matthew809 Frug females like to build nests in the rocks(where they wait for the male frugs), some of them build their nests among the slippery rocks of waterfalls, their predators can't reach them there.So for centuries the frug females have been selected to nest among the slippery rocks. The frug male with slick skin can't reach those females very well, they slip off the rocks into the stream and have to climb the hill again, if they survive the fall. Our frug has a rough patch of skin on it's belly that gives him some traction on the rocks and he can easily reach the females. Soon all frugs will have a rough patch on their belly. It's just that simple. Whether the rough patch helps him in his life(compared to other frugs)or not is irrelevant if it allows him an advantage in reproduction. GrumpyPlease Register or Log in to view the hidden image!