One of the more common Creationist claims today is that while microevolution (variation at the species level) is possible, macroevolution (variation above this level) is not. This is another one of those odd Creationist arguments that get bandied around a lot but don’t really seem to have anything substantial at their core.
Claiming that microevolution does not imply macroevolution means that you’re suggesting some sort of mechanism by which small changes over short amounts of time do not lead to many more small changes over large amounts of time. If that seems ridiculous, keep in mind that no Creationist will ever phrase it that way. I’ve also never seen any mechanism proposed which prevents macroevolution from occurring, without which the entire argument makes no sense.
A related objection is that we’ve never observed macroevolution, but we don’t expect to; if one species could change into another that quickly, it would be pretty strong evidence against evolution. Furthermore, nobody but Creationists have ever claimed that observing macroevolution is a source of evidence for evolution itself.
A large part of this argument stems from incredulity: ‘I can imagine variation within species, but anything more than that sounds impossible.’ Unfortunately, science isn’t about how unlikely something sounds when you first hear about it.