Macroevolution Vs. Microevolution.
Many times I've heard creationists say that microevolution, or "Evolution within a species" can exist, while macroevolution, or "Evolution between species." cannot.
We all know that this is a stupid conclusion, but I've never heard it explained very thoroughly why it is so stupid.
The main issue in this argument is a misconception of meaning. Not the meaning of evolution, but the meaning of "species".
What is a species? A species is a category. It is a classification of organisms based on similarities in such things as genetic qualities, adaptive abilities, reproductive habits, etc.
Who defines what species are? We do.
Human scientists use accepted criteria to define which organisms belong to which species. A species is a man-made definition.
So, evolution can happen within species, but not outside the realms of species? Does genetic mutation really give a d*** about man-made definitions?
The only way that this argument works is if it was declared that evolutionary limits were a known criteria for categorizing species. No such claim has ever been made.
So. Here's how it works. A species experiences microevolution over a series of generations. This microevolution accumulates to changes in the species traits. Eventually, there are enough iconic changes in the species that it can then be defined as a new species.
Since we have only relatively recently began naming species, the point where those species were named became an arbitrarily placed 0 on the evolutionary timeline, with future generations going in the positive direction, and past generations in the negative. Not enough time has passed since then for enough genetic mutation to accumulate to the point where we can name something as a new species.
Really, we're not waiting for the scientists to make a discovery, we're waiting for the scientists to make the decision. Once they make that decision, we will have then recorded macroevolution.
This is the best way I can think of to explain the issue of microevolution vs. macroevolution, assuming that the creationist you use it with accepts microevolution as a valid premise. If not, then don't bother arguing with them. There's no way to help that level of stupidity. Let me know if I've gotten something screwed up with my logic.