Genetic Variation Can Fly
Noah saved a pair of each kind of unclean creature, and seven pairs of every kind of clean creature on the Ark. Generally speaking, the classification of "kind" is broader than today's species. Creationists believe that today's species developed from those creatures, staying within the kind. For example, while there are many species of fruit flies, they are all of the same kind.
But how could all the species we know today have developed so quickly? In 1978, a few European fruit flies were accidentally introduced to Chili. They have now spread their population over much of North America's west coast. In that short time, they differentiated within their kind. For example, those found in more northern latitudes have the longer wingspans, while those in the more southerly latitudes have shorter wingspans. Tests have shown that these differences are due to differences in genetic makeup. Amazingly, the same pattern of longer wing spans in more northerly fruit flies is very much like the pattern observed in European fruit flies. Even more astonishing is the fact that the American fruit flies achieve the longer and short wing spans through the lengthening of a different part of the wing than their European cousins.
That these differences, down to the genetic level, have developed in a mere quarter of a century illustrates how, in the few thousand years since the Flood, the various species we know today could have easily developed from their basic kinds.