page_title: Cline (biology)
section_title: Examples of clines
hierarchical_section_title: Cline (biology) / Examples of clines
context_page_description: In biology, a cline is a measurable gradient in a single character of a species across its geographical range. First coined by Julian Huxley in 1938, the “character” of the cline referred to is usually genetic, or phenotypic. Clines can show smooth, continuous gradation in a character, or they may show more abrupt changes in the trait from one geographic region to the next.
  A cline refers to a spatial gradient in a specific, singular trait, rather than a collection of traits; a single population can therefore have as many clines as it has traits, at least in principle. Additionally, Huxley recognised that these multiple independent clines may not act in concordance with each other. For example, it has been observed that in Australia, birds generally become smaller the further towards the north of the country they are found. In contrast, the intensity of their plumage colouration follows a different geographical trajectory, being most vibrant where humidity is highest and becoming less vibrant further into the arid centre of the country.
context_section_description: Although the term “cline” was first officially coined by Huxley in 1938, gradients and geographic variations in the character states of species have been observed for centuries. Indeed, some gradations have been considered so ubiquitous that they have been labelled ecological “rules”. One commonly cited example of a gradient in morphology is Gloger's Rule, named after Constantin Gloger, who observed in 1833 that environmental factors and the pigmentation of avian plumage tend to covary with each other, such that birds found in arid areas near the Equator tend to be much darker than those in less arid areas closer to the Poles.  Since then, this rule has been extended to include many other animals, including flies, butterflies, and wolves.
  Other ecogeographical rules include Bergmann's Rule, coined by Carl Bergmann in 1857, which states that homeotherms closer to the Equator tend to be smaller than their more northerly or southerly conspecifics. One of the proposed reasons for this cline is that larger animals have a relatively smaller surface area to volume ratio and therefore improved heat conservancy – an important advantage in cold climates. The role of the environment in imposing a selective pressure and producing this cline has been heavily implicated due to the fact that Bergmann’s Rule has been observed across many independent lineages of species and continents. 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. The populations elsewhere along the cline interbreed with their geographically adjacent populations as in a standard cline. In the case of Larus gulls, the habitats of the end populations even overlap, which introduces questions as to what constitutes a species: nowhere along the cline can a line be drawn between the populations, but they are unable to interbreed.
  In humans, clines in the frequency of blood types has allowed scientists to infer past population migrations. For example, the Type B blood group reaches its highest frequency in Asia, but become less frequent further west. From this, it has been possible to infer that some Asian populations migrated towards Europe around 2,000 years ago, causing genetic admixture in an isolation by distance model. In contrast to this cline, blood Type A shows the reverse pattern, reaching its highest frequency in Europe and declining in frequency towards Asia.
caption: The Larus gulls interbreed in a ring around the arctic.
  1: Larus argentatus argentatus, 2: Larus fuscus (sensu stricto), 3: Larus fuscus heuglini, 4: Larus argentatus birulai, 5: Larus argentatus vegae, 6: Larus argentatus smithsonianus, 7: Larus argentatus argenteus