What is a Species?

That question has been around for a while. Mayr had proposed a definition that essentially said a species was a collection of animals or plants which had the capacity of breeding among each other. As Kevin de Queiroz has stated:

Ernst Mayr played a central role in the establishment of the general concept of species as metapopulation lineages, and he is the author of one of the most popular of the numerous alternative definitions of the species category. Reconciliation of incompatible species definitions and the development of a unified species concept require rejecting the interpretation of various contingent properties of metapopulation lineages, including intrinsic reproductive isolation in Mayr's definition, as necessary properties of species. On the other hand, the general concept of species as metapopulation lineages advocated by Mayr forms the foundation of this reconciliation, which follows from a corollary of that concept also advocated by Mayr: the proposition that the species is a fundamental category of biological organization. Although the general metapopulation lineage species concept and Mayr's popular species definition are commonly confused under the name “the biological species concept,” they are more or less clearly distinguished in Mayr's early writings on the subject. Virtually all modern concepts and definitions of the species category, not only those that require intrinsic reproductive isolation, are to be considered biological according to the criterion proposed by Mayr.

In the current Nature there is an ongoing debate as to the many conflicting definitions. The authors state:

The assumption that species are fixed entities underpins every international agreement on biodiversity conservation, all national environmental legislation and the efforts of many individuals and organizations to safeguard plants and animals. Yet for a discipline aiming to impose order on the natural world, taxonomy (the classification of complex organisms) is remarkably anarchic. There is reasonable agreement among taxonomists that a species should represent a distinct evolutionary lineage. But there is none about how a lineage should be defined. 'Species' are often created or dismissed arbitrarily, according to the individual taxonomist's adherence to one of at least 30 definitions. Crucially, there is no global oversight of taxonomic decisions — researchers can 'split or lump' species with no consideration of the consequences.

 The problem is that especially in plants we have a significant amount of inbreeding potential between what we call species. The plants have the same number of chromosomes of the same length with genes in the same positions, almost. Then why call them different species? All too frequently it is in the eye of the beholder.