Physiological Plasticity to Water Flow Habitat in the Damselfish,Acanthochromis polyacanthus: Linking Phenotype to Performance

The relationships among animal form, function and performance are complex, and vary across environments. Therefore, it can be difficult to identify morphological and/or physiological traits responsible for enhancing performance in a given habitat. In fishes, differences in swimming performance across water flow gradients are related to morphological variation among and within species. However, physiological traits related to performance have been less well studied. We experimentally reared juvenile damselfish, Acanthochromis polyacanthus, under different water flow regimes to test 1) whether aspects of swimming physiology and morphology show plastic responses to water flow, 2) whether trait divergence correlates with swimming performance and 3) whether flow environment relates to performance differences observed in wild fish. We found that maximum metabolic rate, aerobic scope and blood haematocrit were higher in wave-reared fish compared to fish reared in low water flow. However, pectoral fin shape, which tends to correlate with sustained swimming performance, did not differ between rearing treatments or collection sites. Maximum metabolic rate was the best overall predictor of individual swimming performance; fin shape and fish total length were 3.3 and 3.7 times less likely than maximum metabolic rate to explain differences in critical swimming speed. Performance differences induced in fish reared in different flow environments were less pronounced than in wild fish but similar in direction. Our results suggest that exposure to water motion induces plastic physiological changes which enhance swimming performance in A. polyacanthus. Thus, functional relationships between fish morphology and performance across flow habitats should also consider differences in physiology.     

Hydrologic regimes as potential drivers of morphologic divergence in fish

Fishes often exhibit phenotypic divergence across gradients of abiotic and biotic selective pressures. In streams, many of the known selective pressures driving phenotypic differentiation are largely influenced by hydrologic regimes. Because flow regimes drive so many attributes of lotic systems, we hypothesized fish exhibit phenotypic divergence among streams with different flow regimes. We used a comparative field study to investigate the morphological divergence of Campostoma anomalom (central stonerollers) among streams characterized by highly variable, intermittent flow regimes and streams characterized by relatively stable, groundwater flow regimes. We also conducted a mesocosm experiment to compare the plastic effects of one component of flow regimes, water velocity, on morphology of fish from different flow regimes. We observed differences in shape between flow regimes likely driven by differences in allometric growth patterns. Although we observed differences in morphology across flow regimes in the field, C. anomalum did not exhibit morphologic plasticity in response to water velocity alone. This study contributes to the understanding of how complex environmental factors drive phenotypic divergence and may provide insight into the evolutionary consequences of disrupting natural hydrologic patterns, which are increasingly threatened by climate change and anthropogenic alterations.     

Integrating environmental variation, predation pressure, phenotypic plasticity and locomotor performance

The Wujiang River, a tributary of the Three Gorges Reservoir, has many dams along its length. These dams alter the river's natural habitat and produce various flow regimes and degrees of predator stress. To test whether the swimming performance and external body shape of pale chub (Zacco platypus) have changed as a result of alterations in the flow regime and predator conditions, we measured the steady (U _crit) and unsteady (fast-start) swimming performances and morphological characteristics of fish collected from different sites along the Wujiang River. We also calculated the maximum respiratory capacity and cost of transport (COT). We demonstrated significant differences in swimming performance and morphological traits among the sampling sites. Steady swimming performance was positively correlated with water velocity and negatively correlated with the abundance of predators, whereas unsteady swimming performance was negatively correlated with water velocity. The body shape was significantly correlated with both swimming performance and ecological parameters. These findings suggested that selection pressure on swimming performance results in a higher U _crit and a more streamlined body shape in fast-flow and (or) in habitats with low predator stress and subsequently results in a lower COT. These characteristics were accompanied by a poorer fast-start performance than that of the fish from the slow-flow and (or) high-predator habitats. The divergence in U _crit may also be due in part to variation in respiratory capacity.     

The influence of gene flow and drift on genetic and phenotypic divergence in two species of Zosterops in Vanuatu

Colonization of an archipelago sets the stage for adaptive radiation. However, some archipelagos are home to spectacular radiations, while others have much lower levels of diversification. The amount of gene flow among allopatric populations is one factor proposed to contribute to this variation. In island colonizing birds, selection for reduced dispersal ability is predicted to produce changing patterns of regional population genetic structure as gene flow-dominated systems give way to drift-mediated divergence. If this transition is important in facilitating phenotypic divergence, levels of genetic and phenotypic divergence should be associated. We consider population genetic structure and phenotypic divergence among two co-distributed, congeneric (Genus: Zosterops) bird species inhabiting the Vanuatu archipelago. The more recent colonist, Z. lateralis, exhibits genetic patterns consistent with a strong influence of distance-mediated gene flow. However, complex patterns of asymmetrical gene flow indicate variation in dispersal ability or inclination among populations. The endemic species, Z. flavifrons, shows only a partial transition towards a drift-mediated system, despite a long evolutionary history on the archipelago. We find no strong evidence that gene flow constrains phenotypic divergence in either species, suggesting that levels of inter-island gene flow do not explain the absence of a radiation across this archipelago.     

Repeatability of Habitat-Associated Divergence in Shell Shape of Turtles

Repeated patterns of phenotypic divergence between environments across disparate taxa provide strong evidence for the generation of adaptive phenotypes. Flow velocity is an important selective force in aquatic habitats; however, among vertebrates, the study of its effects on morphology has been limited almost exclusively to fully-aquatic bony fishes. We tested whether three confamilial species of semi-aquatic freshwater turtle (family Emydidae: Graptemys pseudogeographica, Graptemys nigrinoda, and Pseudemys concinna) displayed similar patterns of phenotypic divergence in carapace shape between fast- and slow-flowing aquatic environments. We used (1) geometric morphometrics to quantify shell shape, (2) multivariate analysis of variance to test the effects of species, sex, and flow, and (3) phenotypic trajectory analysis to compare patterns of divergence for six species-sex groups. We found significant effects on shell shape for all factors. In general, ecomorphs from fast-flowing habitats had flatter shells than those from slow-flowing habitats. Furthermore, results of trajectory analysis indicate that the degree to which, as well as the way in which, ecomorphs differed were concordant across all species. Our findings demonstrate that the effects of flow are not limited to fully-aquatic vertebrates, and provide evidence of the ability of flow to drive repeatable phenotypic divergence in tetrapods.     

Do different rates of gene flow underlie variation in phenotypic and phenological clines in a montane grasshopper community?

Species responses to environmental change are likely to depend on existing genetic and phenotypic variation, as well as evolutionary potential. A key challenge is to determine whether gene flow might facilitate or impede genomic divergence among populations responding to environmental change, and if emergent phenotypic variation is dependent on gene flow rates. A general expectation is that patterns of genetic differentiation in a set of codistributed species reflect differences in dispersal ability. In less dispersive species, we predict greater genetic divergence and reduced gene flow. This could lead to covariation in life‐history traits due to local adaptation, although plasticity or drift could mirror these patterns. We compare genome‐wide patterns of genetic structure in four phenotypically variable grasshopper species along a steep elevation gradient near Boulder, Colorado, and test the hypothesis that genomic differentiation is greater in short‐winged grasshopper species, and statistically associated with variation in growth, reproductive, and physiological traits along this gradient. In addition, we estimate rates of gene flow under competing demographic models, as well as potential gene flow through surveys of phenological overlap among populations within a species. All species exhibit genetic structure along the elevation gradient and limited gene flow. The most pronounced genetic divergence appears in short‐winged (less dispersive) species, which also exhibit less phenological overlap among populations. A high‐elevation population of the most widespread species, Melanoplus sanguinipes, appears to be a sink population derived from low elevation populations. While dispersal ability has a clear connection to the genetic structure in different species, genetic distance does not predict growth, reproductive, or physiological trait variation in any species, requiring further investigation to clearly link phenotypic divergence to local adaptation.     

The interaction of resource use and gene flow on the phenotypic divergence of benthic and pelagic morphs of Icelandic Arctic charr (Salvelinus alpinus)

Conceptual models of adaptive divergence and ecological speciation in sympatry predict differential resource use, phenotype–environment correlations, and reduced gene flow among diverging phenotypes. While these predictions have been assessed in past studies, connections among them have rarely been assessed collectively. We examined relationships among phenotypic, ecological, and genetic variation in Arctic charr (Salvelinus alpinus) from six Icelandic localities that have undergone varying degrees of divergence into sympatric benthic and pelagic morphs. We characterized morphological variation with geometric morphometrics, tested for differential resource use between morphs using stable isotopes, and inferred the amount of gene flow from single nucleotide polymorphisms. Analysis of stable isotopic signatures indicated that sympatric morphs showed similar difference in resource use across populations, likely arising from the common utilization of niche space within each population. Carbon isotopic signature was also a significant predictor of individual variation in body shape and size, suggesting that variation in benthic and pelagic resource use is associated with phenotypic variation. The estimated percentage of hybrids between sympatric morphs varied across populations (from 0% to 15.6%) but the majority of fish had genotypes (ancestry coefficients) characteristic of pure morphs. Despite evidence of reduced gene flow between sympatric morphs, we did not detect the expected negative relationship between divergence in resource use and gene flow. Three lakes showed the expected pattern, but morphs in the fourth showed no detectable hybridization and had relatively low differences in resource use between them. This coupled with the finding that resource use and genetic differentiation had differential effects on body shape variation across populations suggests that reproductive isolation maintains phenotypic divergence between benthic and pelagic morphs when the effects of resource use are relatively low. Our ability to assess relationships between phenotype, ecology, and genetics deepens our understanding of the processes underlying adaptive divergence in sympatry.     

Paternal Genetic Effects on Offspring Swimming Performance Vary with Age of Juvenile Chinook Salmon, Oncorhynchus tshawytscha

While fish swimming behaviour has been extensively studied, the parental genetic basis of this critical behaviour has been rarely examined, especially past the earliest stages of development. We used a quantitative genetic breeding design to measure the critical swimming speed (U-crit) of offspring (15 and 18 weeks post-hatch) from 36 families of Chinook salmon (Oncorhynchus tshawytscha), a species with a nonresource-based mating system. We investigated the roles of dam, sire, and dam × sire on offspring U-crit, and estimated contributions of additive and nonadditive genetic effects and maternal effects to phenotypic variation in U-crit at both ages. We also used existing ‘high-survival’ and ‘low-survival’ lines of Chinook to determine if these two lines show differences in U-crit. At 15 weeks, there were no significant genetic effects, but at 18 weeks there were significant sire effects. Furthermore, additive genetic effects increased from 26 to 100 % from 15 to 18 weeks post-hatch. The two survival lines also showed differences in U-crit at 18 weeks post-hatch, with higher U-crit associated with “high-survival” sires. Collectively, the present study provides evidence for increasing importance of paternal identity (additive genetic variation) on swimming as juvenile offspring age. Given that mortality is high in young Pacific salmon and swimming ability is crucial, the sire effects could potentially shape survival though subsequent developmental stages. The change in the magnitude of effects in the present study indicates that future research should investigate genetic effects across multiple stages for better understanding of how phenotypic traits could respond to selection.     

Shape variation in a benthic stream fish across flow regimes

Evolution of fish body shapes in flowing and non-flowing waters have been examined for several species. Flowing water can select for fish body shapes that increase steady swimming efficiency, whereas non-flowing water can favor shapes that increase unsteady swimming efficiency. Benthic stream fishes often use areas near the substrate that exhibit reduced or turbulent flow, thus it is unclear which swimming forms would be favored in such environments, and how shape might change across flow regimes. To test the relationship between fish body shape and flow regime in a benthic stream fish, we used geometric morphometric techniques to characterize lateral body shape in mountain sucker (Catostomus platyrhynchus) across flow rates, using stream gradient as an indicator of stream flow. Mountain suckers from low-flow environments were more streamlined, consistent with steady swimming body shapes, whereas mountain suckers from high flows had deeper bodies, consistent with unsteady swimming body shapes. In addition, smaller individuals tended to have more robust body shapes. These patterns are opposite to those predicted for stream fishes in the mid-water column. The benthic stream environment represents a distinct selective environment for fish shape that does not appear to conform to the simple dichotomy of flowing versus non-flowing water.     

Morphological plasticity in a native freshwater fish from semiarid Australia in response to variable water flows

In fishes, alterations to the natural flow regime are associated with divergence in body shape morphology compared with individuals from unaltered habitats. However, it is unclear whether this morphological divergence is attributable to evolutionary responses to modified flows, or is a result of phenotypic plasticity. Fishes inhabiting arid regions are ideal candidates for studying morphological plasticity as they are frequently exposed to extreme natural hydrological variability. We examined the effect of early exposure to flows on the development of body shape morphology in the western rainbowfish (Melanotaenia australis), a freshwater fish that is native to semiarid northwest Australia. Wild fish were collected from a region (the Hamersley Ranges) where fish in some habitats are subject to altered water flows due to mining activity. The offspring of wild‐caught fish were reared in replicated fast‐flow or slow‐flow channels, and geometric morphometric analyses were used to evaluate variation in fish body shape following 3, 6, 9, and 12 months of exposure. Water flows influenced fish morphology after 6 and 9 months of flow exposure, with fish in fast‐flow environments displaying a more robust body shape than those in slow‐flow habitats. No effect of flow exposure was observed at 3 and 12 months. Fishes also showed significant morphological variation within flow treatments, perhaps due to subtle differences in water flow among the replicate channels. Our findings suggest that early exposure to water flows can induce shifts in body shape morphology in arid zone freshwater fishes. Morphological plasticity may act to buffer arid zone populations from the impacts of anthropogenic activities, but further studies are required to link body shape plasticity with behavioral performance in habitats with modified flows.     

Phenotypic stability in scalar calcium of freshwater fish across a wide range of aqueous calcium availability in nature

Spatial environmental gradients can promote adaptive differences among conspecific populations as a result of local adaptation or phenotypic plasticity. Such divergence can be opposed by various constraints, including gene flow, limited genetic variation, temporal fluctuations, or developmental constraints. We focus on the constraint that can be imposed when some populations are found in locations characterized by low levels of an essential nutrient. We use scales of wild fish to investigate phenotypic effects of spatial variation in a potentially limiting nutrient—calcium. If scale calcium (we use “scalar” calcium for consistency with the physiology literature) simply reflects environmental calcium availability, we expect higher levels of scalar calcium in fish from calcium‐rich water, compared to fish from calcium‐poor water. To consider this “passive response” scenario, we analyzed scalar calcium concentrations from three native fish species (Lepomis gibbosus, Percina caprodes, and Perca flavescens) collected at multiple sites across a dissolved calcium gradient in the Upper St. Lawrence River. Contradicting the “passive response" scenario, we did not detect strong or consistent relationships between scalar calcium and water calcium. Instead, for a given proportional increase in water calcium across the wide environmental gradient, the corresponding proportional change in scalar calcium was much smaller. We thus favor the alternative “active homeostasis” scenario, wherein fish from calcium‐poor water are better able to uptake, mobilize, and deposit calcium than are fish from calcium‐rich water. We further highlight the importance of studying functional traits, such as scales, in their natural setting as opposed to only laboratory studies.     

Predictability of phenotypic differentiation across flow regimes in fishes

Fish inhabit environments greatly varying in intensity of water velocity, and these flow regimes are generally believed to be of major evolutionary significance. To what extent does water flow drive repeatable and predictable phenotypic differentiation? Although many investigators have examined phenotypic variation across flow gradients in fishes, no clear consensus regarding the nature of water velocity's effects on phenotypic diversity has yet emerged. Here, I describe a generalized model that produces testable hypotheses of morphological and locomotor differentiation between flow regimes in fishes. The model combines biomechanical information (describing how fish morphology determines locomotor abilities) with ecological information (describing how locomotor performance influences fitness) to yield predictions of divergent natural selection and phenotypic differentiation between low-flow and high-flow environments. To test the model's predictions of phenotypic differentiation, I synthesized the existing literature and conducted a meta-analysis. Based on results gathered from 80 studies, providing 115 tests of predictions, the model produced some accurate results across both intraspecific and interspecific scales, as differences in body shape, caudal fin shape, and steady-swimming performance strongly matched predictions. These results suggest that water flow drives predictable phenotypic variation in disparate groups of fish based on a common, generalized model, and that microevolutionary processes might often scale up to generate broader, interspecific patterns. However, too few studies have examined differentiation in body stiffness, muscle architecture, or unsteady-swimming performance to draw clear conclusions for those traits. The analysis revealed that, at the intraspecific scale, both genetic divergence and phenotypic plasticity play important roles in phenotypic differentiation across flow regimes, but we do not yet know the relative importance of these two sources of phenotypic variation. Moreover, while major patterns within and between species were predictable, we have little direct evidence regarding the role of water flow in driving speciation or generating broad, macroevolutionary patterns, as too few studies have addressed these topics or conducted analyses within a phylogenetic framework. Thus, flow regime does indeed drive some predictable phenotypic outcomes, but many questions remain unanswered. This study establishes a general model for predicting phenotypic differentiation across flow regimes in fishes, and should help guide future studies in fruitful directions, thereby enhancing our understanding of the predictability of phenotypic variation in nature.     

Population history and the selective landscape shape patterns of osmoregulatory trait divergence in tidal marsh Savannah sparrows (Passerculus sandwichensis)

A persistent challenge in making associations between phenotypic and environmental variation is understanding how ecological factors and demographic history interact to shape adaptive outcomes. Evaluating the degree to which conspecific populations exposed to similar environmental pressures respond in parallel provides a powerful framework for addressing this challenge. We took this comparative approach with multiple populations of Savannah sparrows (Passerculus sandwichensis) found in tidal marshes along the Pacific coast of North America. The high salinities characterizing tidal marshes select for increased osmoregulatory performance and salinity tolerance. We collected data on physiological traits associated with osmoregulatory performance from 10 tidal marsh and three freshwater-adapted interior populations to evaluate the degree of parallel divergence across populations. All traits showed differences in the magnitude of divergence, but only total evaporative water loss (TEWL) showed differences in the direction of divergence. The drivers of these differences in both the magnitude and direction of divergence varied among traits. For kidney morphology and TEWL, patterns of divergence were best explained by variation in immigration rate from interior populations. Maximum temperature was the best predictor of variation in urine excretion ability, and both gene flow and temperature contributed to variation in plasma osmolality. Finally, analysis of multitrait divergence patterns indicated that differences in the direction of divergence were best explained by population genetic structure, whereas differences in the magnitude of divergence were explained by environmental differences. Together these results show that the influences of demography and the selective landscape can manifest themselves differently across functionally integrated traits.     

Parallel variation among populations in the shell morphology between sympatric native and invasive aquatic snails

A phenotypic response, either plastic or evolved, is often required for successful invasion of novel environments. Populations of the invasive snail Potamopyrgus antipodarum have colonized a wide range of environments in the western U. S. since 1985, but the extent of plastic adjustment and evolved adaptation to local environments is largely unknown. We examined variation in shell morphology among four sites in the Snake River, Idaho, including both still-water and free-flowing river habitats and compared the variation to that of a native snail (Pyrgulopsis robusta) using geometric morphometric techniques. Using Generalized Procrustes analysis, we tested for phenotypic responses by determining (1) whether Po. antipodarum from the four locations differed in shell morphology, and (2) whether these snails exhibited corresponding shell shape variation with sympatric populations of a native snail. Both native and invasive snails exhibited similar variation in shell morphology across three of the four sites. The Canonical Variate assignment test grouped 85 % of both snail species to their rightful sample site. In addition, the Principal Component Analysis displayed similar patterns of shell variation across the four sites, indicating parallel variation in shell shape. For three of the four sites, both the native and invasive snails exhibited differences in shell shape consistent with water flow variation (still-water versus fast free-flowing river). Taken together, these results suggest that the shell shape of the invasive snail has changed either through plasticity or evolution, and that both native and invasive snail populations responded to local environmental conditions in a similar manner.     

Population structure on breeding grounds of Lake Malawi’s ‘twilight zone’ cichlid fishes

Aim Free‐ranging benthopelagic fishes often have large population sizes and high rates of dispersal. These traits can act to homogenize population structure across the distributional range of a species and to reduce the likelihood of allopatric speciation. The apparent absence of any barriers to gene flow among populations, together with prior molecular evidence for panmixia across the ranges of three species, has resulted in Diplotaxodon, a genus of benthopelagic cichlid fishes of Lake Malawi, being proposed as a candidate case of sympatric speciation. Our aim was to further investigate this possibility by testing for intraspecific genetic subdivision among breeding populations, and intraspecific differences in breeding habitat. Location Lake Malawi, central‐east Africa. Methods We analysed eight microsatellite DNA loci to test for spatial genetic differences among populations on breeding grounds of eight Diplotaxodon species. We also tested for temporal population genetic differences within breeding grounds of three species. Records of ripe Diplotaxodon encountered during sampling were analysed to test if spatial variation in assemblage structure was linked to nearshore water depth and geographic proximity of sampling sites. Results Consistent with previous molecular evidence, within four of the eight species tested we found no evidence of spatial genetic structuring among breeding populations. However, within the other four species we found slight yet significant spatial genetic differences, indicating restricted gene flow among breeding grounds. There was no evidence of temporal genetic differences within sites. Analyses of the distributions of ripe Diplotaxodon revealed differences in assemblage structure linked to nearshore water depth. Main conclusions Together, these results demonstrate both the evolution of fidelity to deep‐water breeding locations in some Diplotaxodon species, and differences in breeding habitat among species. These findings are consistent with a role for divergence of breeding habitat in speciation of these cichlids, possibly promoted by dispersal limitation among geographically segregated spawning aggregations.     

Swimming abilities of juvenile estuarine fishes: implications for passage at water control structures

The capability of early life history stage fishes to access nursery habitat within managed salt marshes is dependent on their ability to negotiate water control structures (WCSs). Knowledge of swimming ability and hydrodynamic preferences is essential to assess the impact of WCSs on fish movement in managed marshes. These data, however, are lacking for many common estuarine fishes, and the utility of the data for the few species examined thus far is limited. We examined critical swimming speeds and derived linear relationships between fish size and swimming speed for juveniles of six common estuarine fish species of the southeast U.S. and northern Gulf of Mexico coasts. White mullet Mugil curema displayed the greatest swimming ability among these six species and was able to swim against currents ≥ 30 cm s^?1 higher than the other species examined at the same size. The remaining species displayed lower critical swimming speeds and were classified into groups of moderate (pinfish Lagodon rhomboides, striped mullet Mugil cephalus) or slow (silver perch Bairdiella chrysoura, spotfin mojarra Eucinostomus argenteus, spot Leiostomus xanthurus) swimmers. Our results suggest that high-flow conditions at WCSs would likely preclude the passage of all but the largest juvenile fishes, and passage for most juveniles would occur under low-flow conditions; these flows at WCSs are dictated largely by site-specific tidal and weather conditions.     

Geographically multifarious phenotypic divergence during speciation

Speciation is an important evolutionary process that occurs when barriers to gene flow evolve between previously panmictic populations. Although individual barriers to gene flow have been studied extensively, we know relatively little regarding the number of barriers that isolate species or whether these barriers are polymorphic within species. Herein, we use a series of field and lab experiments to quantify phenotypic divergence and identify possible barriers to gene flow between the butterfly species Lycaeides idas and Lycaeides melissa. We found evidence that L. idas and L. melissa have diverged along multiple phenotypic axes. Specifically, we identified major phenotypic differences in female oviposition preference and diapause initiation, and more moderate divergence in mate preference. Multiple phenotypic differences might operate as barriers to gene flow, as shown by correlations between genetic distance and phenotypic divergence and patterns of phenotypic variation in admixed Lycaeides populations. Although some of these traits differed primarily between species (e.g., diapause initiation), several traits also varied among conspecific populations (e.g., male mate preference and oviposition preference).     

Rapid morphological divergence of a stream fish in response to changes in water flow

Recent evidence indicates that evolution can occur on a contemporary time scale. However, the precise timing and patterns of phenotypic change are not well known. Reservoir construction severely alters selective regimes in aquatic habitats due to abrupt cessation of water flow. We examined the spatial and temporal patterns of evolution of a widespread North American stream fish (Pimephales vigilax) in response to stream impoundment. Gross morphological changes occurred in P. vigilax populations following dam construction in each of seven different rivers. Significant changes in body depth, head shape and fin placement were observed relative to fish populations that occupied the rivers prior to dam construction. These changes occurred over a very small number of generations and independent populations exhibited common responses to similar selective pressures. The magnitude of change was observed to be greatest in the first 15 generations post-impoundment, followed by continued but more gradual change thereafter. This pattern suggests early directional selection facilitated by phenotypic plasticity in the first 10–20 years, followed by potential stabilizing selection as populations reached a new adaptive peak (or variation became exhausted). This study provides evidence for rapid, apparently adaptive, phenotypic divergence of natural populations due to major environmental perturbations in a changing world.     

Body shape differentiation at global and local geographic scales in the invasive cichlid Oreochromis mossambicus

The Mozambique tilapia Oreochromis mossambicus (Teleostei, Cichlidae) has been transplanted worldwide during the 20th century, and now belongs to the list of the most invasive species. Using a geometric morphometric approach, we describe body shape differentiation among 15 populations from native (Mozambique) and invaded (New Caledonia and Guadeloupe) ranges. A dominant phylogeographic signal is detected, despite the broad range of environmental conditions at the local scale. This result suggests that phylogeographic background rather than phenotypic plasticity responding to environmental variation constitutes the main factor correlated with shape divergence. This could result from successive founder events that occurred during the process of colonization of new geographic areas, and therefore strongly suggests heritable phenotypic differentiation. In addition, shape changes along a major axis of divergence hypothetically refer to different swimming abilities, possibly related to divergent functional requirements between the native and invaded ranges. Overall, patterns of contemporary shape diversification in O. mossambicus probably result from both phylogenetic constraints and adaptive divergence processes. We show that critically taking into account recent phylogenetic history of populations as a constraint on rapid phenotypic divergence is necessary for an improved view of contemporary evolution. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, 105 , 369–381.     

Different Sources of Allelic Variation Drove Repeated Color Pattern Divergence in Cichlid Fishes

The adaptive radiations of East African cichlid fish in the Great Lakes Victoria, Malawi, and Tanganyika are well known for their diversity and repeatedly evolved phenotypes. Convergent evolution of melanic horizontal stripes has been linked to a single locus harboring the gene agouti-related peptide 2 (agrp2). However, where and when the causal variants underlying this trait evolved and how they drove phenotypic divergence remained unknown. To test the alternative hypotheses of standing genetic variation versus de novo mutations (independently originating in each radiation), we searched for shared signals of genomic divergence at the agrp2 locus. Although we discovered similar signatures of differentiation at the locus level, the haplotypes associated with stripe patterns are surprisingly different. In Lake Malawi, the highest associated alleles are located within and close to the 5′ untranslated region of agrp2 and likely evolved through recent de novo mutations. In the younger Lake Victoria radiation, stripes are associated with two intronic regions overlapping with a previously reported cis-regulatory interval. The origin of these segregating haplotypes predates the Lake Victoria radiation because they are also found in more basal riverine and Lake Kivu species. This suggests that both segregating haplotypes were present as standing genetic variation at the onset of the Lake Victoria adaptive radiation with its more than 500 species and drove phenotypic divergence within the species flock. Therefore, both new (Lake Malawi) and ancient (Lake Victoria) allelic variation at the same locus fueled rapid and convergent phenotypic evolution.