(A) fossil evidence suggests that the mammals underwent adaptive radiation to produce the range of mammal types extant today.
(B) Adaptive radiation, in the life sciences, refers to the differentiation (or anagenesis) of one or a few species into many to fill a large number of related ecological niches by adaptation.
(C) Thus the first bird species may have given rise to many more bird species by adaptive radiation.
(D) Typically, a species adapts to colonize a new habitat and, this adaptation opening up a new range of niches, adapts again to fill the new niches which are presented.