An adaptation occurs when a species is set through a new constraint. This can be change in climate, defense mechanisms, food source, etc. Which will have to occur before an adaptation is made. For an example: in North America the climate slightly swifted. This meant smaller mammals such as rabbits would need to have a thicker coat of fur or shelter. Let's say all the thin furred rabbits died and all that remained were the thick furred rabbits. Over a span of time the rabbits have adapted through the climate. Meaning most of the population is thick furred through their DNA. Conclusively, it's a genetic characteristic that can happen very rapidly or slowly.
Bohr proposed his quantized shell model of the atom to explain how electrons can have stable orbits around the nucleus. The motion of the electrons in the Rutherford model was unstable because, according to classical mechanics and electromagnetic theory, any charged particle moving on a curved path emits electromagnetic radiation; thus, the electrons would lose energy and spiral into the nucleus.