Incorrect. This piece of creationist rhetoric is demonstrable nonsense. Speciation does occur and new species, orders, classes and phylums do evolve.
I very much doubt that any of the birds alive today can procreate with any of the species which we have classified as dinosaurs. Obviously just because your species evolved from a species in a certain classification does mean that your species belongs to that classification. We are not all protozoa just because our species evolved from species that any biologist today would classify as protozoa. So I disagree with your claim that there are still living dinosaurs.
I think what @T_aquaticus is saying is that no matter how much dogs evolve, their descendants will always be part of the “dog clade”. It isn’t a claim that large changes, even to the point of forming of new species are impossible within that clade. In fact, if I’m not mistaken, the whole notion of fixed species is probably more an antiquated Linnean concept than a cladistic one. But I will happily accept correction here if that is a wayward notion.
First, you don’t evolve out of your ancestry. If an ancestor of a species was a dog, then so too are the descendants. If the current population of dog splits into two new species then they will be two new species of dog. All of their ancestors will be dogs, no matter how much they change.
Second, everything above the species level in Linnaean taxonomy does not exist in nature. There are no orders, classes, or phyla in nature. Those are human inventions.
There were contemporaneous dinosaur species prior to the K/T event that couldn’t procreate with each other, yet they were still dinosaurs. Dinosaur is not the name of a species but of an entire clade of species.
Within cladistics, that is exactly how it works.
Protozoa is not considered to be a valid taxon anymore, but it may still be used for historical reasons. Protists are a paraphyletic group, and systematists try to avoid them where possible. Here are the clades from the Tree of Life Web Project:
Part of the problem with Linnaean taxonomy is that it doesn’t produce a true tree-like pattern. The fixity of species is also a problem, but the poor fit to actual phylogenetic relationships is the bigger problem, IMHO. With Linnaean taxonomy, once there is an arbitrary amount of change within a group then it breaks off and produces a new genus, order, class, and so on. It is a system of buckets within buckets instead of a true tree like pattern.
