There's also typically a ton of standing genetic variation that is relatively fitness-neutral, but can then lead to more rapid or directional evolution if conditions change. For example, standing variation in silk elasticity may have been more fitness neutral when capturing other prey, but after encountering this ant species, spiders with more elastic silk were more successful and had a fitness advantage.
> standing genetic variation
An interesting illustration of this might be variety seen in dogs, which are still one species that can reproduce together.
Presumably there was some ur-mutt, where almost all the variations we see today were already there, latent, until humans nudged the balances and blends.
Attempts to systematically domesticate foxes [0] suggest that certain packages of physical traits come along with selecting for calm or friendly behavior.
https://www.pbs.org/newshour/science/domesticated-foxes-gene...