"There has been a lot of discussion about ‘the species question’ over the past 20–30 years, and several surveys have converged on the essence of what we mean by species: they are evolutionary lineages ... Species thus have a real existence. This settles the ontological status of the species concept, but it does not necessarily solve the question of how to recognise them; the most logical way of defining species operationally is by the so-called Phylogenetic Species Concept: ‘A species is the smallest population or aggregation of populations which has fixed heritable differences from other such populations or aggregations’ ..."