Final answer:
The sister populations on opposite sides of the isthmus would be considered true species under the biological species concept due to reproductive isolation. While the phylogenetic species concept also plays a role in defining species based on unique traits, the biological concept is the primary one in this scenario because it focuses specifically on the breeding behavior and reproductive barriers between populations.
Step-by-step explanation:
The sister populations on opposite sides of the isthmus would be considered true species under the biological species concept. This concept defines a species as a group of interbreeding organisms that are reproductively isolated from other such groups. The biological species concept is focused on breeding behavior and reproductive isolation, meaning that if two populations cannot or do not interbreed in nature, they are considered distinct species. The aforementioned populations, by being on opposite sides of the isthmus, are typically geographically isolated, leading to reproductive isolation which over time can result in speciation.
The phylogenetic species concept addresses the issue of species identification over time, particularly with fossils, by defining species based on the unique characteristics they possess. If two populations possess distinct traits exclusive to each, then they are considered different species phylogenetically. However, the question specifically mentions sister populations that are presumably isolated by the isthmus, which implies a focus on reproductive isolation more than on unique traits.
The morphological species concept relies on the physical characteristics to define a species, but given the context of the isthmus, this concept is not directly addressed in the scenario provided. Hence, while morphological differences may arise, they are not the core criterion for defining species in the context of geographical isolation leading to reproductive barriers.
In conclusion, the most fitting species concept for the sister populations separated by the isthmus would be the biological species concept, due to the emphasis on reproductive isolation as the defining characteristic of a species.