Okay, so if this is going to be a full-out debate about the validity of evolutionary biology, I'll have to bow out soon. A professional biologist will be in a better position to answer questions about cladistics and genetics than I am. But here are two of the important points.
1) No one claims that frogs developed into squirrels. Instead, biologists claim that frogs and squirrels have a common ancestor. Frogs, squirrels, and their common ancestor share many traits in common: Four limbs, two eyes, a bony skeleton with a spine, and so on.
2) Squirrels aren't more evolved than frogs. Rather, squirrels and frogs are both extremely well adapted to their two environments. Frogs are great at catching insects that live near ponds, and squirrels are amazing tree-climbers. There's no particular reason for either species to change as long as we still have ponds and trees. Some species of plants and animals have been around for a very long time. The squirrel population continues to be squirrel-like instead of becoming hedgehog-like, because squirrels are better tree-climbers than hedgehogs, so they're better adapted to the treetop environments they live in. Hedgehogs are also well-adapted, but to a different environment.
(Note: If a squirrel is ever born with a genetic mutation that makes it run away from cars instead of toward them, it'll out-survive all the other squirrels in my neighborhood, and its descendants will take over the suburbs.
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