Populations of mosquitofish have diverged from each other in environments that are different, based on the presence/absence of a predator. Is this an example of allopatry or sympatry?

This is an example of sympatry.

Explanation:

Sympatry: Refers to two similar species that coincide in part or their whole distribution area. There are two ways in which two species can reach sympatry.

Two species that originated in different areas, expanded and met each other in a concrete geographic place. In this case, both species compete for resources and one of them displaces the other one, or the two species might segregate and make use of different niches. Two species suffered sympatric speciation. They both have the same ancestor and originated in the same place. In the distribution area of the original species, there were differences in the populations' microhabitats, and they were submitted to different environmental pressures, such as dense or clear forest, presence or absence of predators, among others. These pressures could lead to certain differences that could have driven to the final isolation of one of the populations. These populations suffer genetic changes due to the environmental conditions that are inheritable.

Allopatry: Refers to the case in which two species are completely separated from each other, with no overlapping. This separation is due to the emergence of a barrier that separated the original species, and after that, the two populations evolved separately in different new species.

In the exposed example there is no barrier, but the presence/absence of a predator might be acting as an environmental pressure of the populations' microhabitats that leads to divergence.