Populations of coastal lodgepole pine are separated along an environmental gradient of soil pH that changes from neutral pH near the shoreline to extremely acidic pH approximately 8 km away from the shore. Populations of lodgepole pine inhabiting the ends of this environmental gradient (i. e. neutral pH versus extremely acidic pH soils) have diverged in a number of phenotypic characters, as well as cannot interbreed. What model of speciation is this pattern consistent with?

Parapatric speciation  

Explanation:

There are different types of speciation. Parapatric speciation consists of new species evolving from a continuous distribution. That is, there is no physical barrier that might impede the gene flow,  in fact, it might continue during the speciation process.  

There is an environmental gradient that determines different phenotypes for different environmental conditions. One phenotype is more adapted to one of the distribution extremes than the other phenotype. In the exposed case, the environmental gradient is given by the differences in pH.

In the border between the environmental extremes, there is the primary hybrid zone, where both phenotypic forms might meet and hybridize.

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