A species of bird had an original range covering the entire eastern half of the U. S. from New England west to the Mississippi and then south to Florida. During a period of glaciation, the eastern part of the population was cut off from the western part for 2000 years. Then the glacier melted and the birds' range was reestablished. During the separation, the western birds evolved a slightly different song and a darker wing color. Ornithologists are now studying this species to determine whether speciation has taken place. What evidence will the look for in their study