As you saw in Part C, the body sizes of sympatric and allopatric P. cinereus and P. hoffmani are consistent with a hypothesis of character displacement resulting from interspecific competition. What additional information would strengthen the argument that character displacement is in fact responsible for body size evolution in these salamanders

Divergence in the body size trait between both species occurred once they were living together. The evolution of the trait difference occurred after sympatry.

Explanation:

Close species, P. cinereus and P. hoffmani, mostly live in different regions. In allopatry, both species have the same -or very similar- body size. Body size in these species is a heritable trait.

However, there are some areas where P. cinereus and P. hoffmani coexist. When they are in sympatry, P. cinereus exhibits a smaller body size and feeds on smaller prey items than allopatric P. cinereus. On the other hand, sympatric P. hoffmani exhibit bigger bodies, feeding on bigger prey items than the allopatric P. hoffmani.

So, when both species are in sympatry, they develop different morphological traits to avoid competition between them and manage to coexist in the same area. Resource availability is limited, so P. cinereus became smaller and feeds on small prey, while P. hoffmani became bigger and feeds on bigger prey. Through this trait divergence, they got to live longer and reproduce more, decreasing interspecific competition and increasing their fitness.

Allopatric species feed on similar prey items and have similar body sizes, reflecting that the divergence occurred after sympatry. If species living in allopatry were places together in the same habitat, they would exhibit strong competition that, over time, would drive to character displacement.