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Modern concepts

The modern concepts of species make use of other criteria than simple morphological comparison, with some of them even completely free of any character examination.

They may be subdivided according to whether or not they take into account the temporal dimension of the species (see row TD, Table 6.5). Only the biological concept of species (see column BIOL, Table 6.5) (and related ones cohesion, recognition, etc.) does not consider the birth and the death of the species.

The modern concepts of species could also be subdivided according to the reproductive strategy of the organisms: biological and Hennigian concepts (Table 6.5) only consider sexually reproducing organisms, the other concepts apply to the entire range of living organisms.

The Biological Concept of Species (“biospecies,” see column BIOL of Table 6.5). This is probably the most often used concept in entomology. Briefly, it considers as species natural interbreeding populations reproductively isolated from other such natural populations.3,96 It has ancient roots in the texts from a contemporary of Carl von Linnaeus: the earl of Buffon (1707-88), maybe the first naturalist to consider the criterion of interfertility in the definition of species. In this concept, temporal dimension is not necessary. Indeed, the reproductive isolation criterion would be poorly contributive when considering fossils or extinct species. Other related or almost similar concepts do insist on prezygotic barriers,97 or on gene flow within species, rather than reproductive isolation.98,99

The biological concept is not a character-related concept: it relies only on reproductive isolation. In addition, like the evolutionary concept but contrary to the phylogenetic concepts (see below), the biological definition of species does not contain the operation to be performed to verify its criterion. Reproductive isolation can be demonstrated in various ways, which makes the biological concept a refutable one. Thus, in spite of ignoring the time dimension of a species, i.e., ignoring evolutionary biology and phylogenetics, the concept is not an arbitrary one.

It is however difficult to reject the propensity to interbreed when natural populations are allopatric ones.100 It might also be confusing to observe partial reproductive isolation more so between separated populations (insular regions). Such situations, not infrequent in Triatominae, may be interpreted as incipient speciation, and invite reconsideration of the use of the subspecies category.36

According to the biological concept of species, many well recognized species of Triatominae would become one single species. As an example, T. platensis and T. infestans are interfertile, in the laboratory as well as in nature, although the latter is not frequently detected because of ecological separation. According to the biological concept, separation, even ecological separation, does not mean reproductive isolation. Triatoma platensis and T. infestans are two morphospecies but one single biospecies. The same conclusion could be derived from the R. prolixus— R. robustus question: these morphologically close species are interfertile, but ecologically and to some extent geographically separated. Again, separation does not mean reproductive isolation in the sense of the biological concept: R. prolixus— R. robustus are one single biospecies. Similar reasoning could be applied to the members of the phyllosoma complex, some members of the brasiliensis complex (see Section 4.1.6), the dimidiata complex (see Section 4.1.8), etc.

The Hennigian Concept of Species (“Hennigian species,” see column HENN of Table 6.5). The Hennigian Concept74 also considers that species are reproductively isolated, but it gives them a beginning and an ending in time. The inclusion of the temporal dimension in the definition of species appears as an obvious reality, but this is exactly where discrepancies begin with other related concepts. The HENN concept considers that speciation is a splitting event (cladogenesis), which means the birth of two reproductively isolated sister species and the dissolution of the stem species.4 Not only the survival of the stem species is not admitted, but, coherently, the idea of phyletic speciation (anagenesis) is rejected. The speciation event is the dissolution of the stem species into a reproductively isolated pair of sister species. Table 6.5 indicates which definitions agree or disagree with this point of view (see row ST, Table 6.5).

Reproductive isolation (from the sister species) is a very important component of the HENN concept, because it avoids the idea of groups linked by netlike relationships. This restriction allows clean definitions of monophyletic groups.

The HENN concept has been applied to draw hypothesized relationships within the genus Panstrongylus. P. lignarius and P. herreri could not be resolved,7 suggesting one single species as supported later by their close genetic101 and morphometric similarity (Gumiel et al., unpublished data).

However, when applied to the Rhodniini (excluding the robustus species since it is interfertile with prolixus) the cladistic tree performed on the basis of isoenzyme characters could not reveal the paraphyly between Psammolestes and Rhodnius genera,102 as revealed later by molecular phylogenetic approaches using more sophisticated algorithms.53—55

The Phylogenetic Concepts of Species (columns PHG I and PHG II of Table 6.5). Two different positions are competing for the title of “phylogenetic” concept of species.103 One is defining species prior to cladistic analysis on the basis of an exclusive combination of character states,6 the other is defending the idea of using synapomorphies alone in order to define monophyletic groups of demes, hence species.5

1. The phylogenetic concept sensu6 The phylogenetic concept of species sensu6 (PHG I in Table 6.5) defines hypothetic species prior to any cladistic analysis, as being “the smallest aggregation of (sexual) populations or (asexual) lineages diagnosable by a unique combination of character states.”6 The “unique combination of character states” is completely independent from the circumstances of the speciation event: either a splitting into sister species with dissolution of the stem species, a vicariance event, an allopatric or a sympatric speciation (hence with survival of the stem species). Neither synapomorphy nor mono- phyly, both of critical importance to cladistics, nor reproductive isolation, are prerequisites to define a phylogenetic species. A new species typically contains a fixed state of a previously polymorphic character in the ancestor species. The concept does not reject the idea of phyletic speciation (anagenesis), it thus accepts the idea of stem species survival, and relies on morphological characters. The authors of this concept recognize that it could dramatically increase the number of species in certain groups, especially in groups where many subspecies were described, which has been the case for Triatominae.18

This approach does not seem to have been used in the systematics of Triatominae, probably because of the paucity of discrete morphological characters.

2. The phylogenetic concept sensu5 This concept of species (PHG II in Table 6.5) provides a species definition quite distinct from the previous one, but it might be considered as very close to the HENN one. It rejects the idea of stem species survival and does not accept the hypothesis of phyletic speciation. Its first criterion for a species is its monophy- letic nature. The important difference with the HENN concept is about the entry groups. According to the HENN concept, the entry groups must be reproductively isolated. This is not required by the PHG II concept: terminal entities may be species, demes, or local populations. This concept makes species dependent on cladistic analysis: species are the result of phylogenetic analysis of infraspecific units, they are defined by their monophyly. The resulting criticism is that it may be difficult to correctly retrieve monophyletic groups when terminal entities are interbreeding units.

Furthermore, even the authors of the concept admit that to define a monophyletic group is not enough to raise it as a species: “any application of fixed names to phylogenetic trees has to be arbitrary to some extent.”5 The definition includes additional steps to rank the monophyletic group as a species (bootstrap percentage, number of synapomorphies, decay index, biological criteria, etc.). Moreover, even if they are well supported, the following monophyletic groups should not be named: groups defined only by “selectively neutral apomorphies,” and groups “marked only by molecular apomorphies and thus nearly impossible to distinguish for practical use.”5

In Triatominae the R. prolixus and R. robustus question offers a good example of this approach. R. prolixus and R. robustus, known as interfertile taxa, appeared as different monophyletic groups in molecular cladistic analyzes, and, in a second step of the same analysis, were claimed to be different species because of relatively large mtDNA sequence divergence between groups.55 Two different steps were (1) to recognize distinct monophyletic groups and (2) to consider them as distinct species. The first step might be considered as a rigorous application of phylogenetic rules, although even on that matter phylogeneticists may disagree because of reticulation.104 The second one is a subjective ranking appreciation (the amount of genetic distance) which is here comparable to the “distance approach,” or the “phenetic concept of species.”

Thus, the phylogenetic concept sensu5 is not free of subjective appreciation, and its main problem remains the reticulate structure of terminal entities. Interspecific or interdeme natural hybridism might be the main obstacle to the blind application of synapomorphies to define monophyletic groups. The problem is worth mentioning for Triatominae where interspecific hybridism is known to be frequent,105-107 and the so-called prolixus group is far from being an exception.108 If the relationships among the terminal taxa are reticulate rather than hierarchical, as they could be in the present concept, apparent synapomorphies might not correspond to monophyletic groups. Reticulate evolution produces confusing patterns of synapomorphy.109,110

The Evolutionary Concept of Species (“evolutionary species,” see column EVOL of Table 6.5). An evolutionary species is “an entity composed of organisms that maintain its identity from other such entities through time and over space and that has its own independent evolutionary fate and historical tendencies.75,76

As for the BIOL concept, and contrary to the other modern concepts, the EVOL concept does not specify a particular operation to verify its properties. Its strength, which is unique among the various concepts presented here, is that its acceptance does not mean rejection of the other (modern) concepts. Biological and Hennigian species are “evolutionary” species, as are phylogenetic species (to the extent that they retrieve correct ancestry patterns).

Its weakness is some arbitrariness in its definition: “historical tendencies,” “evolutionary fate,” are not easy to define or reject. However this objection is also the one making the concept potentially attractive. The definition of the evolutionary species does not dictate any rigorous system nor any kind of rigid algorithm to identify independent units. Their identification require discussion and consensus. But it is true that the phylogenetic approach does not have a preponderant place in the discussion, which is based more frequently on ecology, population genetics, geography and morphology. For instance, the Rhodnius genus has been discussed in terms of an assemblage of evolutionary units derived from each other, where R. pictipes is considered as the survival of the stem species of the genus, and where geography, vicariance, and ecological adaptation are the main forces explaining the groups observed within the genus by other approaches.50 In this study, different criteria were considered and contrasted to evaluate the fate and destiny of each recognized member of the genus.

The example of the T. infestans and T. platensis pair is also a good model of evolutionary units where genetic and ecological criteria were applied together to reach a consensus. In spite of being naturally interbreeding and genetically similar units, their obvious ecological adaptation makes them two evolutionary species. This observation is true for the third species of the infestans subcomplex, T. delpontei, having partial reproductive compatibility with T. platensis but apparently adapted to other birds.

 
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