But since we know that changes occur that result in offspring not able to mate with the parent species it is technically a new species.
Like with the theory of continental drift, no eyewitness has seen one continent break up into two even though we can measure the drift of land masses. But Africa is always Africa and South America is always South America. Likewise we don't see frogs evolving into giraffe.
We can see the process of land changes today, and we have been able to observe them for thousands of years. Islands rise and fall, lands that once existed are covered by sea, others are exposed. Lakes evaporate or are sucked up overnight. Tropical areas become desert and vice versa. Icebergs break from polar regions. Earthquakes and volcanoes produce obvious and dramatic changes in land. Even the recent tsunami in Asia was felt all over the globe literally (it shook the planet and I believe moved it a fraction). All of this is demonstrable and has been witnessed and measured repeatedly. Unlike evolution. There is no comparison.