Final answer:
The main difference between the ecological species concept and the biological species concept is that the former relies on natural selection maintaining species' distinctness despite gene flow, whereas the latter defines species based on interbreeding capabilities.
Step-by-step explanation:
The greatest difference between the ecological species concept and the biological species concept lies in their criteria for defining species. The ecological species concept focuses on the role of natural selection in maintaining species boundaries, regardless of gene flow. Species are often seen as distinct because of their ecological niches, even in the presence of hybrid zones where gene flow does occur. A classic instance is the Macaca species in Sulawesi, which maintain distinctness despite interbreeding.
On the other hand, the biological species concept defines a species by its capacity to interbreed and produce fertile offspring. This is more straightforward but challenging to apply to fossil records, where it's hard to discern if different specimens could have interbred, or in understanding the variation within a species.