Cline

There is a gradual cline of decreasing size to the northward.

Source: tatoeba (10651140)

As one moves further south they enter a different cline.

Source: tatoeba (11535923)

2000 Michael J. O'Brien and R. Lee Lyman: Applying Evolutionary Archaeology →ISBN [A cline is a] character gradient, wherein a character such as length increases or decreases gradually and continuously. A cline distributed over geographic space is a "chorocline"; a cline distributed over time is a "chronocline." Compare with chorospecies and chronospecies. . . Simpson termed the change through time a "chronocline", where a cline represents a character gradient. A chunk of a chronocline comprises a chronospecies. The difficulty with identifying a chronospecies resides, then, in first identifying a chronocline, or temporal gradient in a character or attribute. As pointed out by Kevin Padian, some characters "change more or less uniformly through time, but others change not at all, and still others vacillate with no clear trend. This is ... one reason to be suspicious of the evolutionary utility of clines: no criterion for identifying a cline seems to be in force. A cline is simply a gradient in character state along a continuum, and it may be broken, temporarily reversed, or stepped. Furthermore, there is no criterion for a cline's magnitude and no control on its probability."

Source: wiktionary

2002 The Future of Evolution. Proceedings of the National Academy of Sciences Recent studies have shown that invaders can rapidly adapt to the new environments in which they find themselves. Huey et al. demonstrated how an introduction of a new fruit fly into the west coast of North America resulted in the evolution, in only 20 years, of an apparently adaptive cline related to wing size, throughout the vast new latitudinal range extending from southern California to British Columbia. The cline that developed in North American female flies was similar to that found in the European native populations. Interestingly, the developmental basis for the cline of wing size was different in Europe than for the invader in North America, although the functional result was the same, providing additional evidence for the adaptive advantage of this set of traits.

Source: wiktionary