What is this passage trying to say about Darwin's difficulty in explaining variation?

Question

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:

1. 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 (?)"

2. 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.

Answer 1

1. Remember this is all before the rediscovery of Mendel and the understanding of things like recessive traits and inheritance of discrete rather than blended traits. The problem is that, given an environment in which large beaks are beneficial, over geological time selection will tend to eliminate the small beaks. Then, if the geological environment changes such that small beaks are favored, there are no small-beaked individuals to benefit from the change: they would have been eliminated gradually during the long time when long beaks were preferred. The paradox or difficulty Darwin had was explaining how populations could maintain sufficient variability for selection to act to generate new traits.

2. This issue is similar to the first: the process of natural selection is all about what happens to existing populations. It explains what happens when you have variation existing in a trait. Natural selection by itself does not explain how mutations could occur to produce those new traits.

Darwin was concerned about both of these issues because at the time he saw them as serious threats to his theory (and specifically, like most good scientists, he worried that these things could mean that he was wrong). It seemed like a weakness of exactly the type he saw in prior works on transmutation of species that he was trying to work beyond. It took a few decades and a lot of scientific progress including the rediscovery of Mendel and the Modern synthesis for these problems to be truly worked out. As for Darwin, the best he could come up with was his somewhat-Lamarckian pangenesis idea, which he defended strongly.