Evolution never really stops. Alleles of genes mix in every generation in sexual reproduction. If the organism is well adapted to one or more environments, the resulting mutations in offspring create little change. Once the environment changes—which is pretty much a guarantee on Earth thanks to plate tectonics and ensuing climate change—then those mutations become critical to the organism’s survival. It’s a matter of what traits allow an organism to live long enough to successfully reproduce. That’s the central dogma of natural selection.