At the Edge of Invasion, Possible New Rules for Evolution

Just as Galapagos finches are icons of evolution by natural selection, Australia’s cane toads may someday be icons of “spatial sorting” — a dynamic that seems to exist at the edges of invasion, altering the standard rules of evolution.

Cane toads have evolved in odd ways Down Under. Adaptations that drove their dramatic spread made individual toads less reproductively fit. Evolution through natural selection of hereditary mutations still exists, but no longer appears driven by reproductive imperatives alone. It’s also shaped by speed.
“The possibility that some traits have evolved by ‘mating betwixt the quickest’ rather than ’survival of the fittest’ warrants further attention,” wrote biologists led by the University of Sydney’s Richard Shine in the March 21 Proceedings of the National Academy of Sciences.

Introduced to northeast Australia 75 years ago in an ill-advised attempt at beetle control, cane toads spread like fire, their range expanding at rates that grew daily. As they first arrived at his study area, Shine noticed something strange: As expected, the toads displayed myriad adaptations — longer legs, greater endurance, a tendency to move faster and farther and straighter — that affected their ability to disperse, but dispersal’s benefits were unclear.

The fastest-spreading cane toads also had the highest mortality rates. Longer, stronger toad legs led to spinal injuries. “Most obviously, why did the toads just sprint through our magnificent, food-rich flood plain in a frenetic rush to keep on going?” said Shine. After all, if the toads’ evolution is driven solely by a drive to reproduce, they would have stopped to enjoy the spoils of invasion.

“Much of what they did seemed hard to reconcile with the idea of natural selection enhancing individual fitness,” said Shine. “We started thinking about what other kinds of processes could have caused them to become such driven little robotic dispersal machines.”

In the new study, Shine describes those processes, which fall under the rubric of “spatial sorting” and are most easily understood by analogy: Imagine a race between rowboats crewed by randomly distributed oarsmen. If the race is stopped intermittently, and oarsmen randomly redistributed between boats nearest each other, boats in the lead will accumulate ever-higher proportions of skilled rowers.

Those are the dynamics of spatial sorting. Boats are organisms, rowers are genes and the crew swap is reproduction. Each newly-crewed boat is offspring. Generation by generation, organisms in the lead get faster and faster. Classical natural selection still operates — if a mutation causes an organism’s offspring to go sterile, the lineage soon ends — but it’s no longer the only driver of evolution.

Now space matters, too. Physical proximity produced by dispersal continue to shape that dispersal. Whatever drives creatures to spread farther and faster clusters at the front. If an adaptation improves dispersal but hurts survival, it matters less than usual, because the pool of potential mates is determined by their ability to cover ground.

Read more at Wired Science