 For example, the house sparrow, which was introduced in the early 1850s to the eastern United States, evolved a north-south gradient in size soon after its introduction.This gradient reflects the gradient that already existed in the house sparrow’s native range in Europe.Ring species are a distinct type of cline where the geographical distribution in question is circular in shape, so that the two ends of the cline overlap with one another, giving two adjacent populations that rarely interbreed due to the cumulative effect of the many changes in phenotype along the cline.