A plant species is distributed on the slopes of two mountains
and in the broad valley between them. During a period of climate
warming, the valley floor becomes uninhabitable to the plant, but
the populations persist on the mountains and undergo speciation
(Phase 1). The climate cools again and the new species come into
contact (Phase 2), but the only reproductive barrier between them
is hybrid inviability, a postzygotic barrier.

a) What type of speciation is occurring during Phase 1
(allopatric, parapatric or sympatric)? Explain your answer.

b) During Phase 2, would plants pollinated by members of the
same species be fitter or less fit than those pollinated by the
other species? Explain your answer.

c) What types of traits are likely to be favoured in this
situation?