Species of goby fishes build nests by digging burrows in which their young are born. When individuals of two different groups of gobies mate, their offspring are healthy and fertile, but lack the ability to build nesting burrows. What is this an example of?

Postzygotic reproductive barriers

Explanation:

Individuals from two different species of goby fish will mate to produce a hetrozygote zygote which is unable to build the nest.

This happens because the gene pools of the two types of goby fish remained isolated prezygotically

Problems arising because of formation of hybrid zygote  is known as postzygotic reproductive barriers. This inability leads to reduced ability of survival and reproduction which if persists for long could result into infertility of hybrid species.  

x would be the most and the least would be y because there are more fruit   and seeds than flock z has insects