The gradual change has been correlated with increasing depth of water. The populations intermediate between the two species were presumed to have lasted for 73 000–250 000 years. In this pattern there is no rapid shift in morphology and, in studies of this kind, the species limits are usually recognized by comparing the morphological variation over a period of time with morphological variation of modern species and dividing up the continuum accordingly.
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Answer & Explanation:
The gradual change has been correlated with increasing depth of water. The populations intermediate between the two species were presumed to have lasted for 73 000–250 000 years. In this pattern there is no rapid shift in morphology and, in studies of this kind, the species limits are usually recognized by comparing the morphological variation over a period of time with morphological variation of modern species and dividing up the continuum accordingly.