However, changes in those inherited traits might be valuable, too, if the environment changed. A muscular fin, for example, would be of great value in enabling a lunged fish to crawl out of a drying pond. After generations, the selectively valuable structure might no longer look like the original. The main limbs of whales, mice, bats, and humans have similar components. By contrast, structures with the same function but different evolutionary origin, such as the wings of insects and those of birds, are analogous structures.
Geographic barriers are the best stimulants of evolution.
Formation of a mountain range, for example, can divide a species into isolated units and thus block gene exchange. Also, a few members of a species might wander across a mountain chain and establish an isolated population. Eventually, the mountains might erode enough for descendants of the isolates to regain contact with descendants of the parent group. If they diverged too much genetically but still interbred, most of their offspring would be infertile hybrids. If all members of a species were genetically identical, there would be no natural selection.