Populations of a single species that are quite distinct from one another phenotypically may be connected by populations that have which characteristics?

Intermediate phenotype

Explanation:

The same species might express different phenotypes as a response to the environment where it lives and its pressures.

Let us imagine an altitudinal gradient and the same tree species. The tree is able to live in the whole gradient, but it will be beneficiated by low altitude. This means that it will grow faster and taller, developing bigger foliage and bigger leaves when living in an altitude near the sea level than living at 500 mt over the sea level. At this last altitude, the tree is able to grow, but much slowlier, reaching a short shrub-like high, and poor foliage with small leaves.

Altitude is the pressure that is affecting the tree´s growth and development.  

At an intermediate altitude, let us say 300 meters over the sea level, the tree species will express an intermediate phenotype, not high nor that short, with more or less developed foliage, and medium-sized leaves.