Look at the image below.
It changes colour from red to purple from left to right.
Think of it as every colour represents a new species.
Red is one species, orange another, yellow a third and so on all the way to purple.
Now when does red stop and orange begin?
When are the changes large enough to be a new species?
My point?
Any change is just a variation in relation the parent (starting colour), but given enough small changes they all add up, and at some point you have something so different from the starting point that you can no longer say it is the same species (colour)