Biological species vs. Phylogenetic species

Biological species concept: a group of organisms is a species if it consists of actually or potentially interbreeding individuals, and is reproductively isolated from other such groups (Mayr, 1969). Phylogenetic species concept: a group of organisms is a species if it is the least inclusive monophyletic group definable by at least one autapomorphy (i.e. a derived character state exclusive to a particular taxon) (Mishler and Donoghue, 1982). Closely related to this is the diagnostic species concept (Cracraft, 1983), where the classification is based on character states that are fixed and not necessarily autapomorphic. In practical terms, these two definitions are not as different as they at first seem. Both of them attempt to define a species essentially as an evolutionarily independent unit; in genetic terms, the biological species concept implies that gene flow can occur, whereas the phylogenetic species concept implies that gene flow has occurred. However, both concepts have limitations. The biological species concept is unable to classify asexual species, and neither can it be applied to fossil species; it tends to be overly lumpy and results in groups larger than perhaps are desired; and it can also be argued that, because the ability to interbreed is a primitive trait, the biological species concept may result in grouping of species that are not actually closest genetic relatives. The phylogenetic species concept is limited in ways that are in many respects the flip-side of the above problems. It tends to be overly splitting, resulting in groups smaller than desirable; organisms may be grouped on characteristics of unclear biological relevance; and different species (according to this definition) may interbreed, leading to interspecies gene flow.

Grade vs. Clade

Wessen-Simulating-human-origin-evo.pdf Simulating Human Origins and Evolution by Ken Wessen (2005) p 19: Any group of species may be classified according to the phylogenetic relationships of its members. A group that contains its most recent common ancestor and all its descendants is said to be monophyletic. If some, but not all, descendants are contained, it is a paraphyletic group. If the most recent common ancestor is not in the group, it is said to be polyphyletic. Traditionally, classification has been based on the concept of a grade, i.e. a grouping determined on the basis of overall morphological similarity. Such groupings often do not reflect the precise genetic relationships between the species, and are frequently paraphyletic or polyphyletic groups. The alternative is a clade-based classification, determined on the basis of common genetic origin, or monophyly. Because both morphological similarity and genetic relatedness between species are such primary concerns, both grades and clades remain important for taxonomy (Cronquist, 1987; Sokal, 1985). On the basis of the computer simulations mentioned in Section 1.3, Sepkoski and Kendrick (1993) found that, for incomplete data, polyphyletic groups may be just as useful ‘systematically’ as are clades (monophyletic groups). The species simulations in this book employ both techniques (see Section 3.1).