Cohesion species concept

The cohesion species concept defines species as '... the most inclusive group of organisms having the potential for genetic and/or demographic exchangeability' (Templeton 1989). Templeton (1989) argued that one of the main weaknesses of the biological species concept was its lack of applicability to either asexual organisms or to taxa belonging to syngameons. In Templeton's (1989) terminology the difficulty resulted from either 'too little' or 'too much sex'. Templeton (1989) proposed the cohesion species concept to address problems with the biological species concept as well as other species concepts. The cohesion species concept was thus designed to accommodate all of the microevo-lutionary processes thought to contribute to specia-tion (Templeton 1989). 'Genetic exchangeability' incorporated the process of gene flow and 'demographic or ecological exchangeability' the processes of genetic drift and natural selection (Templeton 1989). Specifically, '. .. genetic exchangeability ... is the ability to exchange genes during sexual reproduction' and '... demographic exchangeability occurs when all individuals in a population display exactly the same ranges and abilities of tolerance to all relevant ecological variables' (Templeton 1989). For the current discussion it is important to ask how the cohesion species concept can accommodate biological instances in which genetic exchange between divergent lineages is detected (i.e. when there is 'too much sex').

Unlike the biological, phylogenetic, and other species concepts, cohesion species are defined on the basis of independent evolutionary/ecological trajectories (Templeton, 1989, 2001). Among other implications from this concept, cohesion species may arise as products of a reticulation event between lineages defined as species (Templeton 1981, 2001). As importantly, cohesion species may participate in some degree of genetic exchange with other lineages and yet maintain their species status (Templeton 1981, 1989, 2001, 2004b). For example, syngameons (Figure 2.2) are a result of species having greater genetic than ecological exchangeability. In contrast to the biological or phylogenetic species concepts, the cohesion species concept does not require that these taxa be reduced to subspecific

Figure 2.2 An illustration of a syngameon involving the Pacific Coast irises, series Californicae. Each small circle represents a separate species. The species designations are as follows: BRA, Iris bracteata; MUN, Iris munzii; TEN, Iris tenax; PUR, Irispurdyi; INN, Iris innominata; TMA, Iris tenuissima; FER, Iris fernaldii; MAC, Iris macrosiphon; CHR, Iris chrysophylla; DOU, Iris douglasiana; HAR, Iris hartwegii. A line connecting two circles indicates natural hybridization between those taxa (from Lenz 1959).

Figure 2.2 An illustration of a syngameon involving the Pacific Coast irises, series Californicae. Each small circle represents a separate species. The species designations are as follows: BRA, Iris bracteata; MUN, Iris munzii; TEN, Iris tenax; PUR, Irispurdyi; INN, Iris innominata; TMA, Iris tenuissima; FER, Iris fernaldii; MAC, Iris macrosiphon; CHR, Iris chrysophylla; DOU, Iris douglasiana; HAR, Iris hartwegii. A line connecting two circles indicates natural hybridization between those taxa (from Lenz 1959).

categories. Rather than viewing hybridization as a problem to be overcome for the process of divergent evolution to proceed, the application of the cohesion species concept indicates that it is more constructive and instructive to incorporate reticulate evolution into an evaluation of evolutionary process and pattern (Templeton 2004b).

Another strength of the cohesion species concept is that it allows the construction of hypotheses that are testable within the widely applied phylogeo-graphic paradigm of Avise (2000b). By constructing such null hypotheses, it is possible to incorporate geography and distribution of population genetic variation into a genealogical analysis (Figure 2.3 Templeton 2001, 2004b). The rejection of the null hypotheses, (i) that geographically associated samples are from a single evolutionary lineage and (ii) that the lineages discovered by (i) are not exchangeable in terms of genetic or ecological characteristics, results in the inference that more than

Figure 2.3 Phylogenetic and reticulate evolutionary patterns overlain on the geographic distribution of the brown trout species complex. The evolutionary patterns were inferred using DNA sequence data from the transferrin (TF) locus. Ovals indicate the approximate geographic locations of clades (as defined by the TF haplotypes, such as TF 100). Solid arrows indicate phylogenetic transitions between clades; dashed arrows indicate recombination events (from Templeton 2004b).

Figure 2.3 Phylogenetic and reticulate evolutionary patterns overlain on the geographic distribution of the brown trout species complex. The evolutionary patterns were inferred using DNA sequence data from the transferrin (TF) locus. Ovals indicate the approximate geographic locations of clades (as defined by the TF haplotypes, such as TF 100). Solid arrows indicate phylogenetic transitions between clades; dashed arrows indicate recombination events (from Templeton 2004b).

one evolutionary lineage (or species) is present in the sample (Templeton 2001). It could be argued that the phylogenetic species concept, but not the biological species concept (without the occurrence of sympatric populations), also allows such a rigorous identification of evolutionary lineages with separate trajectories. However, it is problematic to decide the weight to be given to the characters that identify a 'phylogenetic species' (Avise 2000a). In contrast, the cohesion species concept allows a statistical test of whether evolutionarily distinct lineages exist, and also whether reticulation, past or present, has impacted said lineages (Templeton 2001, 2004b). As such, the application of this paradigm reflects an encompassing, rather than exclusionary, approach for the deciphering of evolutionary pattern and process. In the same way, the prokaryotic species concept can be seen to embody the web-of-life metaphor, rather than a tree-of-life metaphor based on the biological or phylogenetic species concepts.

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  • peyton
    What is the cohesion species concept?
    10 months ago
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    What iscohesion species concept?
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