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Intergradation
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<h2 id="primary-intergradation">Primary intergradation</h2> <p>This occurs in cases where two subspecies are connected via one or more intermediate populations, each of which is in turn intermediate to its adjacent populations and exhibits more or less the same amount of <a href="/facts/Genetic_variability/Fvcqh60e">variability</a> as any other population within the <a href="/facts/Species/oDg6b96r">species</a>. Adjacent populations and subspecies are subject to <a href="/facts/Cline_(population_genetics)/e1h1SPrH">cline</a> intergradation, and in these situations it is usually taken for granted that the clines are causally related (by <a href="/facts/Natural_selection/JXTte6en">natural selection</a>) to <a href="/facts/Environmental_gradient/N6cJsxV3">environmental gradients</a>.<a class="footnote-ref" id="fnref:1" href="#fn:1"><sup>1</sup></a> </p> <h2 id="secondary-intergradation">Secondary intergradation</h2> <p>When contact between a geographically isolated subspecies is reestablished with the main body of the species or with another isolate subspecies, interbreeding takes place as long as the isolate has not yet evolved an effective set of isolating mechanisms. Consequently, a relatively distinct zone or belt of <a href="/facts/Hybrid_(biology)/pzVJ54xU">hybridization</a> will develop depending on the degree of <a href="/facts/Genetics/bu8wne0A">genetic</a> and <a href="/facts/Phenotypic/vxWXKGLj">phenotypic</a> difference that was achieved by the previously isolated <a href="/facts/Subspecies/4L9jFacF">subspecies</a>.<a class="footnote-ref" id="fnref:2" href="#fn:2"><sup>2</sup></a> </p> <h2 id="see-also">See also</h2> <ul><li><a href="/facts/Ring_species/fDAfgocV">Ring species</a></li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>Mayr E, Ashlock PD (1991). Principles of Systematic Zoology. Second edition. McGraw-Hill. ISBN 0-07-041144-1. <a href="/wiki/ISBN_(identifier)" target="_blank">/wiki/ISBN_(identifier)</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>Mayr E, Ashlock PD (1991). Principles of Systematic Zoology. Second edition. McGraw-Hill. ISBN 0-07-041144-1. <a href="/wiki/ISBN_(identifier)" target="_blank">/wiki/ISBN_(identifier)</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> </ol>