A ring species is made up of a series of closely related populations that surround a region of unsuitable habitat. The shading i
n the image below shows the distribution of six populations of greenish warblers, a ring species found in Asia and Europe. Population 1 is the original population. Population 2 formed by branching off from population 1, population 3 formed by branching off from population 2, and so on, until population 6 was established. As a result of this pattern of formation, each population has minor variations in mating calls compared to the population immediately before it in the ring. Based on this information, which population is most likely to be reproductively isolated from population 2?
