Mechanisms and tests for geographic clines in genetic polymorphisms

A continuous spatial gradient in visible traits, which is called a cline, is a natural model system for quantifying the effects of selection and stochastic factors and their relative importance. Geographic clines in phenotypic traits also provide key insights into the evolutionary forces that lead to allopatric speciation in nature. Thus, the underlying mechanisms for establishing clines and their evolutionary consequences remain key topics in evolutionary biology. However, few experimental studies have confirmed the underlying mechanisms of geographic clines in morph/allele frequencies, probably because of the lack of understanding of the theoretical basis of geographic clines in polymorphisms and/or suitable comprehensive tests. Thus, I present a general review of the underlying mechanisms for establishing geographic clines in polymorphisms. I also provide a case study using the female dimorphic damselfly Ischnura senegalensis to illustrate a strategy that confirms the underlying mechanisms of geographic clines in morph frequencies. This review may help to address geographic clines in other polymorphic systems, as well as contribute to a comprehensive understanding of geographic clines in quantitative traits, and thus, their evolutionary consequences in nature.     

PLANT RESPONSES TO CLIMATE IN THE CAPE FLORISTIC REGION OF SOUTH AFRICA: EVIDENCE FOR ADAPTIVE DIFFERENTIATION IN THE PROTEACEAE

Local adaptation along environmental gradients may drive plant species radiation within the Cape Floristic Region (CFR), yet few studies examine the role of ecologically based divergent selection within CFR clades. In this study, we ask whether populations within the monophyletic white protea clade (Protea section Exsertae, Proteaceae) differ in key functional traits along environmental gradients and whether differences are consistent with local adaptation. Using seven taxa, we measured trait–environment associations and selection gradients across 35 populations of wild adults and their offspring grown in two common gardens. Focal traits were leaf size and shape, specific leaf area (SLA), stomatal density, growth, and photosynthetic rate. Analyses on wild and common garden plants revealed heritable trait differences that were associated with gradients in rainfall seasonality, drought stress, cold stress, and less frequently, soil fertility. Divergent selection between gardens generally matched trait–environment correlations and literature‐based predictions, yet variation in selection regimes among wild populations generally did not. Thus, selection via seedling survival may promote gradient‐wide differences in SLA and leaf area more than does selection via adult fecundity. By focusing on the traits, life stages, and environmental clines that drive divergent selection, our study uniquely demonstrates adaptive differentiation among plant populations in the CFR.     

Wing trait–inversion associations in Drosophila subobscura can be generalized within continents,but may change through time

Clinal variation is one of the most emblematic examples of the action of natural selection at a wide geographical range. In Drosophila subobscura, parallel clines in body size and inversions, but not in wing shape, were found in Europe and South and North America. Previous work has shown that a bottleneck effect might be largely responsible for differences in wing trait–inversion association between one European and one South American population. One question still unaddressed is whether the associations found before are present across other populations of the European and South American clines. Another open question is whether evolutionary dynamics in a new environment can lead to relevant changes in wing traits–inversion association. To analyse geographical variation in these associations, we characterized three recently laboratory founded D. subobscura populations from both the European and South American latitudinal clines. To address temporal variation, we also characterized the association at a later generation in the European populations. We found that wing size and shape associations can be generalized across populations of the same continent, but may change through time for wing size. The observed temporal changes are probably due to changes in the genetic content of inversions, derived from adaptation to the new, laboratory environment. Finally, we show that it is not possible to predict clinal variation from intrapopulation associations. All in all this suggests that, at least in the present, wing traits–inversion associations are not responsible for the maintenance of the latitudinal clines in wing shape and size.     

Lack of genetic variation prevents adaptation at the geographic range margin in a damselfly

What limits a species' distribution in the absence of physical barriers? Genetic load due to asymmetric gene flow and the absence of genetic variation due to lack of gene flow are hypothesized to constrain adaptation to novel environments in marginal populations, preventing range expansion. Here, we examined the genetic structure and geographic variation in morphological traits in two damselflies (Ischnura asiatica and I. senegalensis) along a latitudinal gradient in Japan, which is the distribution centre of I. asiatica and the northern limit of I. senegalensis. Genomewide genetic analyses found a loss of genetic diversity at the edge of distribution in I. senegalensis but consistently high diversity in I. asiatica. Gene flow was asymmetric in a south–north direction in both species. Although body size and wing loading showed decreasing latitudinal clines (smaller in north) in I. asiatica in Japan, increasing latitudinal clines (larger in north) in these phenotypic markers were observed in I. senegalensis, particularly near the northern boundary, which coincided well with the location where genetic diversity began a sharp decline. In ectothermic animals, increasing latitudinal cline in these traits was suggested to be established when they failed to adapt to thermal gradient. Therefore, our findings support the possibility that a lack of genetic variation rather than geneflow swamping is responsible for the constraint of adaptation at the margin of geographic distribution.     

Trunk elongation and ontogenetic changes in the axial skeleton of Triturus newts

Body elongation in vertebrates can be achieved by lengthening of the vertebrae or by an increase in their number. In salamanders, longer bodies are mostly associated with greater numbers of vertebrae in the trunk or tail region. However, studies on the relative contribution of the length of single vertebra to body elongation are lacking. In this study, we focus on evolutionary and ontogenetic changes in differentiation of the trunk vertebrae and the relative contribution of individual vertebrae to trunk lengthening in Triturus newts, a monophyletic group of salamanders that shows remarkable disparity in body shape. We compared juveniles and adults of the most elongated T. dobrogicus , which has 17 trunk vertebrae, with juveniles and adults of two closely related species (T. ivanbureschi and T. anatolicus belonging to the T. karelinii species complex) representing a stout and robust morphotype with thirteen trunk vertebrae. We show that trunk vertebrae are uniform in size at the juvenile stage of both analyzed morphotypes. In adults, the trunk vertebrae of the elongated T. dobrogicus are largely uniform, while in those of T. anatolicus , the first two vertebrae differ from the remaining trunk vertebrae. There was no difference in the relative contribution of individual vertebrae to body lengthening between species or stages. We conclude that body elongation in Triturus newts is achieved by increasing the number of vertebrae but not their length.     

ALTITUDINAL CLINAL VARIATION IN WING SIZE AND SHAPE IN AFRICAN DROSOPHILA MELANOGASTER: ONE CLINE OR MANY?

Geographical patterns of morphological variation have been useful in addressing hypotheses about environmental adaptation. In particular, latitudinal clines in phenotypes have been studied in a number of Drosophila species. Some environmental conditions along latitudinal clines—for example, temperature—also vary along altitudinal clines, but these have been studied infrequently and it remains unclear whether these environmental factors are similar enough for convergence or parallel evolution. Most clinal studies in Drosophila have dealt exclusively with univariate phenotypes, allowing for the detection of clinal relationships, but not for estimating the directions of covariation between them. We measured variation in wing shape and size in D. melanogaster derived from populations at varying altitudes and latitudes across sub‐Saharan Africa. Geometric morphometrics allows us to compare shape changes associated with latitude and altitude, and manipulating rearing temperature allows us to quantify the extent to which thermal plasticity recapitulates clinal effects. Comparing effect vectors demonstrates that altitude, latitude, and temperature are only partly associated, and that the altitudinal shape effect may differ between Eastern and Western Africa. Our results suggest that selection responsible for these phenotypic clines may be more complex than just thermal adaptation.     

Geographic clines in wing morphology relate to colonization history in New World but not Old World populations of yellow dung flies

Geographic clines offer insights about putative targets and agents of natural selection as well as tempo and mode of adaptation. However, demographic processes can lead to clines that are indistinguishable from adaptive divergence. Using the widespread yellow dung fly Scathophaga stercoraria (Diptera: Scathophagidae), we examine quantitative genetic differentiation (Q_ST) of wing shape across North America, Europe, and Japan, and compare this differentiation with that of ten microsatellites (F_ST). Morphometric analyses of 28 populations reared at three temperatures revealed significant thermal plasticity, sexual dimorphism, and geographic differentiation in wing shape. In North America morphological differentiation followed the decline in microsatellite variability along the presumed route of recent colonization from the southeast to the northwest. Across Europe, where S. stercoraria presumably existed for much longer time and where no molecular pattern of isolation by distance was evident, clinal variation was less pronounced despite significant morphological differentiation (Q_ST>F_ST). Shape vector comparisons further indicate that thermal plasticity (hot‐to‐cold) does not mirror patterns of latitudinal divergence (south‐to‐north), as might have been expected under a scenario with temperature as the major agent of selection. Our findings illustrate the importance of detailed phylogeographic information when interpreting geographic clines of dispersal traits in an adaptive evolutionary framework.     

Reinforcement shapes clines in female mate discrimination in Drosophila subquinaria

Reinforcement of species boundaries may alter mate recognition in a way that also affects patterns of mate preference among conspecific populations. In the fly Drosophila subquinaria, females sympatric with the closely related species D. recens reject mating with heterospecific males as well as with conspecific males from allopatric populations. Here, we assess geographic variation in behavioral isolation within and among populations of D. subquinaria and use cline theory to understand patterns of selection on reinforced discrimination and its consequences for sexual isolation within species. We find that selection has fixed rejection of D. recens males in sympatry, while significant genetic variation in this behavior occurs within allopatric populations. In conspecific matings sexual isolation is also asymmetric and stronger in populations that are sympatric with D. recens. The clines in behavioral discrimination within and between species are similar in shape and are maintained by strong selection in the face of gene flow, and we show that some of their genetic basis may be either shared or linked. Thus, while reinforcement can drive extremely strong phenotypic divergence, the long‐term consequences for incipient speciation depend on gene flow, genetic linkage of discrimination traits, and the cost of these behaviors in allopatry.     

GENETICAL POPULATION STRUCTURE IN PLANTS: GENE FLOW BETWEEN DIPLOID SEXUAL AND TRIPLOID ASEXUAL DANDELIONS (TARAXACUM SECTION RUDERALIA)

Levels and distribution of genetic variation were studied in central and western European populations of Taraxacum section Ruderalia containing differing mixtures of sexual diploid and asexual triploid plants. All sexual populations were panmictic with their variation partitioned mainly among populations. Genotypic diversity in triploid samples was very high with few clones widespread and many clones restricted to one or a few populations. Extensive amounts of gene (pollen) flow between the diploid and triploid components of a population were inferred from the following data: (1) the two ploidy levels share all major allozyme polymorphisms; (2) the intrapopulational homogeneity in genic variation between diploids and triploids contrasts strongly with the geographic differentiation at each ploidy level separately; (3) population-unique alleles simultaneously occur at the two ploidy levels; (4) not only sexuals but also asexuals generally simulate Hardy-Weinberg expectations. Most likely, intrapopulational gene exchange occurs bidirectionally by mechanisms such as reductional pollen meiosis in apomictic plants, facultative apomixis, and formation of unreduced gametes in sexuals. Thus, diploid and triploid Taraxacum section Ruderalia are less genetically isolated than has previously been supposed and probably form a cohesive evolutionary unit with the level at which gene pools are shared differing by population.     

The evolution of the Streptanthus glandulosus complex (Cruciferae): genetic divergence and gene flow in serpentine endemics

An analysis of the levels and distribution of allozyme variation in the Streptanthus glandulosus species complex was undertaken to test paradigms of speciation processes in the context of serpentine endemism. Electrophoretic analysis of 21 putative enzyme loci in 1,224 individuals representing 56 populations revealed extensive intrapopulational variation and interpopulational divergence. Estimates of gene flow among populations within taxa are typically lower than is theoretically needed to counteract the effects of genetic drift (i.e., Nm values are below 0.5), suggesting that drift may play a significant role in the evolution of the complex. A cluster analysis of genetic identities between populations using UPGMA demonstrates geographically structured groupings, some of which include neighboring populations of different taxa. Moreover, the genetic identity between two populations is correlated with the distance by which they are separated. The results are consistent with a hypothesis of a paleoendemic origin of the complex. The ancestor of the complex (perhaps S. glandulosus ssp. glandulosus) probably was formerly distributed more continuously across serpentine and nonserpentine habitat throughout its range. Elimination of the nonserpentine populations allowed regional and population-level divergence, following a model of geographic speciation.     

MOLECULAR PHYLOGENETICS AND EVOLUTION OF SEXUAL DICHROMATISM AMONG POPULATIONS OF THE YARROW'S SPINY LIZARD (SCELOPORUS JARROVII)

Understanding evolution of geographic variation in sexually dimorphic traits is critical for understanding the role that sexual selection may play in speciation. We performed a phylogenetic analysis of geographic variation in sexual dichromatism in the Yarrow's spiny lizard (Sceloporus jarrovii), a taxon that exhibits remarkable diversity in male coloration among populations (e.g., black, red, green, yellow, blue, brown). An mtDNA phylogeny based on approximately 880 bp from the 12S ribosomal RNA gene and 890 bp from the ND4 gene was reconstructed for 30 populations of S. jarrovii and eight other species of the torquatus species group using maximum-likelihood and parsimony methods. The phylogeny suggests that S. jarrovii consists of at least five evolutionary species, none of which are sister taxa. Although intraspecific diversity in male coloration is less than indicated by previous taxonomy, two species formerly referred to as S. jarrovii exhibit impressive geographic variation in sexual dichromatism. In one of these species, the phylogeny shows the independent evolution of a distinctive blue color morph in different parts of the species range. This pattern suggests that sexual selection may lead to striking phenotypic divergence among conspecific populations and striking convergence. Results also demonstrate the importance of a phylogenetic perspective in studies of evolutionary processes within nominal species and the problematic nature of “polytypic” species recognized under the biological species concept.     

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.     

SYSTEMATIC SIGNIFICANCE OF CHROMOSOME NUMBERS IN ACMELLA (ASTERACEAE)

Chromosome counts are reported for 372 individuals from 202 populations in 26 taxa of Acmella (Asteraceae: Heliantheae). Chromosome numbers for 15 taxa are first reports. A review of previous counts and the new reports supports a basic chromosome number of 13 for the genus. The results show that polyploidy, sometimes accompanied by hybridization and asexual reproduction, is widespread in Acmella and has contributed to the taxonomic difficulties in the genus. These factors have produced a variable polyploid pillar complex in sect. Acmella. In one taxon in this complex, A. oppositifolia var. oppositifolia, intrataxon and even intrapopulational chromosomal variation has been detected. Morphological studies in conjunction with observations of meiotic pairing suggest that most polyploids are alloploid in origin. The occurrence of polyploidy in 16 of the 27 taxa known chromosomally emphasizes the important role this process has had in speciation within Acmella. Although intrataxon chromosomal variation has limited the taxonomic utility of chromosome numbers, a few examples are presented in which these data have been valuable for separating some pairs of closely related taxa (A. decumbens var. affinis from var. decumbens and A. poliolepidica from A. oppositifolia).     

Gene flow between populations of two invertebrates in springs

1. Using allozymes, we analysed genetic structure of the freshwater gastropod Bythinella dunkeri and the freshwater flatworm Crenobia alpina. The two species are habitat specialists, living almost exclusively in springs. The sampled area in Hesse (Germany) covers a spatial scale of 20 km and includes two river drainages. From the biology of the two species we expected little dispersal along rivers. However, the possibility exists that groundwater provide suitable pathways for dispersal. 2. In B. dunkeri heterozygosity decreased from west to east. For some alleles we found clines in this geographic direction. These clines generated a positive correlation between geographic distance and genetic differentiation. Furthermore patterns of genetic variation within populations suggested that populations may have been faced with bottlenecks and founder effects. If populations are not in population genetic equilibrium, such founder effects would also explain the rather high amount of genetic differentiation between populations (10%). 3. For C. alpina the mean number of alleles decreased with increasing isolation of populations. Genetic differentiation between populations contributed 19% to the total genetic variation. Genetic differentiation was not correlated to geographic distance, but compared with B. dunkeri variability of pairwise differentiation between pairs of populations was higher in C. alpina. 4. Overall B. dunkeri appears to be a fairly good disperser, which may use groundwater as dispersal pathway. Furthermore populations seem to be not in equilibrium. In contrast C. alpina forms rather isolated populations with little dispersal between springs and groundwater seems to play no important role for dispersal.     

Natural selection drives the fine-scale divergence of a coevolutionary arms race involving a long-mouthed weevil and its obligate host plant

Background

One of the major recent advances in evolutionary biology is the recognition that evolutionary interactions between species are substantially differentiated among geographic populations. To date, several authors have revealed natural selection pressures mediating the geographically-divergent processes of coevolution. How local, then, is the geographic structuring of natural selection in coevolutionary systems?

Results

I examined the spatial scale of a "geographic selection mosaic," focusing on a system involving a seed-predatory insect, the camellia weevil (Curculio camelliae), and its host plant, the Japanese camellia (Camellia japonica). In this system, female weevils excavate camellia fruits with their extremely-long mouthparts to lay eggs into seeds, while camellia seeds are protected by thick pericarps. Quantitative evaluation of natural selection demonstrated that thicker camellia pericarps are significantly favored in some, but not all, populations within a small island (Yakushima Island, Japan; diameter ca. 30 km). At the extreme, camellia populations separated by only several kilometers were subject to different selection pressures. Interestingly, in a population with the thickest pericarps, camellia individuals with intermediate pericarp thickness had relatively high fitness when the potential costs of producing thick pericarps were considered. Also importantly, some parameters of the weevil - camellia interaction such as the severity of seed infestation showed clines along temperature, suggesting the effects of climate on the fine-scale geographic differentiation of the coevolutionary processes.

Conclusion

These results show that natural selection can drive the geographic differentiation of interspecific interactions at surprisingly small spatial scales. Future studies should reveal the evolutionary/ecological outcomes of the "fine scale geographic mosaics" in biological communities.     

Broad Concordance in the Spatial Distribution of Adaptive and Neutral Genetic Variation across an Elevational Gradient in Deer Mice

When species are continuously distributed across environmental gradients, the relative strength of selection and gene flow shape spatial patterns of genetic variation, potentially leading to variable levels of differentiation across loci. Determining whether adaptive genetic variation tends to be structured differently than neutral variation along environmental gradients is an open and important question in evolutionary genetics. We performed exome-wide population genomic analysis on deer mice sampled along an elevational gradient of nearly 4,000 m of vertical relief. Using a combination of selection scans, genotype−environment associations, and geographic cline analyses, we found that a large proportion of the exome has experienced a history of altitude-related selection. Elevational clines for nearly 30% of these putatively adaptive loci were shifted significantly up- or downslope of clines for loci that did not bear similar signatures of selection. Many of these selection targets can be plausibly linked to known phenotypic differences between highland and lowland deer mice, although the vast majority of these candidates have not been reported in other studies of highland taxa. Together, these results suggest new hypotheses about the genetic basis of physiological adaptation to high altitude, and the spatial distribution of adaptive genetic variation along environmental gradients.     

Evolution of skull and body shape in Triturus newts reconstructed from three‐dimensional morphometric data and phylogeny

To explore the relationship between morphological change and species diversification, we reconstructed the evolutionary changes in skull size, skull shape, and body elongation in a monophyletic group of eight species that make up salamander genus Triturus. Their well‐studied phylogenetic relationships and the marked difference in ecological preferences among five species groups makes this genus an excellent model system for the study of morphological evolution. The study involved three‐dimensional imagery of the skull and the number of trunk vertebrae, in material that represents the morphological, spatial, and molecular diversity of the genus. Morphological change largely followed the pattern of descent. The reconstruction of ancestral skull shape indicated that morphological change was mostly confined to two episodes, corresponding to the ancestral lineage that all crested newts have in common and the Triturus dobrogicus lineage. When corrected for common descent, evolution of skull shape was correlated to change in skull size. Also, skull size and shape, as well as body shape, as inferred from the number of trunk vertebrae, were correlated, indicating a marked impact of species' ecological preferences on morphological evolution, accompanied by a series of niche shifts, with the most pronounced one in the T. dobrogicus lineage. The presence of phylogenetic signal and correlated evolutionary changes in skull and body shape suggested complex interplay of niche shifts, natural selection, and constraints by a common developmental system. © 2014 The Linnean Society of London, Biological Journal of the Linnean Society, 2014, 113 , 243–255.     

Steep,coincident, and concordant clines in mitochondrial and nuclear‐encoded genes in a hybrid zone between subspecies of Atlantic killifish,Fundulus heteroclitus

Steep genetic clines resulting from recent secondary contact between previously isolated taxa can either gradually erode over time or be stabilized by factors such as ecological selection or selection against hybrids. We used patterns of variation in 30 nuclear and two mitochondrial SNPs to examine the factors that could be involved in stabilizing clines across a hybrid zone between two subspecies of the Atlantic killifish, Fundulus heteroclitus. Increased heterozygote deficit and cytonuclear disequilibrium in populations near the center of the mtDNA cline suggest that some form of reproductive isolation such as assortative mating or selection against hybrids may be acting in this hybrid zone. However, only a small number of loci exhibited these signatures, suggesting locus‐specific, rather than genomewide, factors. Fourteen of the 32 loci surveyed had cline widths inconsistent with neutral expectations, with two SNPs in the mitochondrial genome exhibiting the steepest clines. Seven of the 12 putatively non‐neutral nuclear clines were for SNPs in genes related to oxidative metabolism. Among these putatively non‐neutral nuclear clines, SNPs in two nuclear‐encoded mitochondrial genes (SLC25A3 and HDDC2), as well as SNPs in the myoglobin, 40S ribosomal protein S17, and actin‐binding LIM protein genes, had clines that were coincident and concordant with the mitochondrial clines. When hybrid index was calculated using this subset of loci, the frequency distribution of hybrid indices for a population located at the mtDNA cline center was non‐unimodal, suggesting selection against advanced‐generation hybrids, possibly due to effects on processes involved in oxidative metabolism.     

A DEVELOPMENTAL CONSTRAINT IN CERION,WITH COMMENTS ON THE DEFINITION AND INTERPRETATION OF CONSTRAINT IN EVOLUTION

Since orthodox evolutionary theory is functionalist, constraints attain their most important positive meaning as channels of change imposed by historical and formal determinants, rather than by immediate natural selection. Since ontogeny is the usual locus of expression for these determinants, developmental constraint is an appropriate, general term. A particular developmental constraint in Cerion, most variable of West Indian land snails, stands out for two reasons: 1) it is simply and inexorably defined as a consequence both of formal principles (coiling of tube about axis) and of historical contingencies in Cerion's invariant allometry of growth; 2) it is pervasive in its influence, underlying major patterns of variation in every Cerion study I have ever undertaken. I refer to this pattern as the “jigsaw constraint.” When whorls are large and final size is limited, adult shells must grow fewer whorls. In Cerion, this obvious fact is promoted from trivial to important because complex allometries impose a substantial set of further consequences for form upon this basic trade-off of whorl size and whorl number. I show that this complex of consequences dominates patterns of natural variation in Cerion at all levels (among shells within samples, between samples in the geographic variation of single species, and between species in multitaxon faunas). It also sets patterns of hybridization between taxa. This paper is primarily a compendium of such examples. It is designed to illustrate the importance of this constraint by the fundamental criterion of relative frequency.