I've seen it answered many times. Your argument assumes new species can only appear suddenly in one generation.
An analogy:
What is the likelihood that a male and female of some 'new' dog breed would be born at approximately the same time, in the same general location and with the exact same mutation; survive to adulthood; find each other; successfully breed and raise young to adulthood?
For example one day a wolf gave birth to a male poodle and by sheer coincidence another wolf nearby gave birth to a female poodle, and by even more sheer coincidence both those male and female poodles managed to find each other and mate. What's the chance of that? Well near zero of course, but then again this is nothing like how it happened - gradual steps, not sudden steps.
Of course one cannot walk from New York to Chicago; any step leaves you within a fathom of your last position.
Aside from that, you have nothing to offer in the way of answering the question other than to simply speculate that there were gradual steps. In a way, that makes even less sense.
At what point does a transitional form become a new species and why is any such transitional form not a separate species?
And none of ths even addresses the problem evolution has explaining how one classification of life can be transformed into another classification. An example would be how did a cold-blooded animal get transformed into a warm-blodded animal? Any answer to such a question requires a tremendous amount of speculation.
Another perfect analogy is that the English language descended from the ancestral language of Latin.
According to the goofy creationist argument this must have been impossible, unless there came a time when suddenly a new generation of people spoke English and couldn't understand the Latin that their parents were speaking...
Yet again, the "clue for the clueless" is that these things happen very gradually, such that the changes which are introduced in any one generation are too small to cause insurmountable problems or incompatibilities, and yet over large periods of time the small per-generation differences accumulate to the point where the grand total change (in language, or species) are so large (when comparing starting point to ending point) that the final result is almost unrecognizable compared to the original starting point.
This concept is so simple and obvious that even a child can grasp it, but for some reason the anti-evolution creationists never manage to wrap their brains around it. Go figure.
Another instructive point of the language analogy is that French, Spanish, German, and other languages *also* have descended from Latin roots, as well as English. The means by which this occurs -- that is, how multiple different languages descended from a common original language -- are the same means by which lineages of living things can give rise to multiple descendants (e.g., the original primates gave rise to apes *and* monkeys *and* lemurs *and* humans, among many others). Species not only change, they *split*. Thus the "tree of life", which includes many branchings.