Bird guides once listed the myrtle warbler and Audubon's warbler as distinct species because of differences in color patterns of the feathers and size of the beak. Recently these birds have been reclassified as eastern and western forms of a single species, the yellow-rumped warbler, based on the observation that the two forms interbreed often in nature and the offspring have good survival and reproduction. The earlier classification into two species was using a species concept; the reclassification is using the species concept.

morphological; biological

Explanation:

The morphological species concept highlights the importance of physical traits to delimit species. This concept is useful for classifying asexually-reproducing organisms and where lack information on interbreeding (for example, to classify fossils). On the other hand, the biological species concept states that one species represent a group of organisms that can interbreed (or potentially can interbreed) in order to produce fertile offspring. The biological species concept is nowadays the most widely accepted concept to define species.

the correct answer is b