The bulk of extant sister species of birds are allopatric or nearly so; those with some overlap are evolutionarily "older," implying that they evolved sufficient ecological differences to allow coexistence in some areas, usually where the ranges meet (Chesser and Zink 1994).
The predicted power of neutral theory can be taken as evidence for ecological equivalence at the coexistence equilibrium of species with more or less different intrinsic attributes.
I explain its appearance in the steady-state distributions even of extremely dissimilar species in terms of the trivial expectation that species must achieve ecological equivalence at their coexistence equilibrium, which is defined by equal realised fitness for all.
On the other hand, sexual selection models require ecological differentiation for the coexistence of incipient species.
The evolutionary consequences of competition are of great interest to researchers studying sympatric speciation, adaptive radiation, species coexistence and ecological assembly.
These feral populations have to be considered in ecological risk assessment and coexistence measures as a relevant hybridisation partner of transgenic oilseed rape.
In principle, ecological differentiation allowing stable coexistence and reproductive isolation preventing interbreeding can both be achieved if disruptive natural selection and disruptive sexual selection are acting at the same time.
In a sense, coexistence reflects the ecological balance.
