The idea of a species - How do we decide between the species concepts?
Ecological versus reproductive species concepts
There is a theoretical controversy between advocates of the ecological and reproductive species concepts.
The reproductive and ecological aspects of species are probably correlated in nature: if the members of a species are interbreeding with too wide a range of mates, and producing maladaptive offspring, selection should adjust their mating preferences. Because of the correlation, it is difficult to test whether gene flow or selection provides a better explanation of species integrity.
Gene flow (migration) can rapidly unify the gene frequencies of separate populations if selection is weak. But if selection is strong it can keep populations distinct, and it is an empirical question how important the two processes are in nature.
Some test cases
Here we shall look at a few test cases, in which the processes come into conflict. The reproductive and ecological concepts make different predictions when:
• Interbreeding is present but selection favors divergence.
• When interbreeding is absent but selection favors uniformity.