In yet another example of evolution in action, investigators have documented morphological changes in the cane toads (Bufo marinus) that infest parts of Australia. They’re an invasive species that was introduced in a misbegotten attempt to control beetles that were damaging the sugar cane crop; as it turns out, they are aggressive predators that eat lots of other native fauna, and they secrete toxins that kill animals that try to eat them.
Another feature that contributes to their unwanted success is their rapid dispersal. Individuals can move up to 1.8km per night, occupying new territory at a distressing pace. In the 70 years since they were introduced, they’ve taken over a million square kilometers of Australia. Since dispersal into virgin territories is a significant advantage for the toads, it was predicted that there would be selection for faster migration rates in the population. The authors have several lines of evidence to show that this is the case.
One, they can correlate rates of movement with leg length; as you might expect, long-legged toads travel faster than short-legged toads.
Toads were captured at a study site that was positioned right at the leading edge of the invasion front; they measured leg lengths in the first toads to arrive, and then in later waves. Long-legged frogs were the first to show up.
So far, these data show that there is an advantage to having long legs. Are there any evolutionary changes in different populations? They compared leg lengths in older populations, back near the location of the first cane toad release, and compared them with the younger populations at the wavefront, and the answer is…yes, the new generation is longer limbed and faster.
That means, unfortunately, that toads are becoming better adapted at rapid dispersal, and the invasion rate is accelerating—slight changes in leg morphology are translated into greater effects on the population.
It’s kind of cool, in a train-wreck sort of way. It’s clear, empirical evidence of evolution shaping a population…but the consequences aren’t exactly beneficial to the native species of Australia, or that other invasive population, Homo sapiens.
Phillips BL, Brown GP, Webb JK, Shine R (2006) Invasion and the evolution of speed in toads. Nature 439:803.