This is how most evolution happens. Over a long period of time, lots of mutations will accumulate in the population. Genetic diversity increases, and you get lo

This is how most evolution happens. Over a long period of time, lots of mutations will accumulate in the population. Genetic diversity increases, and you get lots of variations of different traits. But these new genes don't necessarily improve fitness a lot, they just don't decrease it enough to be selected away. Then something happens that changes the environment. Selection pressure increases, and the mutated genes are suddenly selected for at a much higher rate and come to fixate in the population. And if that happens to multiple genes, the changes can be quite dramatic. Even though all the genes existed before in the population, they didn't all exist together, or in most of the population.