... which can act as a welcome antidote to an overdose of phylogenetic species concepts:
Avise, J.C. (2000) Cladists in wonderland. Evolution, 54(5): 1828-1832.
And, in my view the best and most thoughtful contribution to the whole species cocnept debate:
De Queiroz, K. (1998) The general lineage concept of species, species criteria, and the process of speciation. Pp. 57-75. In Howard, D.J. & Berlocher, S.H. (Eds.), Endless Forms - Species and Speciation. Oxford University Press, New York.