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[edit] Subspecies

Further information: species
Further information: subspecies
  • "Subspecies are groups of actually or potentially interbreeding populations phylogenetically distinguishable from, but reproductively compatible with, other such groups. Importantly the evidence for phylogenetic distinction must normally come from the concordant distributions of multiple, independent, genetically based traits." Avis and Bell.
  • "A phenotypically and/or geographically distinctive subspecific group, composed of individuals inhabiting a defined geographical and/or ecological region, and possessing characteristic phenotypic and gene frequencies that distinguish it from other such groups. The number of racial groups that one wishes to recognize within a species is usually arbitrary but suitable for the purposes under investigation."King and Stansfield 1990 (Race)
  • "1. A taxonomically recognized subdivision of a species. 2. Geographically and/or ecologically defined subdivisions of a species with distinctive characteristics." King and Stansfield (subspecies)
  • "Races, then, can be defined and picked out in a number of ways. Several ways of picking out races will likely overlap because of the nature of biological organisms; for example, if a population is ecologically distinct (e.g., it lives at high elevations) it is also likely to be geographically isolated (by virtue of occurring in a location at high elevation) and to be somewhat genetically differentiated. But while genetic and phenotypic differences between local populations will often be associated with phylogenetic distinctiveness, such differences do not imply phylogenetic distinctiveness, nor, a fortiori, do they imply incipient speciation. For a lineage to acquire phylogenetic distinctiveness, gene flow with other closely related populations must essentially cease. If gene flow is still significant, the lineage is evolving according to a reticulate, not cladistic (branching) pattern. While it is still possible for such an entity to maintain ecological distinctiveness (see below), its historical roots are continuously reshuffled by migration events. Thus, while eco-geographical-genetic differentiation tend to correlate with each other they do not imply cladogenesis and speciation, though the latter two are themselves associated." Massimo Pigliucci1,2 and Jonathan Kaplan2,3
  • "A great division of mankind, characterized as a group by the sharing of a certain combination of features, which have been derived from their common descent, and constitute a vague physical background, usually more or less obscured by individual variations, and realized best in a composite picture." Hooton
  • "An aggregate of phenotypically similar populations of a species, inhabiting a geographic subdivision of the range of a species, and differing taxonomically from other populations of the species." Mayr
  • "Races are genetically distinct Mendelian populations. They are neither individuals nor particular genotypes, they consist of individuals who differ genetically among themselves." Dobzhansky Templeton
  • "A subspecies (race) is a distinct evolutionary lineage within a species. This definition requires that a subspecies be genetically differentiated due to barriers to genetic exchange that have persisted for long periods of time; that is, the subspecies must have historical continuity in addition to current genetic differentiation." Templeton
  • "Members of a subspecies share a unique geographic range or habitat, a group of phylogenetically concordant phenotypic characters, and a unique natural history relative to other subdivisions of the species. Because they are below the species level, different subspecies are reproductively compatible. They will normally be allopatric and they will exhibit recognizable phylogenetic partitioning, because of the time-dependent accumulation of genetic difference in the absence of gene flow. Most subspecies will be monophyletic, however they may also derive from ancestral subspecies hybridization." O'Brien and Meyr. (1991)


With the advent of the modern synthesis in the early 20th century, many biologists sought to use evolutionary models and populations genetics in an attempt to formalise taxonomy below the species level. The term subspecies is used by biologists when a group of organisms are classified in such a way. Species are usually described as genetically discrete, but subspecies are not seen as discrete groups, gene flow does not normally occur between species, whereas it can occur between subspecies.[1] In biology the term "race" is very rarely used because it is ambiguous, "'Race' is not being defined or used consistently; its referents are varied and shift depending on context. The term is often used colloquially to refer to a range of human groupings. Religious, cultural, social, national, ethnic, linguistic, genetic, geographical and anatomical groups have been and sometimes still are called 'races'".[2] Generally when it is used it is synonymous with subspecies.[3][2][4] One of the main obstacles to identifying subspecies is that, while it is a recognised taxonomic term, it has no precise definition.[3] "The traditional meaning of a subspecies is that of a geographically circumscribed, genetically differentiated population."[3] Or to put it another way "the designation 'subspecies' is used to indicate an objective degree of microevolutionary divergence"[2] One objection to this idea is that it does not identify any degree of differentiation, therefore any population that is somewhat biologically different could be considered a subspecies, even to the level of a local population.[3] As a result it has become necessary to impose a threshold on the level of difference that is required for a population to be designated a subspecies.[3] This effectively means that populations of organisms must reach a certain measurable level of difference in order to be recognised as subspecies. It has been suggested that if two populations differ from each other by 70% or more, but by less than totality, then they should be designated as subspecies.[1] Sometimes only a single characteristic is used for comparison of populations, but usually it is a set of characters.[1] A species may occupy a continuous rage with the extremes of this range displaying a great enough degree of differentiation to be considered subspecies, while the intermediate populations, those displaying less than 70% differentiation are not designated as subspecies (see diagram). One problem with using observed morphological differences to classify subspecies is that these very morphological differences are completely irrelevant when it comes to classifying different species, and may have no evolutionary importance. Indeed in determining whether two populations represent discrete species, only features that inhibit reproduction between two groups are considered (that is species are assigned based only on a lack of reproduction between the two populations): "These reproductive traits have priority in defining a species when in conflict with other traits, such as morphology (Mayr 1970). Unfortunately, there is no such guidance at the subspecies level, although in practice easily observed morphological traits (the very ones deemed not important under the biological species concept) are used. There is no evolutionary justification for this dominance of easily observed morphological traits; indeed, it merely arises from the sensory constraints of our own species".[3] Due to this consideration many zoologists have rejected the concept of subspecies altogether.[2][3] Objections to the concept of subspecies have been summed up thus:

A parapatric subspecies. Populations A and C display a greater than 70-75% divergence, and are therefore considered subspecies. The intermediate population, B, is not designated a subspecies.
A parapatric subspecies. Populations A and C display a greater than 70-75% divergence, and are therefore considered subspecies. The intermediate population, B, is not designated a subspecies.[1]
  • Visible physical differences do not correlate with one another, leading to the possibility of different classifications for the same individual organisms.
  • Parallel evolution can lead to the existence of the appearance of similarities between groups of organisms that are not part of the same species.
  • The existence of isolated populations within previously designated subspecies.
  • That the criteria for classification are arbitrary.[2]

Several attempts have been made to define subspecies on a more biologically objective basis.

Biological definitions of race.[4]
Concept Reference Definition
Essentialist Hooton (1926) "A great division of mankind, characterized as a group by the sharing of a certain combination of features, which have been derived from their common descent, and constitute a vague physical background, usually more or less obscured by individual variations, and realized best in a composite picture."
Taxonomic Mayr (1969) "An aggregate of phenotypically similar populations of a species, inhabiting a geographic subdivision of the range of a species, and differing taxonomically from other populations of the species."
Population Dobzhansky (1970) "Races are genetically distinct Mendelian populations. They are neither individuals nor particular genotypes, they consist of individuals who differ genetically among themselves."
Lineage Templeton (1998) "A subspecies (race) is a distinct evolutionary lineage within a species. This definition requires that a subspecies be genetically differentiated due to barriers to genetic exchange that have persisted for long periods of time; that is, the subspecies must have historical continuity in addition to current genetic differentiation."

It is apparent that the Essentialist and Taxonomic definitions of Hooton and Mayr respectively, represent similar concepts to the traditional concept, that is that certain traits can be used to identify specific morphological types that vary by geography, although neither definition specifically requires a 70% differentiation between groups. Likewise the Dobzhansky's Population definition gives no guidance as to how much differentiation populations need to display in order to be considered distinct subspecies. Templeton's Lineage definition requires that subspecies display not only genetic differentiation (as opposed to morphological differentiation), but also that this genetic differentiation has persisted for long periods. This takes into account the possibility that differentiation may be transient: "The additional requirement of historical continuity is particularly important because many traits should reflect the common evolutionary history of the subspecies, and therefore in theory there is no need to prioritize the informative traits in defining subspecies. Indeed, the best traits for identifying subspecies are now simply those with the best phylogenetic resolution. In this regard, advances in molecular genetics have greatly augmented our ability to resolve genetic variation and provide the best current resolution of recent evolutionary histories (Avise 1994), thereby allowing the identification of evolutionary lineages in an objective, explicit fashion."[3] Templeton argues that "A standard criterion for a subspecies or race in the nonhuman literature under the traditional definition of a subspecies as a geographically circumscribed, sharply differentiated population is to have FST values of at least 0.25 to 0.30" This means that about 25-30% of genetic variation in a species must be observed between populations for the populations to be considered subspecies. He also argues that the FST observed between human populations amounts to only about 15%. This observation was first made by Richard Lewontin and is a reproducible statistic. There have been many criticisms of the use of FST as a way of measuring differentiation between populations as a way of defining lineages. Long and Kittles themselves point out that "High within-group variance is also consistent with the population lineage concept, because mutation will introduce novel variation within a divergent lineage over a long time. Therefore, gene diversity within old lineages is expected to be near its mutation/drift equilibrium. The relative proportion of variation within and among groups therefore appears to be meaningless as a criterion for judging the validity of races or subspecies as defined by biologists."[4] Long and Kittles conclude "However, none of the race concepts is compatible with the patterns of variation revealed by our analyses."[4]

The global human population displays many of the characteristics of a parapatric species, as described in the diagram. The question remains regarding the level of differentiation that various human populations exhibit. It has been observed that the human population displays a marked lack of diversity relative to other large land mammals and that human genetic variation displays an unusual pattern:

  • "First, compared with many other mammalian species, humans are genetically less diverse... For example, the chimpanzee subspecies living just in central and western Africa have higher levels of diversity than do humans (Ebersberger et al. 2002; Yu et al. 2003; Fischer et al. 2004)." Human variation is also distributed in an unusual and not easily understood fashion compared to other mammalian species: "The details of this distribution are impossible to describe succinctly because of the difficulty of defining a 'population,' the clinal nature of variation, and heterogeneity across the genome (Long and Kittles 2003).... This distribution of genetic variation differs from the pattern seen in many other mammalian species, for which existing data suggest greater differentiation between groups (Templeton 1998; Kittles and Weiss 2003)."[5]
  • "Since the 1980s, there have been indications that the genetic diversity of humans is low compared with that of many other species. This has been interpreted to mean that humans are a relatively young species, so populations have had relatively little time to differentiate from one another. For example, 2 randomly chosen humans differ at ~1 in 1,000 nucleotide pairs, whereas two chimpanzees differ at ~1 in 500 nucleotide pairs.[6]
  • "'Race' is a legitimate taxonomic concept that works for chimpanzees but does not apply to humans (at this time). The nonexistence of 'races' or subspecies in modern humans does not preclude substantial genetic variation that may be localized to regions or populations....The DNA of an unknown individual from one of the sampled populations would probably be correctly linked to a population. Because this identification is possible does not mean that there is a level of differentiation equal to 'races'. The genetics of Homo sapiens shows gradients of differentiation."[7]
  • "Humans are ~98.8% similar to chimpanzees at the nucleotide level and are considerably more similar to each other, differing on average at only 1 of every 500−1,000 nucleotides between chromosomes. This degree of diversity is less than what typically exists among chimpanzees.[8]
  • "The average proportion of nucleotide differences between a randomly chosen pair of humans (i.e., average nucleotide diversity, or π) is consistently estimated to lie between 1 in 1,000 and 1 in 1,500. This proportion is low compared with those of many other species, from fruit flies to chimpanzees...."[9]