Species group

A species group is an informal taxonomic rank into which an assemblage of species within a genus are grouped because of their morphological similarities. Different definitions exist. Some authors use the term for species that share a geographic range and cannot interbreed.^[1] Others use the term for a group of related species with separate geographic ranges that might interbreed if opportunity arises.^[2] Others use the term for a group that has not yet been sufficiently studied, which might or might not have a monophyletic origin.^[3]

Use

The use of the term reduces the need to use a higher taxonomic category in cases with taxa that exhibit sufficient differentiation to be recognized as separate species but possess inadequate variation to be recognized as subgenera. Defining species groups is a convenient way of subdividing well-defined genera with a large number of recognized species. The use of species groups have enabled systematists to consolidate polytypic species species into nominal species which in turn can be grouped into the larger array of the species group.^[3]

Range

In regards to whether or not members of a species group share a range, sources differ. A source from Iowa State University Department of Agronomy says that members of a species group usually have partially overlapping ranges but do not interbreed with each other.^[1] A Dictionary of Zoology (Oxford University Press 1999) describes a species group as complex of related species that exist allopatrically and explains that this "grouping can often be supported by experimental crosses in which only certain pairs of species will produce hybrids."^[2] The examples given below may support both uses of the term "species group."

Arthropod examples

• The fruit fly subgenus Sophophora contains the Drosophila melanogaster species group which itself contains 12 subgroups. The Drosophila obscura species group belongs to the same subgenus and contains 6 subgroups.^[4]
• In Vespula, a genus of wasps, only a few species have a scavenging habit (as opposed to a strictly predatory habit) and thus are considered major pests. The most abundant and bothersome of these are the three species belonging to the "Vespula vulgaris species group" which includes the "common wasp" or "yellowjacket" (Vespula vulgaris), the "German wasp" or "European wasp" (Vespula germanica), and the "western wasp" or "western yellowjacket" (Vespula pensylvanica).^[5]
• The Central American bark scorpions Centruroides limbatus and Centruroides bicolor belong to the "Gracilis species group". All of the species in this group are characterized by their long, narrow pedipalps and overall relatively large size.^[6]
• The arachnids to which the common name "Black widow spider" is given are in a species group that includes the "southern black widow" (Latrodectus mactans), the "northern black widow" (Latrodectus variolus), and the "western black widow" (Latrodectus hesperus').^[7][8]
• The Neotropical butterfly Morpho adonis is in a species group with Morpho eugenia and Morpho uraneis. Morpho marcus is also included in the group but might actually be the same species as Morpho adonis^[9]
• Members of the butterfly genus Charaxes are arranged in "species groups" (these are not superspecies, or clades but an informal phenetic arrangement). The groupings of morphologically similar species named as species groups in the literature may or may not be consistent with clades identified by molecular phylogenetic studies.

Vertebrate examples

• Brachygobius, a small genus of gobies which are popular as aquarium fish, are informally divided by taxonomists into two species groups. The dwarf "Brachygobius nunus species group" contains Brachygobius nunus, Brachygobius aggregatus, and Brachygobius mekongensis while the bigger "Brachygobius doriae species group" contains the bigger species of Brachygobius doriae, Brachygobius sabanus, and Brachygobius xanthomelas.^[10]
• The chameleon Brookesia minima has been characterized as belonging to a species group with other "Madagascan Dwarf Chameleons" such as Brookesia dentata, Brookesia tuberculata, and other new or unidentified species such as a recently described chameleon from Tsingy de Bemaraha Strict Nature Reserve.^[11]
• Peromyscus, a genus of deer mice, has been divided into subgenera Peromyscus and Haplomylomys and these subgenera are subdivided further into thirteen species groups.^[3]
• Recent cytogenetic studies have shown that the Middle East Blind Mole Rat (Spalax ehrenbergi) may actually be a species group containing several cryptic species that can be distinguished by chromosome numbers.^[12]

Other uses

The term "species group" is also used in a different way so as to describe the manner in which individual organisms group together. In this non-taxonomic context one can refer to "same-species groups" and "mixed-species groups." While same-species groups are the norm, examples of mixed-species groups abound. For example, zebra (Equus burchelli) and wildebeest (Connochaetes taurinus) can remain in association during periods of long distance migration across the Serengeti as a strategy for thwarting predators. Cercopithecus mitis and Cercopithecus ascanius, species of monkey in the Kakamega Forest of Kenya, can stay in close proximity and travel along exactly the same routes through the forest for periods of up to 12 hours. These mixed-species groups are cannot be explained by the coincidence of sharing the same habitat. Rather, they are created by the active behavioural choice of at least one of the species in question.^[13]

See also

• Cryptic species
• Parapatric speciation
• Ring species
• Speciation

References