This passage, from the book "How humans evolved" by Robert Boyd, says that one of the problems Darwin had at the time was that he struggled to convincingly explain how variation could be maintained; this was needed to theorize that evolution is the result of cumulative adaptations over many generations. According to the book, there are four problems, one of them being that selection removes variation:
For example, suppose the environment changes so that individuals (finches) with small beaks are less likely to die than those with large beaks. The average beak size in the population will not decrease because there are no small-beaked individuals. Natural selection destroys the variation needed to create adaptations.
I understand the idea, but don't really understand what the example is trying to illustrate. The example seems to be saying if small beak finches survive better, then there would be no small-beaked finches...?
The second difficulty Darwin had was in explaining how animals "might evolve beyond it's range of variation (?)"
Selection can cull away some characters from a population, but how can it lead to new types not present in the original population?
The bit above I understood; however, I struggle to see the point of the example that was given for it:
How could elephants, moles, bats and whales all descend from an ancient shewlike insectivore unless there were a mechanism for creating new variants not present at the beginning?
Please help rephrase or explain again what these two difficulties actually are, or help make sense of the two examples that the book gave.