006, measurable differences in hindlimb length had already emerged among the experimental colonies.
Between 2006 and 2009, body-size-adjusted hindlimb length dropped by 6.5% among males and 4.0% among females. This downward trend occurred at a rate roughly five times greater than that observed on the reference islands.
The physical changes traced back to the distinct environments of the new homes. While Iron Cay offered broad surfaces suited for long limbs, the experimental islets featured low, scrubby vegetation and narrow twigs. Long legs provide speed on broad surfaces, but shorter limbs improve agility and balance on narrow branches.
Founder Effect Preserves Relative Rankings Among Seven Colonies
Although natural selection pushed all seven populations toward shorter hindlimbs, the colonies did not converge on a single body shape. Each population maintained its original relative standing in the length hierarchy. The colony that started with the longest-legged founders retained that advantage four years later.
This outcome pointed to the role of genetic sampling chance during colonization, a concept known as the founder effect. A small group carries only a fraction of the original population’s genetic variation, establishing a unique baseline.
Led by Jason Kolbe and published in Science in 2012, the research demonstrated that both random founding events and natural selection operate simultaneously during biological invasions. The four years of field data were funded by the National Science Foundation and the National Geographic Society.