Isaak here conveniently fails to mention whether by “change in a gene pool over time” he means exactly that (i.e., genetic variation, which is often called “micro-evolution”), or whether he means “macro-evolution”—which is something entirely different
Isaak (http://www.talkorigins.org/faqs/faq-misconceptions.html) does indeed mean exactly that. The macro/micro dichotomy is a false one - little changes lead, over time, to large changes. While it can be useful to talk of micro- or macro- evolution to differentiate between the small changes seen in a population and the accumulation of those changes causing speciation, it does not mean that they are different concepts. Is Mr Wallace wanting to see two different theories to account for what he sees as two different levels of change? That would be rather like asking for a different version of the atomic theory to account for the behaviour of water and ice... or indeed, the behaviour of something light, like hydrogen and something heavy, such as uranium.
I do, indeed, appreciate that it may be difficult to visualise that something that appears to be a set of discrete codings could ever produce something that is different to the initial available possibilities. However, as has been demonstrated with the decrease in the number of chromosomes in the human genome (24 down to 23), and the change of a bacterium to being able to consume nylon, such 'new information' can and does appear.