What is a characteristic feature of a ring species?

A characteristic feature of a ring species is that neighboring populations can interbreed, but distant populations cannot. This phenomenon occurs due to the gradual changes in the population's genetics and traits across geographical distances. As populations spread around a barrier, they may adapt to their specific environments, leading to variations that can affect their ability to interbreed with more distant members of the species. Although adjacent populations may still share enough genetic similarity that they can breed successfully, those at the ends of the "ring" may be sufficiently different that interbreeding is not possible, thereby illustrating the concept of speciation occurring through geographic and reproductive isolation.

The other contexts, such as the ability for all populations to interbreed or being geographically isolated, do not accurately describe a ring species. All populations interbreeding (the first choice) would imply no diversity or variation that could lead to speciation. Geographic isolation (the second choice) generally prevents populations from interacting altogether, while a linear distribution (the last choice) does not necessarily capture the circular or ring-like nature of the geographical distribution, which is crucial for the ring species concept.