Non-parallel morphological divergence following colonization of a new host plant

Adaptation to new ecological niches is known to spur population diversification and may lead to speciation if gene flow is ceased. While adaptation to the same ecological niche is expected to be parallel, it is more difficult to predict whether selection against maladaptive hybridization in secondary sympatry results in parallel divergence also in traits that are not directly related to the ecological niches. Such parallelisms in response to selection for reproductive isolation can be identified through estimating parallelism in reproductive character displacement across different zones of secondary contact. Here, we use a host shift in the phytophagous peacock fly Tephritis conura, with both host races represented in two geographically separate areas East and West of the Baltic Sea to investigate convergence in morphological adaptations. We asked (i) if there are consistent morphological adaptations to a host plant shift and (ii) if the response to secondary sympatry with the alternate host race is parallel across contact zones. We found surprisingly low and variable, albeit significant, divergence between host races. Only one trait, the length of the female ovipositor, which serves an important function in the interaction with the hosts, was consistently different between host races. Instead, co-existence with the other host race significantly affected the degree of morphological divergence, but the divergence was largely driven by different traits in different contact zones. Thus, local stochastic fixation or reinforcement could generate trait divergence, and additional evidence is needed to conclude whether divergence is locally adaptive.

     

CLINAL VARIATION ASSOCIATED WITH EDAPHIC ECOTONES IN HYBRID POPULATIONS OF GAILLARDIA PULCHELLA

The variety pulchella of the outcrossing annual plant species Gaillardia pulchella consists of two edaphic races in central Texas which are divergent for one morphological and four electrophoretic characters. Reduced pollen stainability in F₁ hybrids suggests the races are also divergent in chromosome structure. The recent proliferation of this species on roadsides and in pastures has led to hybridization between these races. An analysis of character variation in three hybrid populations revealed significant clinal variation associated with edaphic ecotones, and the width of these clines was found to vary among characters in a consistent pattern. It is argued that this pattern is the result of different characters experiencing different effective selection regimes, with narrower clines reflecting greater differentials in effective selection. Several mechanisms are discussed by which selection may impede the transgression of alleles across the ecotones in these populations. The results of this study are compared to the results of parallel studies on the autogamous annual species Avena barbata in California, and it is suggested that the difference between these two species in the width of clines separating edaphic ecotypes may be accounted for by their different breeding systems.     

On the reality of local and ecological races in lymnaeid snails (Mollusca,Gastropoda, Lymnaeidae)

The reality of ecological and local races was investigated in two widespread Palearctic species of lymnaeid snails (Lymnaea stagnalis and Radix auricularia). Several methods of statistical analysis were used, including two-way ANOVA, cluster analysis, and discriminant analysis for six plastic shell characters. It was shown that none of the methods used could convincingly demonstrate that ecological and local races were present in the studied species. Even in cases when statistically significant differences among the populations in the morphology of shells were revealed, these differences were very small and did not correspond to the meaning that was put into the concept of “race” by malacologists in the past. Perhaps, the formation of such races in pond snails is possible only in case of populations being formed in suboptimal conditions, for example, in thermal springs or at large depths. This being the case, the distinguishing of races requires using other shell characters, both quantitative and qualitative ones.     

Use of genetic effects and genotype by environmental interactions for the classification of mexican races of maize

To examine the questions of whether the additive and dominance effects present for morphological characters in racial crosses are of sufficient consistency and magnitude to allow such genetic effects to be used for racial classification, we used a diallel experiment among the 25 well-defined Mexican races of maize, which include the ancestral stocks of most commercial and genetic maize types. With such an experiment, genetic effects and genotype by environmental interactions for one or more characters can be used to measure genetic and adaptational or environmental similarity. We used average parental effects (general combining abilities), specific effects, and genotype by environmental effects of 21 characters from the diallel (grown at three locations) to group the Mexican races of maize. The groupings based upon average genetic effects and upon genotype by environmental interactions are more satisfactory than groupings based upon specific effects. The standard errors for genetic distances based upon specific (largely dominance) effects seem to be too high for practical use. Principal components analyses of the same data suggest a similar conclusion.-The groupings based upon average genetic effects are in general agreement with previous studies, with the exception of Maíz Dulce, which is grouped with the Cónicos, rather than being isolated from the other Mexican races of maize.     

Hybridization, ecological races and the nature of species: empirical evidence for the ease of speciation

Species are generally viewed by evolutionists as 'real' distinct entities in nature, making speciation appear difficult. Charles Darwin had originally promoted a very different uniformitarian view that biological species were continuous with 'varieties' below the level of species and became distinguishable from them only when divergent natural selection led to gaps in the distribution of morphology. This Darwinian view on species came under immediate attack, and the consensus among evolutionary biologists today appears to side more with the ideas of Ernst Mayr and Theodosius Dobzhansky, who argued 70 years ago that Darwin was wrong about species. Here, I show how recent genetic studies of supposedly well-behaved animals, such as insects and vertebrates, including our own species, have supported the existence of the Darwinian continuum between varieties and species. Below the level of species, there are well-defined ecological races, while above the level of species, hybridization still occurs, and may often lead to introgression and, sometimes, hybrid speciation. This continuum is evident, not only across vast geographical regions, but also locally in sympatry. The existence of this continuum provides good evidence for gradual evolution of species from ecological races and biotypes, to hybridizing species and, ultimately, to species that no longer cross. Continuity between varieties and species not only provides an excellent argument against creationism, but also gives insight into the process of speciation. The lack of a hiatus between species and ecological races suggests that speciation may occur, perhaps frequently, in sympatry, and the abundant intermediate stages suggest that it is happening all around us. Speciation is easy!     

Evolution of the chromosomal races of Mus musculus domesticus in the Rhaetian Alps: the roles of whole-arm reciprocal translocation and zonal raciation

The evolution of ten Robertsonian (Rb) races of the house mouse (Mus musculus domesticus) in the Rhaetian Alps of northern Italy and southern Switzerland is reconsidered. The mechanisms of centric fusion, zonal raciation and, for the first time, whole-arm reciprocal translocation (WART), are used in this non-mathematical approach to produce a phylogenetic tree (using chromosome fusions as characters) with the smallest number of steps. The shortest tree that we found (16 steps) is at least two to nine mutations shorter than previously published models. Three other trees (17 or 18 steps) are also considered, since they are geographically more sensible. In general, these four scenarios correspond more closely to the present distributions of the ten Rb races than previous trees. Our results suggest that zonal raciation and WARTs play an important role in the evolution of Rb races of the house mouse.     

The myology of the manus of microtine rodents

A study of the muscles of the manus of 13 genera and subgenera of microtine rodents revealed considerable variation which could be organized into 23 primitive-derived evolutionary hypotheses (transmutation series). The results, when compared with those for other mammals, produced functional/ecological hypotheses which were tested by comparing their predictions with the literature. The microtine manus is morphologically distinguished by nine characters and is adapted for a terrestrial/semi-fossorial niche. In particular, Dicrostonyx and Lemmus (distinguished by eight characters) possess the most highly fossorial manus, while the manus of Arborimus (distinguished by eight characters) contains a combination of primitive characters and derived characters which give the Tree vole the most highly developed 'clasping' ability among the microtines studied. The study has revealed characters which have potential systematic value but it is probable that parallelism has occurred in the microtine manus. When the manual characters are combined with characters from other functional complexes, their systematic value can be more clearly determined.     

Variation in Mentha arvensis L. (Labiatae). I. The North American populations

Most North American populations of Mentha arvensis differ from those in Europe in leaf and calyx characters, but the extremes of variation overlap. However the former have a diploid chromosome number of 96 and the latter 72. The North American populations are morphologically very variable and much of this variation is genotypic. However, none of it is discrete and, as it shows little geographic or ecological coherence, taxonomic recognition appears undesirable.
The following chemical races based on essential oil composition, occur in North America:
Type 1–high in pulegone, isomenthone and menthone.
Type 2–high in linalool, cfr-ocimene and trans-ocimene.
Type 3–high in cw-isopulegone and fnmi-isopulegone.
Type 4–high in cis- and trans-ocimene and in 1,8-cineol.
These four chemotypes are not correlated with any of the morphological variations. The recognition of chemical races is discussed and it is concluded that where possible they should be based on diagnostic components, rather than on quantitative differences or the mere presence or absence of given compounds. The literature relating to biosynthesis and essential oil composition in Mentha arvensis is reviewed.     

PARALLEL RACE FORMATION AND THE EVOLUTION OF MIMICRY IN HELICONIUS BUTTERFLIES: A PHYLOGENETIC HYPOTHESIS FROM MITOCHONDRIAL DNA SEQUENCES

Mimicry has been a fundamental focus of research since the birth of evolutionary biology yet rarely has been studied from a phylogenetic perspective beyond the simple recognition that mimics are not similar due to common descent. The difficulty of finding characters to discern relationships among closely related and convergent taxa has challenged systematists for more than a century. The phenotypic diversity of wing pattens among mimetic Heliconius adds an additional twist to the problem, because single species contain more than a dozen radically different-looking geographical races even though the mimetic advantage is theoretically highest when all individuals within and between species appear the same. Mitochondrial DNA (mtDNA) offers an independent way to address these issues. In this study, Cytochrome Oxidase I and II sequences from multiple, parallel races of Heliconius erato and Heliconius melpomene are examined, to estimate intraspecific phylogeny and gauge sequence divergence and ages of clades among races within each species. Although phenotypes of sympatric races exhibit remarkable concordance between the two species, the mitochondrial cladograms show that the species have not shared a common evolutionary history. H. erato exhibits a basal split between trans- and cis-Andean groups of races, whereas H. melpomene originates in the Guiana Shield. Diverse races in either species appear to have evolved within the last 200,000 yr, and convergent phenotypes have evolved independently within as well as between species. These results contradict prior theories of the evolution of mimicry based on analysis of wing-pattern genetics.     

Edaphic races and phylogenetic taxa in the Lasthenia californica complex (Asteraceae: Heliantheae): an hypothesis of parallel evolution

Lasthenia californica sensu Ornduff consists of two races that differ in their flavonoid pigments and edaphic tolerances. Recent phylogenetic studies of Lasthenia have revealed that members of L. californica sensu Ornduff belong to two phylogenetic species. The relationship of the edaphic races to these new species and to each other is the focus of this study. Characterization of flavonoid profiles and phylogenetic placement of 33 populations demonstrates that races and phylogenetic taxa are not concordant, suggesting that one or both edaphic races evolved in parallel in the two clades. We hypothesize an edaphically linked ecological role for flavonoid differences that first revealed the existence of two races.     

The Species Problem in Plant Taxonomy in China

Nooteboom (1992) and Peter Raven (pers. comm. ) have pointed out that Chinese taxonomists often hold a narrow species concept and that this may due to the small volum of collections, especially type specimens, available to them which led to the unadequate study on the variability of the species. Raven remarked that “this leads me to believe that the actual concept of species used in plant systematics in China tends to be fairly typological”. What they said are by no means unreasonable. Indeed, the taxonomical status of a considerable number of species in the Chinese flora is probably open to question. New species based on a single character or solely on vegetative characters are of frequent occurrence. Evidences from a very limited number of researches on the patterns of plant variation heretofore available in China have shown that some “species” are, in fact, ecological races ( Clinopodium ), geographical races (Cunninghamia & Indigofera ), or taxa with topoclinal variation (Lespedeza & Rhododendron ). Species based on plasticity of phenotype variation have been or still regarded as “good species” ( Rorippa ). Segregates of an interspecific hybrid with diverse leaf characters have been given different species names ( Ilex ). The originally complicated situation in taxonomy of an agamic complex becomes even more complicated after the publication of additional new species (Malus). A careful analysis of a species with rather complicated patterns of variation leads to the combination of 25 specific names, of which 10 were published in the 80’s by Chinese taxonomists ( Clematoclethra ). Examples of these kinds will greatly increase with the broadening of research work at the species level. Orthodox plant taxonomy is based largely or solely on morphological characters. The exomorphic characters have the practical advantage that they are relatively easy to observe and to record. The taxonomical species concept can meet the needs of general purpose classification. But the notion that the taxonomical species concept is a solely intuitive judgement or preference of an individual worker and one could hardly say what is right and what is wrong is quite problematical. The species category today is much more capable of objective interpretation than ever before. A correct species concept stems from a correct and thorough understanding of the nature of variation pattern of plants and its taxonomical value. Hence, as a herbarium taxonomist, the first thing is to study as many collections as possible. Secondly, the incorporation of evidence from other sources whenever possible is highly desirable. These evidences,if they are not very useful as taxonomical criteria, are frequently of great significanee in contributing to a better understanding or interpretation of the variation pattern of a given taxon. The taxonomist might find the discontinuities he seeks better expressed in either the phenotypic or the genetic variation. A logical application of these two sorts of criteri-a would lead to a more rational classification at the specific level in a great many genera.     

Eine Methode zur Abgrenzung infraspezifischer Sippen anhand quantitativer blütenökologischer Merkmale bei Aegilops tauschii Coss

Differences in characters of pollination ecology allow to distinguish two races of Aegilops tauschii ssp. tauschii most clearly by means of anther length. The facultatively allogamous primitive race has long anthers (>2 mm) and is closely related to ssp. strangulata. It is supposed that the race with long anthers participated in the evolution of Triticum aestivum as donor of genome D. The derived race is strongly autogamous and has short anthers (≤2 mm). Differences within the geographic distribution of the two races have been pointed out (cf. Hammer 1978b). A distinction of the races by anther length is not always possible because of technical or other reasons. That's why a regression analysis was carried out to test the joint influence of some quantitative relatively easy measurable characters (suitable qualitative characters could not be found) on anther length (AL). A relatively close connexion (table 3) exists between this character, glumae length (HL), and plant height (PH) — the regression equation is AL₅ = −0,623 + 0,161 HL + 0,028 PH (B = 0,694). By this equation anther length may be calculated with sufficient exactness. For easier determinations nomographs (fig. 2) were constructed showing the anther length for given values of glumae length and plant height (fig. 2a) or glumae length, plant height, and mass of 1000 spikelets (fig. 2b) for a population of Ae. tauschii. The described method is proposed for the easier distinction of races (mainly on infraspecific level) by quantitative characters.     

A comparative study of morphological integration in Apis mellifera (Insecta, Hymenoptera)

Morphometric correlation matrices from 11 Mediterranean and European honey bee races have been compared. The degree of integration, measured by the eigenvalue variance of the correlation matrix or by Cheverud's index of integration, varies considerably between races but covaries neither with overall body size nor with the variance of the respective sample. Hence, the degree of morphological integration does not depend on body size or the level of variability The patterns of morphometric correlation are significantly similar among all races, but some minor differences in the patterns could be detected: The similarity between the correlation matrices is independent of the degree of phylogenetic relatedness between the respective races. In all races characters belonin to the same functional and/or developmental unit (leg, wing, abdominal characters) have Eiter correlations than the average. Most of the variation in the pattern of correlation can be expfained as a side effect of variation in the degree of integration. Races with high levels of integration tend to have hiher correlations between the main groups of characters, and races with low levels of integration tave smaller correlations between the main grous of characters, while the basic pattern remains undisturbed. In summary, the comparative analysis of morphometric correlation matrices reveals a picture of stability with respect to the pattern of integration and of variation in the degree of integration, which is random with respect to body size, degree of variability, and phylogeny.     

Incipient sexual isolation in thenasuta-albomicans complex ofDrosophila: No-choice experiments

Drosophila nasuta nasuta andDrosophila nasuta albomicans are cross-fertile races ofDrosophila. Hybridization between these races in the laboratory has given rise to new races (Cytoraces), among which karyotypic composition differs from one another and also from those of the parental races. In this study, we search for the evidence of incipient reproductive isolation among the parental races and four Cytoraces by assessing the fraction of no-matings, mating latency and copulation duration in all possible types of homo- and heterogamic crosses (N = 4184). In no-choice conditions, the latency time (time to initiation of copulation) is lower in homogamic crosses than in heterogamic crosses for both parental races and Cytoraces. Latency time and copulation duration are negatively correlated, whereas fraction of no matings is positively correlated with latency time. Thus these six closely related races of thenasuta-albomicans complex show the initiation of the earliest stages of pre-zygotic isolation, manifested as a tendency for matings to be initiated earlier and more often, and for a longer duration, among homogamic rather than heterogamic individuals.     

The genetic architecture of ecological specialization: correlated gene effects on host use and habitat choice in pea aphids

Genetic correlations among phenotypic characters result when two traits are influenced by the same genes or sets of genes. By reducing the degree to which traits in two environments can evolve independently (e.g., Lande 1979; Via and Lande 1985), such correlations are likely to play a central role in both the evolution of ecological specialization and in its link to speciation. For example, negative genetic correlations between fitness traits in different environments (i.e., genetic trade-offs) are thought to influence the evolution of specialization, while positive genetic correlations between performance and characters influencing assortative mating can accelerate the evolution of reproductive isolation between ecologically specialized populations. We first discuss how the genetic architecture of a suite of traits may affect the evolutionary role of genetic correlations among them and review how the mechanisms of correlations can be analyzed using quantitative trait locus (QTL) mapping. We then consider the implications of such data for understanding the evolution of specialization and its link to speciation. We illustrate this approach with a QTL analysis of key characters in two races of pea aphids that are highly specialized on different host plants and partially reproductively isolated. Our results suggest that antagonism among QTL effects on performance in the two environments leads to a genetic trade-off in this system. We also found evidence for parallel QTL effects on host-plant acceptance and fecundity on the accepted host, which could produce assortative mating. These results suggest that the genetic architecture of traits associated with host use may have played a central role in the evolution of specialization and reproductive isolation in pea aphids.     

Variation and evolution inLinum tenuifolium (Linaceae)

The phenetic variation inL. tenuifolium s.l. was assessed using multivariate analyses of 27 characters found to be variable within the species. These results are discussed and interpreted in the context of the reproductive biology, chromosome number and ecological responses of the taxa. Evidence suggests that both the self-compatible and tetraploid races were derived independently from a self-incompatible, diploid type. Although no taxonomic scheme is compatible with all of the evidence presented, the study supports a recent decision to recognize four taxa at subspecific level.     

The races of maize: III. choices of appropriate characters for racial classification

Analyses of variance for 111 characters from 55 races and subraces of maize from eastern South America grown at Piracicaba, S. P., Brazil, between 1960 and 1965, indicated that those characters which were least affected by environmental factors and interactions were reproductive characters. In particular, the component of variance due to differences among races for certain ear and kernel characters was greater than the sum of the corresponding components due to differences among years and race by year interactions. The converse was true for all vegetative characters. Tassel characters tended to be intermediate between ear and plant characters. While some indices had larger components of variance attributable to racial differences than to the effects of environment and/or environmental interaction, some commonly used ones, such as cob/rachis and rachilla/kernel indices, proved to be quite susceptible to environmental influences. Again, indices based upon solely vegetative characters were consistently influenced more strongly by environmental factors and interaction than were those based on reproductive characters.     

Allozyme variation of the pygmy wood mouse Sylvaemus uralensis (Rodentia, Muridae) and estimation of the divergence of its chromosome forms

The genetic divergence between the eastern European, southern European, and Asian chromosome forms of the pygmy wood mouse Sylvaemus uralensis, whose karyotypes differ from one another in the amount of pericentromeric heterochromatin, has been reevaluated using allozyme analysis. In general, Asian S. uralensis living in eastern Kazakhstan, eastern Turkmenistan (the Kugitang Ridge), and Uzbekistan are more monomorphic than European populations of this species. However, the allozyme differences between all chromosome forms of the pygmy wood mouse is comparable with the interpopulation differences within each form and are an order of magnitude smaller than those between "good" species of the genus Sylvaemus. Thus, the chromosome forms of S. uralensis cannot be considered to be separate species. The concept of races as large population groups that have not diverged enough to regard them as species but differ from one another in some genetic characters is used to describe the differentiation of S. uralensis forms more adequately. The currently available evidence suggests the existence of two S. uralensis races, the Asian and the European ones, and two chromosome forms (eastern and western) of the European race. The possible historical factors that have determined the formation of the races of the pygmy wood mouse are considered. According to the most plausible hypothesis, the shift and fragmentation of the broad-leaved forest zone during the most recent glacial period (late Pleistocene) were the crucial factors of the formation of these races, because they resulted in a prolonged isolation of the European and Asian population groups of S. uralensis from each other.