Problem and Approach. Wüster et al. used morphological data to test whether the conventional subspecies constitued coherent evolutionary groups and whether the phylogenetic patterns in the morphological data were supported by variation in venom characteristics.
Characterization of Species Complex. Inconsistency between the existing taxonomy and the apparent evolutionary units was impeding the understanding of the evolution of the group. The inability to consistently distinguish between the subspecies showed that the existing treatment was not adequate, (this reader recognizes that there may be evolutionary lineages which are not distinguishable by morphological characters). The significance of this work is that the authors discuss the sources of taxonomic problems, including the equal potential for differentiation between similar populations by phylogenesis (separate ancestry) or by ecogenesis (differentiation by selection in sister groups).
Effect on taxonomic system. The authors found evidence to support the recognition of additional taxa but abstained from making changes to the nomenclature which would have effects on biomedical literature dealing with venom. The authors proposed continued recognition as two subspecies.
Authors writing on species concepts and implementation of species complex. The authors discuss why the BSC is inapplicable and show how application of the Phylogenetic Species Concept (citing Cracraft, 1989) would split the group into two species. The authors state' " In our opinion, an understanding of the pattern of geographic variation in this group is more important than a decision on taxonomic rank, which is dictated by a subjective decision of the species concept to be used."
Problem and Approach. Giannattasio and Spooner studied the nature, extent, and geographical partitioning of morphological differences between populations with a high degree of phenetic resembalance but believed to be different species. The existing taxonomy was used as a testable hypothesis. In this phase of their study, the authors used morphological data from common-garden plants. The analysis was shown to be sensitive to the quality of data, in which populations could be more effectively characterized by ordination when the characters which did not significantly differ between population were excluded from analysis and when the individuals were characterized by measurement of multiple serial organs at the same stage of growth. The potential for differences to be not expressed in common-garden plots relative to differences that would be expressed in natural populations was discussed.
Characterization of Species Complex. The problems they encountered in this analysis were a high degree of intraspecific variability and weakly differentiated taxa. The degree of variation between the species was not defined by gaps in any character set. The resulting pattern is of two weakly differentiated taxa with contiguous to slightly overlapping distribution.
Effect on taxonomic system. Because of the weakness of differentiation and the taxa having slighly overlapping geographical ranges, the pattern seemed to be better reflected by subspecies designations. The authors used this analysis to justify lowering the ranks of these taxa to subspecies.
Authors writing on species concepts and implementation of species complex. The authors state they are using a "Morphological Species Concept" (Phylogenetic) because using a Biological Species Concept would lump many taxa which have distinct morphological (and therefor potentially agriculturally important) characters.
Problem and Approach. Glannattsio and Spooner used RFLP data to look at phylogenetic relationships between these species, but failed to find taxon-specific markers and a high degree of variation with each taxon. RFLPs, like other work with CpDNA and nDNA, failed to show support for a hypothesis of hybridization.
Characterization of Species Complex. The cumulative results of the morphological and molecular study showed: 1) much intraspecific variation within taxa; 2) no species specific markers; 3) distinguishability by multivariate analysis; and 4) the UPGMA branch lengths distinguishing taxa are small relative to the lengths of the ultimate branches. It seems without taxon-specific markers for codominantly-inherited evidence, that conclusions about hybridization would be difficult to make.
Effect on taxonomic system. (see previous citation)
Authors writing on species concepts and implementation of species complex. (see previous citation)
This study shows clearly that while one level of genetic structure might be in HWE, another can show evidence of the Wahlund effect. The analysis must be sensitive to the scale of the question.
Problem and Approach. Soltis and Soltis used enzyme analysis to study the intra- and inter-population genetic structure of six populations of a relatively invariant species as a way to look at the contrast in patterns resulting from random distribution versus geographical effects. Geographical effects would result in small differences as a result of founder-effects, while random distribution would imply that the populations had not diversified as a result of ongoing gene flow.
Characterization of Species Complex. This concerned a genetically depauperate group of organisms in which the populations were not well differentiated. The author's suggest that this is either a result of geneflow between populations or could simply reflect the fact that divergence had simply not developed. There was a lack of significant deviation from Hardy-Weinberg expectations within subpopulations, but a lack of heterozygotes at the population level suggesting local inbreeding. The authors felt this was consistent with subpopulation outbreeding and short range spore dispersal.
Effect on taxonomic system. No changes to the extant taxonomy.
Authors writing on species concepts and implementation of species complex. This study did not concern taxonomic decisions.
Problem and Approach. Matos studied the morphological variation among three species of pine, one of which was indicated to be a hybrid of the other two by cpDNA analysis. This study used only specimens from one mountain range where the three species are sympatric, rather than including specimens from the entire range of these species.
Characterization of Species Complex. No gaps were found in the distribution of either multivariate or univariate characters. Matos determined that the characters which differentiated these taxa were also varying by an elevational gradient within population. Univariate comparisons between populations disclosed these trends.
Effect on taxonomic system. Matos concluded the data supported the recognition of only one species.
Authors writing on species concepts and implementation of species complex. Matos points out that without the presence of morphological or ecological differences that by the principle of Ockham's razor, only one variable species should be recognized. If there are two species, then the specific morphological character suites which are indicative of particular species should be found in suites.
Problem and Approach. The phylogenetic relationships (as levels of evolutionary divergence) between eight previously-described taxa were investigated using both electrophoretic enzyme data and hybridization.
Characterization of Species Complex. Wain used electrophoretic enzyme data to study the relative genetic divergence between subspecies in this complex. He found much less genetic divergence between species than was documented in the literature for plants outside the Asteraceae family. There was a lack of correlation between genetic distances and crossibility. The resultant pattern was determined to be consistent with the expectation for a species undergoing rapid speciation.
Effect on taxonomic system. No changes made in extant taxonomy
Authors writing on species concepts and implementation of species complex. The author cites Heiser's (1956) definition of subspecies as "populations or series of populations which replace each other geographically an at the same time show conspicuous morphological differences."
Problem and Approach. Desrochers and Bohm used morphological, isozyme, pollen and karyological data to study whether the described races warranted taxonomic recognition, whether there was consistent patterns in the different data sets, and to get information about the mechanisms creating the variation between populations.
Characterization of Species Complex. The lack of congruence between data sets suggested to the authors that speciation occurred not by cladogenesis, but rather by isolated populations becoming fixed independently for each type of character, or "lineage shattering". A high degree of variation between local populations without consistent character correlations is indicative of this model of speciation.
Effect on taxonomic system. The authors concluded groups should not be recognized.
Authors writing on species concepts and implementation of species complex. The species concept was not defined but the conclusion that no taxonomic units can be recognized despite considerable differentiation for pappus shape, flavonoid chemistry, and isozymes. Because discriminant function analysis of morphological data could not differentiate between groups defined by the non-morphological data, the authors concluded groups should not be recognized. This is clearly a morphological Phenetic Species Concept, without regaurd for differences in distribution or genetic identity.
Problem and Approach. Morphology and enzyme electrophoresis used to compare species limits based on each type of data.
Characterization of Species Complex. Ross and Trager used morphological and enzyme electrophoresis to compare species limits based on different types of data in ant species which are typically more variable within than between species. The degree of variability and the presence of cryptic species suggests that the ants are in a phase of active radiation and that speciation may be decoupled from morphological evolution. The authors point out that taxonomically difficult groups are usually evolutionarily young and may be particulary infomative about the processes which lead to speciation. There was general agreement between the data sets and the populations were found to be in HWE. Gene flow from the introduced Argentina ants was limited in native populations, though hybrid colonies were found. The expected effects of a genetic bottleneck as a result of introduction was not evident in the heterozygosity of the non-native species. The authors suggested that this could be through the ameliorating effects of rapid population growth. The authors futher suggest that the effects of founding effects might be very different in simple coding loci than in genetic systems involving epistatis or overdominance. The lack of drift and local adaptation suggests that these creatures may be highly vagile.
Effect on taxonomic system. Although crypic lineages are disclosed in this analysis, new taxa are not described. Mention is made of a recent taxonomic revision by one of the authors. This isozyme study supports that revision.
Authors writing on species concepts and implementation of species complex. Their species delimitations are based on discrete recognizable entities that maintain their genetic integrity through rangesof species overlap. The reproductive barriers allow some gene flow, but not enough to "degrade the internal cohesion of the species, or to prevent their further differentiation". The authors interpret this as representative of independent evolutionary lineages.
Problem and Approach. Haufler et al. used enzyme electrophoresis to determine the number of evolutionary units and their phylogenetic relationships within a group containing allopolyploid lineages which had been previously divided on the basis of cytotype variation. The authors used additivity to explore origins within the complex. Deviations from additivity can be the result of either multiple non-unique (reoccuring) hybridization or can be the result of reciprocal gene silencing in various lineages. Anomalies in the segregation ratios of progeny showed support for the hypothesis of gene silencing.
Characterization of Species Complex. The authors hypothesized an origin of species which were allopatric in the ice-ages then hybridizing across the range. The authors felt that their results were not totally consistent with the geographic distribution.
Effect on taxonomic system. Hypotheses concerning the parental taxa involved in the origins of allopolyploids were tested and modified. Two previously unrecognized but distinct lineages were shown to be present, important in the origins of some allopolyploid lineages, and geographically defined.
Authors writing on species concepts and implementation of species complex. The extant taxonomy of the European and American taxa were based on different lines of evidence. The European taxa had been divided on the basis of hybridization studies and resulted in a usable system of Biological Species. The American groups had been studied using morphology and were divided into one large eastern species with multiple ecotypes and 6 western species with uniform cytotypes. This work was aimed at identifying enzymatic lineages and studying how they had been combined through alloploid events. The discussion is of particular note in contrasting patterns resulting from cladogenic divergence with thoses stemming from reticulation. The authors show how additivity, especially fixed heterozygosity, provides clear evidence of ancestral-descendant relationships. Where heterozygosity is not fixed, confusion can result from secondary patterns of drift and fixation, introgression and persistence of parental taxa.
Problem and Approach. Haufler et al. attempted to resolve the phylogenetic relationships and origins of the allopolyploid fern complex by using cpDNA restriction site data.
Characterization of Species Complex. The greater variability of fern cpDNA, relative to seed plants, enabled the authors to look at the origins of the polyploids from the diploids. Intraspecific variation was rare for the diploids, but not in the polyploids, apparently because of having multiple (reciprocal) maternal origins.
Effect on taxonomic system. No effect on extant taxonomy, assumes species to be real entities.
Authors writing on species concepts and implementation of species complex. This study does not concern taxonomic decisions.
Problem and Approach. Peterson et al. used enzyme electrophoresis to look at the intra- and inter-specific variation in allozymes and morphology as a way to test the validity of infra-specific taxa which had been recognized on the basis of variable vegetative characters.
Characterization of Species Complex. The observed pattern was one of clear divisions between the species and very little structure within the species, suggesting that the reproductive isolation of different flowering phenology and habitat differences between the species kept them from interbreeding, but that the populations within each species either tended to be panmictic and have gene flow between them, or that they populations were too recently derived to have differentiated.
Effect on taxonomic system. The results suggested that the varieties were not clearly differentiated and showed that hybridization and introgression were uncommon between the recognized species. This work supported a subsequent revision which did not recognize the subspecific taxa as valid.
Authors writing on species concepts and implementation of species complex. The species concept used was not defined, but can be assumed to use gaps in the distribution of characters, evidence of reduced gene flow, and genetic differentiation as criteria for species. This might be classified as a Phylogenetic Species Concept.
Problem and Approach. Taylor et al. used morphological and chemotaxonomic (phenolics and terpenoids) characters to investigate the taxonomic boundaries of the Picea englemannii complex in which the ranges of all characters overlapped between taxa. The extreme variation noted in small populations was accepted by the authors as anomalous and they justified excluding them from analysis as deviant populations. It is of note that the ordinations were done with other taxa which would set the direction of the vectors in ways that might not maximize the differences between the taxa of interest.
Characterization of Species Complex. Specific reasons for the variation in this group were listed as: 1) interspecific introgression; 2) fragmented distribution and restricted gene flow promoting genetic drift and other non-selective genotypic fluctuation; 3) environmental selection causing ecotypes; and 4) phenotypic plasticity causing ecophenes. It was shown that all populations did not have equivalent genomes. These reasons might be summarized as results of restricted gene flow and breakdown of polymorphism into small fixed units and the environment driving true genetic differentiation and expression of environmentally controlled phenotypic expression.
Effect on taxonomic system. In a previous work the authors had submerged a species to varietal status because of lack of clear differentiation. This action was challenged by a report of inability to cross-breed the varieties. In this paper, the varietal status was supported by lack of a clear distinction between varieties.
Authors writing on species concepts and implementation of species complex. The morphological similarity is deemed by these authors to be more important than possible reproductive isolation, which is not mentioned beyond being the basis of the challenge.
Problem and Approach. Lutzonia and Brodo set out to delimit homogeneous groups within the complex, assess their phylogenetic relationships, and the evaluate photobiont identity as a diagnostic feature. Cluster analysis was used to disclose morphologically similar groups. A second cluster analysis, based on electrophoretic results, was compared to the morphological results using the methods of Rodrigo et al. (1993) and general congruence between the data sets was taken as an indication of general validity of the units. The combined data set was used to assess intergeneric relations.
Characterization of Species Complex. The previous delimitation relied on limited data and erroneous characters. The author stated that the heterogenietiy within the complex is due, in part, to the readily observable characters. Six homogenous units were identified from cluster analysis, although they did not appear as discrete entites in ordination. Apparently their differences are principally in their chemistry.
Effect on taxonomic system. Prior work with these lichens had used a genus concept bases solely on identity of the photobiont algae. This analysis showed that the photobiont algae, which had been the primary diagnostic feature, varied within morphologically defined species groups. The authors used this new information to totally revise the boundaries of two genera.
Authors writing on species concepts and implementation of species complex. This work seems to be based on a phylogenetic species concept where consistent differences are used to identify taxa. Appropriate species in lichens or other symbionic organisms may not be relevant to a concept of monophylly.
Problem and Approach. Spooner et. al. examined species boundaries by analysis of chromosomes in artificially produced F1 hybrids. Various statistical analysis of a morphological data set are explore the patterns of character state distribution and used to validate recognition of species and intrageneric relationships of those species.
Characterization of Species Complex. This complex contains taxa which are not resolvable by univariate character analysis and did not yeild discrete character sets useful for identification. However the groupings were evident in multivariate analyis. Three hypotheses for complex were discussed: 1) convergence due to upland habitat; 2) common ancestor; and 3) progenitor descendant relationship.
Effect on taxonomic system. These 6 hexaploid species had been previously defined with morphological characters. The authors stated that the results indicate that species boundaries should be broadened and that other types of data will be needed to unravel the patterns of character state variation
Authors writing on species concepts and implementation of species complex. While the species concept is not defined, apparently the author is testing the extant taxonomy against a Phylogenetic Species Concept, where the criteria involve distinct differences between species.
Problem and Approach. Schippers et al. used phenetic methods and morphological characters to determine what the recognizable infraspecific taxonomic units. The variability of each potential character was studied to select morphological characters with a maximum genetic information content. The resulting data was used in cluster analysis to identify morphological groups, which was interpreted as varieties.
Characterization of Species Complex. Four types of characters were identified: 1) differ significantly between clones, but stable to age and environmental effects; 2) significantly different between clones but also subject to age and environmental plasticity; 3) do not differ significantly between group understudy but are useful taxonomic characters outside the group; and 4) show only patterns of variation due to age and environment. Ordination and clustering resulted in clear distinctions between taxa, but individual character variation within each taxon was very high.
Effect on taxonomic system. Combined several taxa as synonomous with others. Provided clear support for the use of characters.
Authors writing on species concepts and implementation of species complex. The authors support the traditional recognition of taxa at the varietal level, rather than as forms. They seem to be using a Phylogenetic Species Concept in which populations would be accorded specific ranks if they showed consistent differences, but because they only differ by multivariate character distributions and geographical boundaries these taxa are recognized by varietal ranking.
Problem and Approach. Peterson et al investigate the phylogenetic relationships among the members of the complex using cladistic analysis of discrete morphological and electrophoretic characters.
Characterization of Species Complex. This species complex is held together by sharing a particular stamen length, but otherwise, the authors contend that the subunits are distinct entities.
Effect on taxonomic system. Former varieties are given specific rank on the basis of being uniquely identifiable and allopatric, with different habitat requirements.
Authors writing on species concepts and implementation of species complex. Citing differences observed and data from other studies, they move varieties to specific status, justifying it with cladistic analysis. Authors say it is justified by morphological, geographical, ecological, and reproductive isolation. No statement is made as to why complex exists.
Problem and Approach.Rogstad examines the occurrence of phenetically similar taxa which occur in sympatry to determine if they are distinct and if they are violating Gause's Principle of Competitive Exclusion
Characterization of Species Complex. Many of the characters previously used to delimit the species have overlapping distributions. Rogstad found that the species could not be defined with multivariate techniques using only continuous variables, but were revealed as distinct taxa by patterns in the non-continuous characters. Six distinct species were defined, showing that groups of discrete, closely related species do occur in sympatry in contrast to the Principle of Competitive Exclusion.
Effect on taxonomic system. From evidence of ordinations, Rogstad delimits 6 species within the complex.
Authors writing on species concepts and implementation of species complex. He cites the evolutionary species concept of Simpson, 1961 (p.153) but infact recognizes taxa on the basis of clear discontinuities in phenetic ordination. He uses a suite of 7 characters to show that the entire complex is monophyletic. No statment made as to why the complex exists.
Problem and Approach. Using a combination of hybridization, morphological, and pollination analysis, McDade searched for the best hypothesis of phylogenetic relationships between members of a species complex.
Characterization of Species Complex. The complex is a monophyletic assemblage of about 40 species, which McDade characterizes as large and heterogeneous. Few natural hybrids were found despite partial interfertility and geographic proximity. The application of the term "complex" to this group seems to imply a subgeneric aggregation of well-defined species, rather than a group with poorly defined subunits.
Effect on taxonomic system. McDade recognizes 3 additional taxa in addition to the previously described species and drops one taxon.
Authors writing on species concepts and implementation of species complex. The species concept is not explicitly stated by the reasons for delimiting new taxa are based on discontinuities in the data sets. The units with the complex are morphologically defined. No statement as to why the complex exists.
Problem and Approach. Yoon et al. examined the question of whether one species actually represented a developmental stage of a "related" species. They used electrophoretic data to show the two species were identical.
Characterization of Species Complex. Similar isozyme genetic identities together with a mosaic of geographical patterns were seen as evidence that two morphologically distinct entities were only age/stage forms of the same organization. The singular genetic identity was taken as evidence of greater weight than the morphological divergence.
Effect on taxonomic system. The genetic identity of fungal populations are used to show that separate phenotypic forms may be age/stage classes. Non-identity would support recognition of multiple taxa according to the authors, but non-identity in some but not all populations did not support subdivision.
Authors writing on species concepts and implementation of species complex. Authors appear to use a species concept based on genetic identity, ie. enzyme electrophoretic similarity. No statments are made as to why complex exists, but the geographical pattern was consistent with age of the population.
Problem and Approach. Wolfe and Elisens asked if patterns of hybridization and introgression evident in allozyme and rDNA data were similar to those of cpDNA. Multiple lines of evidence were compared and contrasted to assess validity of phylogenetic patterns in each data set. This study shows the weakness of studying single lines of evidence to determine lineages where gene flow may occur between taxa.
Characterization of Species Complex. Distributions of chloroplast cytotypes were incongruent with pattern apparent in rDNA, allozmes, and morphology. These data sets were not correlated supporting a hypothesis of multiple origins by local introgression. The results suggested that different taxa had been derived by different speciation processes, being in some cases sister taxa, hybrid derivitives, and the result of chloroplast capture.
Effect on taxonomic system. No changes were made to existing taxonomy.
Authors writing on species concepts and implementation of species complex. The species concept was not specified but apparently rests on some element of either the Phylogenetic or Quantitative Species Concepts not requiring monophylly. The authors discuss how variation in the genome will be affected by the type of speciation process taking place.
Problem and Approach. The data are examined by ordination both without and with apriori classification. The extant taxonomic system is tested by looking at the variance explained by CDF vectors. Because most of the variation is explained, the taxonomy is judged to be valid. This test does not actually address whether or not they can be clearly recognized by an observer as per their criteria for species.
Characterization of Species Complex. This group has a high degree of overlapping characters between populations/taxa, which lead to varying concepts of the included taxa. The author's classification is supported by evidence that the species are characterized not only by morphological differences but by discreet differences in habitat.
Effect on taxonomic system. The authors are validating a taxonomy which the senior author had developed in which species had been described, epithets restored and subspecific taxa raised to the level of species..
Authors writing on species concepts and implementation of species complex. The authors specify that tye are using a biological species concept but then define it as follows: "a set of geographical populations in which individuals possess many common characteristics that differentiate them (more or less) clearly from allied populations in the same genus that can be clearly recognized by the observer", citing Bremer & Wanntorp, 1979. The authors actually use a combination of phenetic and ecological separation as a criteria/justification for their species.
Problem and Approach. Paler and Barrington were interested in being able to recognize reproductively isolated lineages, as defined by the presence of marker alleles. Specimens were identified to group using electrophoretic patterns. The groups corresponded aproximately to the described species. DFA was used to study patterns in the morphological data that would allow the identification of these groups in the field.
Characterization of Species Complex. The lineages in this group lack easy to discern morphological distinction, but vary in electrophoretic patterns of PGI and TPI enzymes.
Effect on taxonomic system. Provides morphological characters to identify electrophoretically distinct lineages.
Authors writing on species concepts and implementation of species complex. The approach implies a Monophyletic (& Phylogenetic) Species Concept, with the intent of recognizing separate evolutionary lineages.
Problem and Approach. Wheeler used morphological and ecological characters and phenetic analysis to work toward a revision of a section of South American carex.
Characterization of Species Complex. This author interprets the patterns of data, ie. proximate allopatry and minor morphological differentiation between populations, as supporting a vicariace model as opposed to a model of adaptive radiation into particular habitats.
Effect on taxonomic system. This work resulted in a revision of the section including the recognition of new species and the combination of some older taxa.
Authors writing on species concepts and implementation of species complex. The species were geographical entities with phenetic distinction in multivariate space, a Phenetic Species Concept.
Problem and Approach. Anderson and Balick used morphological inspection without statistical analysis to resolve a longstanding taxonomic problem with agricultural implications.
Characterization of Species Complex. Characters within this complex have high levels of morphological intra-specific variability. The authors suggested that there were probably three sources of morphological variability contributing to the complexity observed: 1) the wide geographic range of each of the species; 2) the propensity to hybridize between species; and 3) intrinsic genetic variation. The geographical distribution of these species was suggested to be the result of dispersal by rivers, tied to the evolution of the riverine systems of the Amazon.
Effect on taxonomic system. This work was used to revise the complex, combining 9 older taxa into two species and some hybrid populations.
Authors writing on species concepts and implementation of species complex. The author's do not provide a discussion of their species concept, but appear to have focused on finding populations of less variability at the ends of the ranges of character variation, and identifying the degree of variation and intermediacy of the other populations. This might be interpreted as a Phylogenetic Species Concept, in wanting the species to have clear distinction and assigning all less distinctive populations to hybrid status.
Problem and Approach. Bayer studied the chromosome numbers and isozyme variation among populations to look at levels of genetic divergence and relationships with the presumed progenitors of the complex.
Characterization of Species Complex." Antennaria rosea is a compilospecies, having arisen though multiple hybridization and introgression events from among several sexual species." This taxon is an obligate gametophytic apomict (without male function), with more inter- and intra-populational gene diversity than it's sexual relatives. Morphological characters that are unique to each of the sexual progenitor species are found in various combinations in this taxon. Large amounts of morphological divergence have occurred with only moderate amounts of divergence at enzyme loci.
Effect on taxonomic system. No change to extant taxonomy. This paper justifies not recognizing the microspecies as species.
Authors writing on species concepts and implementation of species complex. Bayer recognizes that each populations may have its own phylogenetic history, but because of the problem of proliferation of taxa, all the microspecies should be recognized as a single variable species. This implies that while Bayer would like to use a Monophyletic Species Concept, practicality requires using a more Pluralistic Species Concept.
Problem and Approach. Cosner and Crawford explored the allozyme divergence within and between 9 morphologically well-defined taxa.
Characterization of Species Complex. This section of a genus contains taxa with clear morphological distinctions and varying degrees of reproductive isolation. They show diversity in chromosome number, morphology, habit, annual/perennial and geographic distribution. The taxa were found to have extremely high genetic identity (above .90) with no isozyme markers characterizing any of the taxa. The authors used this as evidence that the taxa were products of a recent divergence,supporting a previous hypothesis that this group is characterized by rapid morphological divergence between species and is one of the most evolutionary advanced groups in North America.
Effect on taxonomic system. No changes were made to the existing taxonomy.
Authors writing on species concepts and implementation of species complex. The authors do not address this issue.
Problem and Approach. Doyle and Brown used variation in the 5S nuclear ribosomal gene to study the progenitors of the polyploid cytotypes.
Characterization of Species Complex. This alloploid species is heterogeneous in chromosome number, morphology, isozymes and nucleotide sequences of the18S-25S rRNA gene, apparently a mixture of several diploid genomes. Several of the groups within the complex are reproductively or geographically isolated. Chromosome numbers vary from diploids, eutetraploids and tetraploid levels.
Effect on taxonomic system. No changes were made to the taxonomy as a result of this study.
Authors writing on species concepts and implementation of species complex. The authors do not address taxonomic concerns, however, retaining the complex as one species, despite it being polyphyletic indicates an underlying Pluralistic Species Concept.
Furlow, John J. The Carpinus carolinia complex in North America. II. Systematics. Systematic Botany 12(3): 416-434.
Problem and Approach. Furlow studied the morphological variation across the entire distribution of the species complex using phenetic methods.
Characterization of Species Complex. Taxonomists had created a number of controversial intraspecific taxa on the basis of correlations between morphology and geography, while other taxonomists considered it to be a single variable assemblage. The results suggested that the group was comprised of racial structures resulting from local adaptation and differentiation to diverse environments, but no sharp morphological discontinuties between geographic segments were found. The boundaries between the groups he recognized as species were sharper in having greater change over smaller geographic space, that the intergradations between species.
Effect on taxonomic system. Two new subspecies were erected and the boundaries between the species were clarified.
Authors writing on species concepts and implementation of species complex. Furlow contends that treating the entire complex as a single species with no infraspecific taxa is an unwarranted oversimplification that conceals important and obvious natural relationships. The differing degrees of differentiation (patterns of evolutionary divergence) were best reflected by recognizing species and subspecies, "while mainintaining a reasonable sense of utility". The subspecies definition was enumerated to be two widespread, intergrading, but coherent geographical races of each species."
Problem and Approach. Gallardo and Palma explored contrasting patterns of protein variation, morphology and karyology and hypothesized reasons for the decoupling of phylogenetic signals.
Characterization of Species Complex. These mammals occur in small isolated populations with great fluctuations in population sizes. While there was general concordance of karyological and morphological patterns, the allozymes did not fit the general pattern. Conflicts in patterns of separate enzymes and a deficiency of heterozygotes were indicative of random extinctions, establishment of new populations, non-random mating, and small effective population sizes.
Effect on taxonomic system. No changes were made to the extant taxonomy.
Authors writing on species concepts and implementation of species complex. The authors did not address these issues.
Problem and Approach. Goldblatt used morphology and karyology to find the subunits of the complex.
Characterization of Species Complex. The two subspecies are cytologically heterogenious with considerable variation between karyotypes. The 2n=20 karyotypes were more similar between species than to their own respective species. Goldblatt suggests that this complex has particular labile chromosome numbers and that chromosomal evolution has followed morphological diversion, breaking up morphological entities with aneuploid reduction.
Effect on taxonomic system. Author makes no changes to extant taxonomy.
Authors writing on species concepts and implementation of species complex. Not addressed in this paper.
Problem and Approach. Theiler and Blanco used breeding experiments between populations with varying karotypes in order to look for evidence of reproductive isolation between morphologically indistinguishable and enzymatically similar populations.
Characterization of Species Complex. These mammals occur in montane populations with chromosome numbers of 2N=36,37,38, and espinal populations with 2N=42. The chromosomal polymorphism was assumed to be the result of a Robertsonian fusion of acrocentric chromosomes of the 2N=42 karyotype. Other studies has shown the genetic similarity between these karyotypes to be very high. The results of this study suggest that reproductive isolation, hence speciation, can occur without genetic or morphological differentiation.
Effect on taxonomic system. No changes were made to the extant taxonomy.
Authors writing on species concepts and implementation of species complex. Not addressed.
Problem and Approach. Hedin studied the genetic structure of a species complex which consists of epigean and troglobitic spiders. Mitochondrial DNA of the ND1 gene was sequenced to look for correspondences with population structure as determined by habitat use and morphology. Twenty-five localities were sampled and 6-12 individuals from each locality were used, providing an exceptional measurement of intra-populational variability.
Characterization of Species Complex. Sequence data distribution patterns were not concordant with either extant taxonomy or geography. Near monophylly within populations and large genetic distances between populations was consistent with the authors expectation for speciation by population fragmentation through habitat vicariance.
Effect on taxonomic system. Hedin makes no changes to classification or taxonomy.
Authors writing on species concepts and implementation of species complex. Writing on the formation of ephemeral lineages, Hedin says, "...species fragmentation [...] might be viewed as a evanescent, yet recurring, phenomenon in 'species time'". Species, therefor, should be identifiec by stable morphological entities as finer structures (lineages) may be extremely temporal. He contrasts the predictions for vicariant speciation (ie. a distance effect within each vicariant area), predictions for founder event speciation (ie. high levels of homozygosity and restricted genetic backgrounds), and predictions for lineage shattering (ie. the "shot-gun effect" of stochastic distribution of characters).
Problem and Approach. Ford and Aquadro contrasted divergence patterns between different gene loci (x-linked and autosomal) within these taxa. They used RFLPs from 9 enzymes on 5 gene loci, then looked at the data using statistical tests of neutrality. They found a pattern of x-linked loci having more inter-taxon variation, and autosomal loci having more intra-taxon variation, which they attributed to a combination of a "selective sweep", in which strongly advantageous alleles at low frequency increase rapidly and replace all other alleles (reinforcement), and a hitchiking effect of genes linked by co-occurance on the same chromosome.
Characterization of Species Complex. These taxa are considered to be a complex of semi-species because they show some partial reproductive isolation by male courtship song, although they have low mtDNA divergence and high genetic similarity in allozymes and autosomal DNA.
Effect on taxonomic system. The authors make no changes to taxonomy.
Authors writing on species concepts and implementation of species complex. The author's do not address this, but the implication of this study for species complexes, is that different parts of the genome may under quite different evolutionary trajectories as a result of linkage. Even within a genome some parts may show less distinct boundaries between taxa than other parts of the genome.
Problem and Approach. Palopoli et al. examined the effects of artificial introgression of genes upon male fertility between three taxa for which 5 genetic data sets yeilded an unresolved tricotomy. The hybridization helped resolve the polytomy, though the authors suggested that the results could be the results of differences in mutation rates (hence population sizes) or could be because the species boundaries between taxa were not equally permeable.
Characterization of Species Complex. The species boundaries were not apparently in question, although distribution of characters and gene frequencies would not allow resolution of phylogeny.
Effect on taxonomic system. No changes were made to the classification or taxonomy of this group.
Authors writing on species concepts and implementation of species complex. Two quotes are extremely relevant to consideration of species concepts. " Speciation is often portrayed as a point event, however, there may be a protracted period of time when a limited amount of genetic exchange is possilbe, even in the absence of extended hybrid zones between taxa." "A general implication of this hypothesis is that a low level of gene flow during the time when incipient species remain incompletely isolated could blur the resulting phylogeny and confound sequence comparisons." This paper suggests that the length of incomplete divergence will be inversely proportional to the amount of phylogenetic resolution possible.
Problem and Approach. Normark used amplified mitochondrial CO-I sequences and maximum parsimony with evolution of parthenogenesis optimized as irreversible to look at the evolution of parthenogenetic lineages (morphotypes).
Characterization of Species Complex. This group is now considered a single species with 28 synonyms, and until recently was considered to be a complex of 6 apomictically reproducing species. The analysis showed that some of the morphotypes were polyphyletic, with 3 to 6 independent origins of parthenogensis in the group.
Effect on taxonomic system. No changes were made to classification or taxonomy.
Authors writing on species concepts and implementation of species complex. Authors relied on morphological identification of individuals by an expert taxonomist, indicating a reliance on a morphological species concept (ie. what a good taxonomist says it is). This is an example of a morphotype not being monophyletic.
Problem and Approach. Foltz et al. were attempting to confirm the taxonomic implications of a previous allozyme analysis, which had disclosed electrophoretic groups living sympatrically (non-panmictly). This study involved extremely large samples from within each populations. The authors amplified the control region of the mtDNA, then used restriction site analysis to look for a pattern corresponding to that of the allozymes. The patterns were examined with both clustering and ordination methods.
Characterization of Species Complex. This complex is comprised of mixed populations containing multiple allozyme types and multiple haplotypes. The authors used electrophoretic identification rather than morphology. The electrophoretic species were found to share some of the haplotypes, although certain haplotypes were characteristic of each of these "species". The sharing of these haplotypes was considered to be the result of ancestral polymorphism in sister taxa, or the effects of introgression.
Effect on taxonomic system. The authors conclude that a complete taxonomic revision of the genus will be required to reconcile the results.
Authors writing on species concepts and implementation of species complex. This work follows up a series of taxonomic decisions based on the allozyme work, which apparently did not use marker alleles, but rather frequencies. In the end, it seems as if a lack of correlation between patterns in the data set suggests that the taxonomy was not clarified by this effort.
Problem and Approach. Arnold et al. looked for evidence of introgression in an area of sympatry using nuclear DNA RAPDs and cloroplast DNA RFLPs from species which were characterized by numerous species specific markers.
Characterization of Species Complex. Introgression by pollination was shown by a mixture of species specific nuclear markers on a background of single cpDNA types.
Effect on taxonomic system. No changes made to classification or taxonomy.
Authors writing on species concepts and implementation of species complex. This work was based on taxa for whom the taxonomy has been the subject of long standing study. The value of this work towards the understanding of species complexes is that it demonstrates that introgression can cause contrasting phylogenetic signals. This paper offers a good discussion of the adaptive value of introgression.
Fu, Yun-Xin and Jonathan Arnold. 1991. On the association of restriction fragment length polymorphism across species boundaries. Proc. Nat'l. Acad. Sci. 88(9): 3967-3971.
Problem and Approach. Fu and Arnold used mathematical modeling to study gametic-phase disequilibrium within and between reproductively isolated subpopulations.
Characterization of Species Complex. This study did not use organisms.
Effect on taxonomic system. None.
Authors writing on species concepts and implementation of species complex. The authors show that you can not determine the organismal pedigree from an RFLP gene geneology because of unequal breakdown of linkages between restriction sites in a subdivided population, causing variance in disequilbrium. This paper was difficult to understand( by an organismal biologist such as myself) but suggests that lack of correlation between patterns of genetic loci can arise from the inescapable development variance in linkage-disequilibrium.
Problem and Approach. Wüster et al. were attempting to resolve some of the taxonomic complexity within the complex using 87 morphological characters and locality-average OTUs for a CVA phenetic analysis. This study was to be followed by a molecular mtDNA study to resolve the remaining issues.
Characterization of Species Complex. The taxonomic difficulty of this group stems from, according to authors, ontogenetic variation, sexual dimorphism, and superficially striking microgeographic variation. The results of this study showed that there was weak morphological differentiation between nominal species as well as lack of congruence of geographic variation between males and females.
Effect on taxonomic system. Preliminary to larger study. No change.
Authors writing on species concepts and implementation of species complex. The authors suggested that these patterns could be the result of three evolutionary processes: 1) weakly differentiated selection causing microgeographic variation; 2) the result of convergence to an optimal phenotype; or 3) contact zone hybridization. A good case was made for the plausibility of convergence on the basis that the low vagility and ambush predation lifestyle of this species would lead to intensive selection for camoflage characters related to local conditions.
Problem and Approach. Berlocher and McPheron studied the population structure of this group across a latitudinal cline using 17 allozymic loci from 17 populations.
Characterization of Species Complex. Of five loci showing latitudinal clines only 2 of them showed evidence of linkage disequalibrium. "Under the climate-linkage hypothesis, adaptation to local climatic regimes indirectly affects allozyme frequencies owing to linkage-disequilibrium between and unknown number of diapause-controlling genes and linked groups of allozyme loci [...]. This species host, Crataegus, is another taxonomically problematic group. Compared to sister taxa, this species is enzymatically diverse, has greater variance in emergence times, and has successfully switched hosts from Crataegus to Malus.
Effect on taxonomic system. No changes were made.
Authors writing on species concepts and implementation of species complex. Host races in this group are not recognized at the species level. This paper is important to species complex theory in demonstrating that each loci may have it's own pattern of variation, that the linkage between them is often weak. This paper also may also document the adaptiveness of variability.
Problem and Approach. Gomez and Snell looked for evidence of reproductive isolation between cryptic species by testing mate choice between allozymically differentiated groups.
Characterization of Species Complex. The evidence that this species consisted of cryptic species included allozyme distribution patterns suggesting that sympatric allotypes were not panmictic and there was evidence of ecological preferences for different allotypes. The results were that there were more discernable mating units than determinable by either allozymes or morphology. Mate recognition occurs by contact pheromones in this species.
Effect on taxonomic system. No taxonomic changes were made in this paper.
Authors writing on species concepts and implementation of species complex. This paper addresses the problem of documenting reproductivly isolated lineages without a way to differentiate them. Species boundaries, as reproductive barriers, could be manifest to the organisms while cryptic to human observation. It further shows that even allozyme patterns may not mark the boundaries of reproductively isolated units.
Problem and Approach. Seigler and Ebiner sought clarification of taxonomic relationships within this group by phenetic analysis of morphological characters and the presence of cyanogenic compounds. The authors were looking for gaps in multivariate analysis.
Characterization of Species Complex. 190 specimens were selected from 900 available to represent typical characters of the group, effectively engineering clean species boundaries within the group.
Effect on taxonomic system.
Authors writing on species concepts and implementation of species complex. This type of analysis is not sensitive to the issues of species complexes, by artificially contriving taxa around defined phenotypes. By whiting-out the gaps between the taxa, the authors have can contribute no information about the nature of the species complex itself.
Problem and Approach. Sullivan used morphological and flavinoid data in a cluster analysis and test-crossing between groups. Her questions concerned the appropriate taxonomy of the group, the range of morphological variability, and the role of hybridization in formation of the complex. Her phenetic analysis used 70 specimens selected from 6500 available.
Characterization of Species Complex. Author concludes that the complexity is the result of a combination of hybridization and local population differentiation.
Effect on taxonomic system. Author used gap patterns in the data to revise groups of previous work. The changes made were a regrouping of infraspecific taxa. Some older taxa were found to be environmental forms.
Authors writing on species concepts and implementation of species complex. The author uses phentic gaps to define taxa. Hybridization is frequently suspected in the formation of species complexes. Using test-crossing is a powerful tool, but does not go far towards documenting how much gene flow is actually affecting the populations. Allozyme analysis is a much stronger tool for answering this type of question about species complexes.
Problem and Approach. Wilson compared the patterns in 9 polymorphic loci with the morphometric characters of seeds in a study of a crop-weed complex in order to determine if there was incongruity between these data sets, and further, to determine which diagnostic characters might be of value in germplasm collections.
Characterization of Species Complex. The free-living taxa grade allozymically into the domestic species. A complex pattern was thought to caused by the domestic varieties being derived from the weeds, or that the weeds are feral domestic species, but the author showed that the two groups were unified by a history of sporadic cyclic introgression with strong selective forces (human crop values in the domestics, and bitter-fruits and seed dormancy in the weeds) at play in each of the end taxa. Wilson called this complex evolutionary process cophylesis.
Effect on taxonomic system. No changes were made to taxonomy.
Authors writing on species concepts and implementation of species complex. This paper clearly shows that characters and character-sets can be under different evolutionary pathways creating incongruence in character patterns. In this paper, bimodal selction combined with sporadic gene flow resulted in increasing neutral variation while achieving differentiation in selected characters.