Morphological Clines Research Articles

Parallels between island lizards suggests selection on mitochondrial DNA and morphology

Parallels between species on historically independent islands have been shown to be a powerful means of testing for the action of natural selection on morphology. Mitochondrial DNA evolution is often assumed to be selectively neutral, but this is difficult to test empirically. Here we show that parallels are present between two species of lizards on the geologically distinct but ecologically similar Lesser Antillean islands of Dominica and Basse Terre (Guadeloupe). These parallels are found not only in morphological variation but also in mitochondrial DNA variation and these are correlated with similar ecological gradients. A morphological cline in Anolis oculatus along the Caribbean coast of Dominica is compared to sequence variation in the cytochrome b gene, which shows exceptionally high variability. Patterns of sequence variation and morphology (especially scalation) are congruent, and are significantly correlated with moisture gradients. Anolis marmoratus on the adjacent island of Basse Terre (Guadeloupe) shows parallel patterns of morphological and cytochrome b variation along the Caribbean coast, and these are also significantly correlated with moisture gradients.

Phenotypic, Genetic and Ecological Variation in the Eucalyptus saligna̵1E. botryoides Complex

Analysis of variation within the E. saligna-E. botryoides complex indicates a continuum of morphological variation, particularly in bud, fruit and bark characters, between the taxa. The continuum occurs between populations north of Sydney (which had always been recognised as E. saligna), and populations restricted to the coastal fringe (which had always been recognised as E. botryoides). There is a suggestion of a latitudinal cline in morphology of the intermediate populations, although it is complex, and shows significant local variation. Analysis of allozyme data indicates that most of the genetic variation within the complex is contained within rather than between populations, and that genetic distances between populations are more typical of within-species rather than between-species differences. The lack of allozyme divergence is attributed to either relatively rapid and recent differentiation or to extensive introgression following secondary contact of the taxa. Analysis of environmental variables reveals two discriminating factors in relation to the current distributions of populations within the complex. Firstly, the morphological gradient appears to follow an underlying temperature gradient, and secondly, E. botryoides occupies a separate niche associated with the distinct coastal environment on which it occurs. These key environmental parameters may have been important in the evolution of the complex along axes of latitude or temperature, and coastal versus inland habitat. Additionally, changes in distribution of the taxa as a result of climatic changes associated with glacial-interglacial cycles may also have played an important role in the formation of the complex.

Constraint and the square snail: life at the limits of a covariance set. The normal teratology of Cerion disforme

Two major developmental constraints, identified and assessed by the covariance structure of ontogenetic measures, strongly influence the form and variation of Cerion in every study I have ever undertaken on this protean genus: the jigsaw constraint of large-and few vs small-and-many whorls for shells reaching a similar final size; and the compensatory constraint of later heights balancing juvenile widths to bring adult shells into a limited range of final proportions. This study explores a set of taxa (the Cerion dimidiatum complex of eastern Cuba) growing at one extreme of the compensatory constraint–maximally flat juvenile shells followed by abrupt allometric transition to orthogonal adult whorls. I show that the most curious of all Cerion taxa, the teratologously uncoiling C. disforme, owes its unique and distinctive form to growth at an extreme of the compensatory constraint: the juvenile cross-section goes beyond mere flatness to actual concavity; and the adult shell, after maximally abrupt allometric transition to downward growth, loses contact with the juvenile whorls and begins to unwind in teratologic fashion. I present several categories of evidence to demonstrate that C. disforme is the extreme in a morphocline of taxa, whose correlated features express increasing discordance (and correlated sequelae of size and shape) under the compensatory constraint: C. geophilus–C. dimidiatum–C. alberli–C.disforme. I provide data for clines in morphology, in amount and character of variation, and in mode of growth. This genre of constraint–Darwin's old “correlations of growth”–must be given equal weight with immediate adaptation in a unified explanation of growth and form.

Offshore Distribution, Size, Age, and Lateral Plate Variation of Late Larval/Early Juvenile Sticklebacks (Gasterosteus) off the Atlantic Coast of New Jersey and New York

Late larval and early juvenile sticklebacks, Gasterosteus aculeatus (14–28 mm SL) and G. wheatlandi (11–19 mm SL), were collected over shelf waters in the New York Bight during May, June, and July 1988. Gasterosteus aculeatus occurred up to 110 km offshore, with peak abundances found from 3 to 75 km. Gasterosteus wheatlandi was most prevalent within 10 km of shore, but some individuals were collected up to 55 km offshore. The further offshore distribution of G. aculeatus should increase its potential for dispersal, perhaps explaining their capacity to found freshwater populations in recently deglaciated habitats. The relatively nearshore distribution of G. wheatlandi should minimize dispersal (especially around Cape Cod) and may maintain a distinct, morphological step cline by the relative isolation of two subpopulations above and below Cape Cod.

The taxonomy of savannah baboons: An odontomorphometric analysis.

The subgeneric relationships of savannah baboons have long been a point of contention among taxonomists. Recent documentation of morphological clines and evidence of interspecific hybridization have further complicated this problem. In this investigation, the dental dimensions of 443 savannah baboons, representing three of the generally recognized species, Papio anubis, P. cynocephalus, and P. ursinus, were analyzed using univariate and multivariate statistical procedures. A small number of possible P. ursinus/P. cynocephalus hybrids was included in the analysis. The results suggest that the three morphotypes are clearly distinct. Furthermore, the hybrid group possesses a dentition incorporating different elements of the two interbreeding species. However, these individuals are not simply intermediate but quite different. These findings, considered in conjunction with other morphological and distributional data, tend to warrant maintenance of the present specific distinctions.

Morphological clines across a karyotypic zone of house mice in Central Italy

AbstractMultivariate morphometric differentiation between chromosomal races of the mouse Mus domesticus in Central Italy was investigated using a population of 2n = 22 “CB” karyotype, three populations of standard 2n = 40 karyotype, five populations of 2n = 22 “CD” karyotype and three populations from the hybrid zone between the latter two karyotypes. Whilst populations of different karyotype generally have significantly different morphometry, canonical analysis does not reveal that the populations ordinate into distinct aggregations based on karyotype, largely because the 2n = 22 “CD” populations are so diverse. Nevertheless, canonical analysis does reveal a significant cline in morphology across the contact zone between the 2n = 40 and 2n = 22 “CD” mice. The nature of this transition, i. e. a cline 1. within the 2n = 40 range, 2. within the hybrid range (but unrelated to chromosome number) and 3. within the 2n = 22 “CD” range, tends to indicate that the morphometric divergence is due to adaptation to the different ecological regimes across which these mice are distributed rather than the phylogenetic divergence of the karyotypic races.

A hybrid zone between Chorthippus parallelus parallelus and Chorthippus parallelus erythropus (Orthoptera: Acrididae): behavioural characters

Two subspecies of the grasshopper Chorthippus parallelus have ranges that abut in the Pyrenees. Males produce two types of song—calling song and courtship song. We have examined the use and structure of these songs in males from nine sites near Mont-Louis, Pyrenees-Onentales, France. These sites were previously used to identify the position of the hybrid zone on morphological characters. The subspecies differ in the use and structure of both types of song. Clines in these characters correspond in position with the morphological cline. At least one character may be relevant to mate choice but none of the characters show evidence of reinforcement.

Morphological clines in reduced areas. The case of Henrotius jordai (Reitter), cave-dwelling beetle from Majorca Island

The present paper shows a statistically significant correlation between the geographical latitude and the morphological variation of the pronotum of Henrotius jordai (Reitter) (Col. Caraboidea), cave-dwelling beetle from Majorca island, after studying the linear and curvilinear regression between these two variables. The existence of specimens situated at different heights of the regression lines leads to the conclusion that morphological variation is clinal. The phenomenon of “semi-isolation” to which the studied populations are subjected, because of their cavernicolous character, allows to explain the existence of a cline in such a restricted area as that occupied by this beetle (ca. 500 Km2). It is worth pointing out the interest in the study of these reduced clines of cavernicolous populations, because they can provide a restricted “observation field” - easier to deal with - to investigate these genetic phenomena and their evolutive implications.

Morphological trends in the genus Heptacarpus (Decapoda, Caridea) and their phylogenetic significance

ABSTRACT Morphological trends or clines in a variety of morphometric, meristic, and two-state characters are described and analysed for the caridean shrimp genus Heptacarpus. One objective of this study was to determine the evolutionary polarity (ancestral to derived) in these various morphoclines. Evidence from functional morphology and out-group comparison strongly support the hypothesis that a high epipod (epipod-setobranch complex) number is primitive in Heptacarpus and related genera and that a lower number represents the derived or advanced condition (epipod loss). The evolutionary polarity in epipod number is used to hypothesize the direction of evolution in morphoclines that are significantly correlated with epipod number. Using this criterion, Heptacarpus species with high epipod number, a short high caparace, short and high abdominal segments, a short rostrum, large first chelae, a high number of antennular spinules, and a branchiostegal setal fringe are considered primitive with respect to these characters. Heptacarpus species which are farthest from the hypothesized ancestral form show a reduced epipod number, a slender or elongate carapace shape, elongate abdominal segments, a proportionately long rostrum, small first chelae, a reduced number of antennular spinules, and no branchiostegal setal fringe. Species groups are formed on the basis of morphological similarity from the 16 Heptacarpus species examined in detail. An index of affinity, based on 17 characters, was calculated for all possible pairs of species. A group is formed by species which show a significant morphological similarity (index of affinity greater than 0.5) to all other members of the group. The phylogenetic validity of species groups formed by this phenetic method was tested by examining the concordance in epipod number among members of a particular group. Evidence is given that epipod number is an indicator of phylogenetic relationships.

VARIATION AND POLYPLOIDY WITHIN LOWLAND POPULATIONS OF THE BETULA PENDULA/B. PUBESCENS COMPLEX

SUMMARYVariation and cytotype distribution have been studied in secondary birch woodland where intermediates between Betula pendula and B. pubescens appear to predominate. Trees at three sites in lowland eastern England, with contrasting soil conditions, were examined.Thirty‐eight of the trees studied proved to be tetraploid (2n= 56) and the remaining 12 were diploid (2n= 28); no triploids or aneuploids were detected.Principal Components Analysis of up to 20 biometric characters of leaves, catkins, fruits and achenes revealed an essentially continuous pattern of variation. The diploid trees displayed rather limited variation and corresponded fairly well with B. pendula. In contrast, the tetraploids showed very great variation which included that normally associated with both species and most of the possible kinds of intermediate. The same pattern of variation was evident in several discontinuous variables and also in measurements of pollen diameter and guard‐cell length made on 20 trees at one site (Holme Fen).Variation was in part site‐dependent, suggesting that the different environments were influencing phenotype. Twenty trees at one site (Dersingham) were located on a topographic gradient from bog to heath; here there was a strong correlation between the loadings of individual trees on the principal axes of variation and the elevation of those trees on the gradient. This indicates a morphological cline which might be determined by site‐wetness. The cline was not associated with polyploidy, as it was mostly attributable to variation within the tetraploids.

Genetic Analysis of Culex pipiens Populations in the Central Valley of California

In the Central Valley of California two subspecies of the mosquito Culex pipiens exist— C. p. quinquefasciatus in the warm southern regions and C. p. pipiens in cooler northern regions. The criterion used to distinguish subspecies is a measurement of the male genitalia, the DV/D ratio. The geography of the Central Valley causes a unique temperature pattern which is not a simple south-north gradient. Based on DV/D measurements, the subspecies of C. pipiens track this unique temperature gradient. Three of 10 allozyme loci had electrophoretically detectable alleles which paralleled the morphological cline. One of the three (Pgm) exhibited a cline which reversed at the same point where the temperature and DV/D reverse, i.e., just north of Sacramento. Further analysis revealed that the genes controlling DV/D are independent of Pgm . It seems likely that natural selection directly or indirectly affects Pgm allele frequencies. Some populations along the cline contained males which ranged in their DV/D from C. p. quinquefasciatus to C. p. pipiens with up to 50% intermediate between the usual criterion separating the subspecies. We concluded that these intermediate populations, possibly of hybrid origin, are stable, freely interbreeding units. This last conclusion has important implications for the systematic status of intermediate groups of the C. pipiens complex found elsewhere in the world.

Variation in the Deep-Sea Isopod Eurycope Iphthima (Asellota, Eurycopidae): Depth Related Clines in Rostral Morphology and in Population Structure

In an especially good sample series of the deep-sea isopod Eurycope iphthima Wilson, substantial variation occurs in the cephalic rostrum and in several aspects of its population structure: the size-frequency distribution, size of maturation of males and females, propor­ tions of female stages, proportions of juvenile stages, and maximum size of adults. Com­ parisons of samples from different depths (2,500 to 4,800 m) and different localities (north­ eastern and central Atlantic Ocean) show the variation to be distinctly depth-related: populations from similar depths are more similar than those from different depths. Because of continuity in other taxonomically important characters, the variation must be clinal and reflects adaptation to local environments, directed genetic variation, or both, The parallel clines in rostral morphology and population structure are most apparent on the continental slopes and rises off Ireland and in the Bay of Biscay. Bimodality in the population structure of the deeper populations is unusual; two hypotheses for its explanation are discussed, although evidence to support either is currently unavailable. Clinal variation in terrestrial organisms is well known (Mayr, 1970; Endler, 1977); it is little known in deep-sea animals. Clines should exist in the deep sea, especially in strong environmental gradients such as those found on the conti­ nental slopes. As depth increases, so does hydrostatic pressure, whereas tem­ perature and the quality and quantity of food decreases. Different levels of the slopes are subject to varying current fields and diurnal tidal rhythms. Most im­ portant, the slope faunas change greatly with depth (e.g., Hartman and Fauchald, 1971; Sanders and Hessler, 1969; Rex, 1979; Grassle et ai., 1979), both signaling this environmental change and becoming part of it. Genetic studies on deep-sea benthic megafauna (Doyle, 1972; Siebenaller, 1978) have demonstrated shifts in allozyme freq~enc. ies between depth-separated pop­ ulations. However, such studies are techn~lly unfeasible on smaller deep-sea invertebrates and must give way to morphological investigations. Individual mor­ phological variation may be important in deep-sea populations (e.g., Schopf, 1976; Hansen, 1967); often there are large differences between localities. For example, Rex (1979) analyzed two populations of Aivania peiagica, a shallow bathyal gas­ tropod found above and below the shelf-slope break off New England, and showed abrupt differences between the popUlations. In contrast, Gardiner (1975) noted striking vertical morphological similarity in specimens of the tanaid Neotanais americanus over a depth range of 2,800 to 5,020 m in the same area. Nevertheless, he found variation with depth in two characters. These two studies lack either verification of conspecificity of the populations or enough specimens for a pop­ ulation analysis. In addition to large samples of conspecific populations, the identification of a cline further requires at least three or more popUlation samples, a rare occurrence in the vagaries of deep-sea sampling. This study uses abundant material of the dominant deep-sea eurycopid isopod EuryciJpeiphthima Wilson (1981) (Table 1), to establish the presence of, clinal variation in one morphological character and in various aspects of the population structure of this species.

Morphological and genetic variation in Japanese populations of the anemonefish Amphiprion clarkii

Geographic populations of the anemonefish Amphiprion clarkii (Bennett) from 6 widely separated locations off the coast of southern Japan are morphologically different, exhibiting (1) latitudinal clinal patterns in color pattern and meristics and (2) degrees of genetic differentiation. Electrophoretic examination of 7 polymorphic loci (95% level) among 6 populations collected between July 1979 and January 1980 revealed an average genetic distance value of 0.008. Significance was demonstrated for 39 out of 105 (37%) tests of heterogeneity. Averaged over the 6 populations, the percentage of polymorphic loci ( $$\bar P$$ 0.95) was 24.6% and the proportion of heterozygous loci per individual ( $$\bar H$$ ) was 0.0613. The results indicate that genetic clines are absent and suggest that the morphological variation may not be genetic. Relative isolation of genetic populations may be maintained by (1) localized larval dispersal resulting from a relatively short larval stage, and (2) current gyres tending to trap larvae, increasing the return of juveniles to their adult coastal habitat. Morphological clines may be due to clines in ecological parameters related to latitude.

PARALYCOPODITES MOREY & MOREY, FROM THE CARBONIFEROUS OF EURAMERICA—A REASSESSMENT OF GENERIC AFFINITIES AND EVOLUTION OF “LEPIDODENDRON” BREVIFOLIUM WILLIAMSON

A study of the morphologies and evolutionary relationships of Upper Carboniferous coal swamp lycopods indicated that plants traditionally treated as Lepidodendron hrevifolium were distinct from Lepidodendron at the generic level, and were not part of a morphological cline between Lepidodendron and Lepidophloios. The description of a new genus and species by Morey and Morey (1977), based on a specimen identical to L. brevifolium distal branches, results in the inclusions of L. brevifolium in Paralycopodites, in which it has nomenclatural priority over P. minutissimus. The small trees or shrubs had deciduous lateral branches and permanently retained leaves; in these characters, Paralycopodites is most similar to the compression‐impression genus Ulodendron as delimited by Thomas. Bisporangiate Lepidostrobus cones are closely associated with the vegetative remains; the cones can be assigned to three stratigraphically complementary species throughout the range of Paralycopodites—Visean to Westphalian D (Middle Pennsylvanian). Extreme evolutionary conservatism is indicated by the low vegetative morphological variability and the great similarities in morphology of associated cones; in combination with the sudden Visean appearance of P. brevifolius and the lack of intermediates among major lycopod genera, this lends support to punctuated equilibria hypotheses. Paralycopodites was generally rare in coal swamps, possibly related to its production of megaspores lacking specialized adaptations for flotation. Its pattern of late Middle Pennsylvanian extinction parallels Lepidodendron and Lepidophloios.

Clinal variation of some mammals during the Holocene in Missouri

Eastern cottontail ( Sylvilagus floridanus), fox squirrel ( Sciurus niger), and gray squirrel ( Sciurus carolinensis) were examined for clinal variation during the Holocene. Modern samples of all three species displayed strong east-west patterns along the western edge of the eastern deciduous forest: S. floridanus and S. niger decrease and S. carolinensis increases in size. Archeological samples of S. carolinensis from Rodgers Shelter (23BE125), Benton County, Missouri, and Graham Cave (23MT2), Montgomery County, Missouri, indicated an increase in size from early to middle Holocene. Sylvilagus floridanus from Rodgers Shelter decreased in size from early to middle Holocene and then increased during the late Holocene to modern proportions. A literature survey reveals that clinal variation is a common phenomenon among modern homeotherms. In introduced species, clinal variation has developed after relatively few generations, indicating rapid adaptations to environmental conditions; often winter climatic variables are implicated. Morphological variation in the study species during the Holocene is interpreted as a response to changing climates. Studies of morphological clines may lead to another valuable data source for reconstructing past ecologies.

Origin and Maintenance of Clinal Variation in Nuphar (Nymphaeaceae)

Natural populations ofNuphar luteum in the coastal plain of North Carolina present a morphological and geographical cline with ssp.macrophyllum representing one extreme and ssp.sagittifolium the other. Plants of intermediate morphology exist in abundance in areas between the two extremes. Data involving 1) analysis of variance within and among clones, 2) analysis of variation in seedling populations cultured under uniform conditions, 3) analysis of seedlings produced through artificial hybridization, and 4) reciprocal transplantation indicate the morphological cline to be genetically controlled with the leaf length-width ratio being the most constant and reliable feature for identification. An analysis of competitive and non-competitive populations within one pond provides evidence that natural selection of specific mature clones occurs under competitive conditions. Likewise, an analysis of the seedling potential of adult populations along the cline indicates that, in nature, a strong selection mechanism is operative during seed germination and/or seedling establishment. Chemical analyses of the aquatic habitats along the cline show several gradients to be correlated with the morphological cline. By extrapolation of the data, the variability evident inNuphar, not only among subspecies but also among populations and clones, is considered to result from adaptation to environmental selection pressures.

Causes and Significance of Morphological Variation in Fishes

MANY ICHTHYOLOGISTS have tended to assume extreme positions as to whether observed differences between populations of a given species of fish are caused by genetic differences or merely by dissimilarities of environments. Early workers were inclined to regard differences, especially countable ones, as indicative of genetic divergence alone. Many of these forms were formally recognized as subspecies, or species (examples are cited in Jensen, 1944). The weight of thought swung the opposite direction after the discovery that characters could be altered through manipulation of the environment. Now ichthyologists commonly assume differences between populations of a species are environmentally induced unless a genetic basis can be demonstrated experimentally. The bulk of recent literature, however, is concerned primarily with the elucidation of the effect of environment on a given genetic stock. The problem of genetic divergence between populations of a fish species, or within a single population, has not received as much attention and evidence along these lines, therefore, is meager (but see Gordon, 1957). It is essential to the progress of systematic ichthyology, as well as to the planning of experiments on this problem, that the nature of morphological variation in fishes be properly understood. Since the early papers by Hubbs, only Taning (1952) has reviewed this subject. Taning stressed the role of the environment in determining meristic characters, and also fostered the misleading notion of daydegrees. The primary objective of the article at hand is to put forth arguments to support what is meant to be a more balanced point of view. Morphological clines, especially for those forms which have somewhat disjunct populations, are regarded as the consequences of environmental modifications coupled with adaptive genetic changes. In the course of writing the article it also became necessary to reconsider the problem of the production of variation in fishes, particularly, meristic variation.' As a result, a refinement of Hubbs's (1926) hypothesis has been developed to account for the genesis of meristic variation in the hope it may give direction to the design of future experiments.