Box 20.2: The Species Concept

A species is the central taxonomic rank of biological systematics, and species are one of the fundamental units in biology, similar to genes, cells and organisms. Species names are binomial, meaning they include the genus (e.g. Abies) and the species rank (e.g. alba), when naming silver fir, for instance. In botany, species can be further subdivided into subspecies, varieties and forms (so-called infraspecific taxa). The way to name species is defined in the “International Code of Nomenclature for algae, fungi, and plants (Melbourne Code)”, adopted by the Eighteenth International Botanical Congress Melbourne, Australia, July 2011.

Despite this clear taxonomic classification, it is not always easy to determine what a species actually is. A central question related to species concepts is thus the determination of clear boundaries between species, which must be based on well-defined criteria: What degree of morphological differences is enough to separate morphospecies? How large must the genetic difference be to separate two species? Evolutionary processes (speciation) are dynamic and ongoing, and so boundaries between species are constantly shifting.

In principle, to separate a group of individuals into two species, the variability of genotypes or phenotypes between species should be larger than between and within subpopulations within a species. However, answers to such questions have changed over time, and the nomenclature therefore also sometimes changes with the emergence of new knowledge. For example, major shifts in taxonomic units, also at the levels of families or orders, took place following the adoption of molecular techniques. Thus, species names are primarily a hypothesis, which can change if new data become available.

While early definitions of species were mostly based on similarities in morphological or other observable traits (morphological or phenetic species concept), the biological species concept defines a species as “groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups” (Mayr 1942). This concept works well in most cases, especially for sexually reproducing organisms. However, for plants, the biological species concept is somewhat problematic because of hybridisation between species, rarely also between genera (e.g. Triticale), and in the case of asexually reproducing organisms, for example, in apo- mictic plants (e.g. Rubus). More than 20 different species concepts have been proposed, including an ecological species concept (emphasising the occupation of ecological niches) and evolutionary or phylogenetic concepts.

In particular, the use of modern molecular techniques opened the door to distinguishing taxonomic units based on genetic distances. All these definitions are based on distinct biological properties, which are acquired by lineages during the course of divergence (e.g. phenetically distinguishable, reproductively isolated, monophyletic) (de Queiroz 2007). Some concepts are partially incompatible with each other (often referred to as the “species problem”), and the adoption of different concepts allows for the occurrence of different species boundaries and, thus, different numbers of species. Nevertheless, the biological species concept is still the most often used species concept.

A proposed solution to the species problem suggests that the only necessary property of a species is the existence of a separately evolving metapopulation lineage (i.e. an ancestral- descendant sequence of sets of subpopulations connected through the exchange of genes; such groups of connected subpopulations are termed metapopulations). This property is inherent to all contemporary species concepts (de Queiroz. It is a further development of Darwin’s revolutionary idea to see species as branches in the line of descent, that is, the evolutionary basis for the concept of species. All other proposed properties to qualify being a species, such as being phenetically distinguishable, monophyletic (i.e. a group of species with an ancestral species and all its descendants), reproductively isolated or ecologically divergent, are considered as secondary, contingent properties of a species. These properties may (or may not) be acquired during the course of species existence and can be seen as different operational criteria to assess lineage separation and, hence, as evidence for the existence of a species. This general and unified species concept also works for asexually reproducing species if they form metapopulations as the result of some processes other than the exchange of genetic material or interbreeding, for example, by natural selection.