How real are the different species concepts?

The other species concepts have sounder theoretical foundations, and the question of nominal and real classification can be answered more conclusively for them.

• For the reproductive species concepts, the question is whether species exist as reproductive communities. It could be that there is a gradual blurring off of interbreeding as we move to more and more distant forms; but in practice interbreeding within a species is typically extensive and hybrids between species are rare. It seems that real species do exist in the reproductive sense.

• For the ecological species concept, we have no external measure of the adaptive zones in nature and we cannot say for sure whether they are discrete and separated by gaps, or blur into each other. Actually, the strongest reason to think that niches are discrete is that species form phenetic clusters - but this argument could only persuade someone who accepts the ecological species concept to begin with: others might interpret the evidence differently.

• There is a strong argument that species really exist in the cladistic sense. The reality of ecological and reproductive lineages at any one time will guarantee that the phylogenetic hierarchy has real branches in it, and that there are cladistic species.

To deny that there are real cladistic species would be to deny the existence of a phylogenetic tree of life. We can therefore conclude that species, in the reproductive, (probably) the ecological, and the cladistic senses are real and not nominal units in evolutionary theory.

Finally, the biologist Michael Donoghue explains why he is convinced the concept of a species is a real one.