The problematics of any metapopulation lineage-based general species concept. Arrowheads represent populations or subpopulations that might or might not comprise a single species. Often, phylogenetic relationships between such clusters of individuals will be unknown or ambiguous: Common memberships in a “metapopulation lineage” cannot be established (left panel). As well, there is in principle no way of knowing at what degree of divergence subpopulations assume independent “evolutionary roles and tendencies,” and thus no way of recognizing minimally inclusive groupings (that is, of distinguishing species from higher taxonomic groupings) (middle panel). And, when individuals are the product of extensive gene exchange, the very notion of lineage becomes problematic (right panel).
