Does Evolution Progress at a Snail’s Pace, or can it Leap Forward Rapidly?

A new study that documented rapid changes in the physiology of snail kites in the Florida Everglades has prompted researchers to question exactly how fast evolution can occur in a longer-lived animal – in this case: instead of occurring over a long period of time, over numerous successive generations; the changes in these birds, prompted by the introduction of a new species of prey available to them, took place in less than one-and-a-half generations.

The snail kite is a bird-of-prey that falls in the same family as eagles and hawks, and derives its name from the large snails that it eats as the majority of its prey. Found mostly in South America and the Caribbean, the snail kite population in the Florida Everglades is considered endangered, with fewer than 400 breeding pairs left.

But the 2004 introduction of the large apple snail, considered an invasive species in Florida, offered new prey for the kites that could grow up to five times larger than the snails the birds were used to eating. At first, the raptors had trouble managing the larger mollusks’ shells, being both heavier and deeper, making it difficult for the birds to reach deep enough into them to retrieve all of the meat. But several years later, University of Florida biologist Robert Fletcher Jr. found that the snail kite population began to grow, and speculated that the birds had adapted physically to take advantage of their new prey.

"Nobody would believe me," exclaims Fletcher. "They said, ‘No, that cannot be. It’s too quick," as science assumes that evolution progresses at a snail’s pace: the snail kite’s sudden surge in prosperity occurred in under ten years, within only one and a half generations. But later measurements of the birds revealed that their bodies were not only getting larger, their bills had also increased in size to reach deeper into the snails’ shells.

"The bills of these birds are now larger than would be expected relative to body mass," according to the paper published by Fletcher’s team.

In-of-itself, this newfound growth isn’t evidence that the Floridian snail kites have undergone evolution — that would be defined by a long-term change in their genetic structure — but Fletcher speculates that the snail kites are displaying "phenotypic plasticity", meaning that they may have had the genetic potential to grow larger beaks, but hadn’t done so until faced with the need to capitalize on the larger snails. This type of adaptation is typically considered a short-term adaptation to changes in the organism’s environment, making use of an already available genetic trait that wasn’t being expressed before. But if the environmental conditions persist, it could mean that, according to the paper, "evolutionary change may be imminent and underscore that even long-lived vertebrates can respond quickly to invasive species."

This is also not the first time a species has displayed such a rapid physical adaptation in response to a changing environment: Just looking at birds, Darwin’s finches on the Galapagos Islands alter their sizes in times of prolonged drought, and urban environments have prompted species such as house finches and great tits to grow longer beaks to take advantage of backyard birdfeeders; and Puerto Rico’s crested anole lizards are growing longer limbs and stickier toes for climbing buildings, just to name a few examples.