A helpful analogy to help people Creationists (or others) better understand how evolution works: Language.

A parent language will be split up into various dialects of the same language as populations of the speakers disperse.
Pretty soon each population will have words that the others do not. Eventually the languages will become recognizably similar but too different to really be understood by the other population (eg> French and Spanish) but at a certain point the daughter languages are so different from the parent language and each other that they are all but unrecognizable as sharing a heritage (Eg> English and whatever the hell they speak in Boston).


All this to say that there are small changes over time and accelerated in isolated populations. These small changes compound to the point that the segregated population is dramatically and unequivocally distinct from the ancestral population.

So if the creationist can accept and recognize the concept that small, compounded changes result in dramatic, virtually unrecognizable change...what is causing them to reject this self-evident and proven principle as it applies to biological diversification?