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More about Dollo's Law:
In future videos, we will talk about the difference between cladogenesis (new species splitting off an existing one) and anagenesis (the formation of a new species by the transformation of an existing one). Most creationists assume evolution proceeds entirely by anagenesis, when it is almost entirely by cladogenesis.
I'm is often forced to use terms that are imprecise in order to convey scientific ideas. The idea of the ancestral lineage of animals becoming more "specialized" is in the sense of "more like their own species". This means that a "generalized" bird is one that represents the common ancestor of the "genus". Each new adaptation that is acquired along an ancestral line goes into the evolutionary history of that species, but this acquisition is one-sided, not an equilibrium of change.
If it helps, think of Dollo's law as the "arrow of time" for evolutionary change.