Chapter 22 Notes: Species and Speciation: Figure WN22.10 - Dobzhansky–Muller divergence within a population spread over a broad area (i.e., in parapatry).

Figure WN22.10 - Dobzhansky–Muller divergence within a population spread over a broad area (i.e., in parapatry).

Figure WN22.10. Dobzhansky–Muller divergence within a population spread over a broad area (i.e., in parapatry). (A) Initially, the population carries alleles a, b, c, ... . A favorable allele A rises in one place and begins to increase at the expense of a; meanwhile, an allele B at another locus starts to spread somewhere else. (B) If A and B are incompatible, as in the Dobzhansky–Muller model (Fig. 22.24), two stable clines will form where the alleles meet each other. (C ) If an allele C is favorable when combined with Ab but is incompatible with aB, then it will strengthen divergence by forming a third stable cline. We have shown only alleles that contribute to incompatibility: Most favorable alleles are likely to spread through the whole population and so will not contribute to divergence. If there is a barrier to gene flow across the species’ range, it will slow the spread of favorable alleles, making it more likely that they will meet an incompatibility before fixing throughout the range.

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