Full Idea
Candidate species concepts can be intrinsic: morphological, physiological or genetic similarity; or relational: biology such as interbreeding and reproductive isolation, ecology, such as mate recognition in a niche, or phylogenetics (ancestor relations).
Gist of Idea
Concepts for species are either intrinsic structure, or relations like breeding or ancestry
Source
Kathrin Koslicki (The Structure of Objects [2008], 8.4.1)
Book Reference
Koslicki,Kathrin: 'The Structure of Objects' [OUP 2008], p.212
A Reaction
She says the relational ones are more popular, but I gather they all hit problems. See John Dupré on the hopelessness of the whole task.