I feel there is a part of the theory of evolution I'm not understanding correctly or maybe I'm missing it, but would like cleared up.
From what I understand, over time random mutations occur which in some cases provide an advantage allowing that animal to survive and reproduce. Its children will have that mutation allowing them to survive and so on over many many years until it evolves into a new species and / or the old ones become extinct.
For example the rock pocket mouse.
The part I'm getting stuck on is that unless these mutations happen instantly and in a large number, given its over such a big time scale how do animals who are not well adapted get enough time to evolve to survive?
Another example is fish that evolve to be able to "walk" out of the sea to escape predators.
If moving out allowed them to survive how did they get enough time to evolve and not be made extint in the time they didn't not have these limbs.
Hopefully I have explained myself clearly :)