Natural selection via reproduction of genetic material. A group of mammals that carried this gene that coded for this response to running water reproduced and passed on this gene (along with the rest of their genetic material). “Beavers” eventually evolved. It’s important to not view their existence in isolation from other dam-building mammals. Although their behaviour seems peculiar now, other mammals will have done it in the past. That beavers do it now is simply because animals that did it in the past passed the gene down successfully.

Evidence suggests beavers don’t build dams because they enjoy it, but because running water precipitates the influx of potential predators in their environments. Big pike eat baby beavers for example. If there are pike around, there are probably other fish around too, and if there are lots of fish around, there are probably bears and wolves not far away either - which are apex predators in the beaver’s ecosystem. Dam-building is therefore an auditory response to running water that contributes to the survival of the species carrying the gene that codes for it.

Dam-building helped animals carrying the gene for it to survive. Beavers are around now because the genes responsible for building dams have helped them survive. If a group of beavers were forced out of their aquatic environments, the genes coding for this behaviour would become less and less useful to their offspring, and their offspring’s offspring, ad infinitum. Different genes would become more useful to this group. This is a very rudimentary explanation about how speciation occurs. Speciation involves the interactions between animals’ genetic material and their environments. Those best adapted to their environments survive to pass their own genes on. Thanks Darwin!

Why and how there has actually come about genetic material that causes a response in animals to running water that makes them build dams, I have no idea. I suspect it is not genetic material too dissimilar from that which incites birds to build nests, or even that provokes humans to build shelters