Studies of parapatric distributions are frequently concerned with the

Studies of parapatric distributions are frequently concerned with the

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Studies of parapatric distributions are frequently concerned with the concordance of clines . Selection acting on one locus/trait can impose a cline on another character if the two characters/loci are linked. Are clines superimposed, shifted, different slopes. Slatkin (1973) has shown that the width of a cline is: See fig. 16.9, page 439; text uses different letters for equation). Different loci may have different cline shapes due to different strengths of selection acting on them. The text is a bit misleading about Parapatric speciation. It might lead one to believe that when a hybrid zone is observed, parapatric speciation is involved. This is not true since the hybrid zone may be the result of secondary contact after allopatry, rather than primary differentiation at the hybrid zone interface. Here again we need to determine the relative importance of the allopatric phase and the parapatric interaction
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Unformatted text preview: in determining the outcome of speciation (or fusion). The cricket hybrid zone (fig. 16.8, page 438) is in fact the result of allopatry followed by secondary contact (my personal knowledge), but Ridley does not let you know this. Non-allopatric models of speciation are controversial but not impossible. Sympatric speciation can be modeled with a two locus polymorphism, one locus (A) affecting fitness (in this case by affecting fitness in terms of survival on one of two alternative hosts/patches), and another locus (B) affecting mate choice which is crucial in the evolution of assortative mating , a barrier to gene exchange (proposed by John Maynard-Smith in 1966) AA Aa aa BB Bb bb host 1 1+s 1 1 mate w/ AA no preference mate w/ aa host 2 1 1 1+t...
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