The study of natural plant populations has provided some of the strongest and most convincing cases of the operation of natural selection currently known, partly because of amenability to reciprocal transplant experiments, common garden work, and long-term in situ manipulation. Genetic differentiation among plant populations over small scales (a few cm to a few hundred cm) has been documented and is reviewed here, in herbaceous annuals and perennials, woody perennials, aquatics, terrestrials, narrow endemics, and widely distributed species. Character differentiation has been documented for most important features of plant structure and function. Examples are known for seed characters, leaf traits, phenology, physiological and biochemical activities, heavy metal tolerance, herbicide resistance, parasite resistance, competitive ability, organellar characters, breeding systems, and life history. Among the forces that have shaped these patterns of differentiation are toxic soils, fertilizers, mowing and grazing, soil moisture, temperature, light intensity, pollinating vectors, parasitism, gene flow, and natural dynamics. The breadth and depth of the evidence reviewed here strongly support the idea that natural selection is the principal force shaping genetic architecture in natural plant populations; that view needs to be more widely appreciated than it is at present.
This paper shows that genetic differentiation between populations is the expected pattern in populations. The big question is: when does it constitute a speciation event?
Adaptedness is both complexly inherited and much affected by environment: consequently the genetic mechanisms that have led to improvements in adaptedness have been difficult to identify and to quantify. Recently it has been shown that 'marker assisted dissection' of adaptedness based on changes in the frequencies of discretely inherited alleles of loci of various kinds (e.g. allozyme, restriction fragment, microsatellite loci) is practicable. I will illustrate marker assisted analysis of the genetic basis of adaptedness with a sample of allozyme data from three species groups, two heavily selfing groups (two wild Avena species and barley) and one outcrossing species (corn, maize). The results lead to three main conclusions: (1) that the single most important genetic mechanism in all three species groups was the assembly of favorable epistatic combinations of alleles of different loci by means of recurring cycles of selection, intercrossing superior selects, and inbreeding to near homozygosity leading to stable superior multilocus genotypes adapted to specific habitats; (2) that exploitation of favorable interactions among alleles of the same locus played a significant role in tetraploid A. barbata and probably also in single-cross maize hybrids; (3) that purifying selection (elimination of deleterious alleles) played a small role in all three species groups. These results indicate that marker alleles provide applied breeders with effective ways to identify, track, and incorporate regions of chromosomes with favorable effects of adaptedness into improved cultivars.
This paper shows that adaptedness is not a simple matter, but rather involves epistatic combinations of alleles and response to cyclical selection regimes. The development of adapted ecotypes (or cultivars) is more complicated than simple changes in gene frequency.
The comparative growth and osmotic acclimation of ten culture strains of the marine benthic cyanobacterium Microcoleus chthonoplastes Thuret isolated from microbial mats in Germany, Spain, Egypt, the United States, Mexico, Chile, and Australia were investigated in salinities ranging from freshwater to twice seawater. All isolates showed a broad growth versus salinity response consistent with the dominance of this species in intertidal and hypersaline microbial communities. Growth optima, salinity preferences, and maximum growth rates differed for each isolate and could be related to the habitat from which they were isolated. This is most obvious when comparing strains from brackish habitats with those from a hypersaline lake. While the former isolates exhibited sharply pronounced growth optima under hyposaline conditions, cultures from the hypersaline environment grew best in salinity more than double seawater. The major low-molecular weight organic compounds present in all M. chthonoplastes strains were the carbohydrates glycosylglycerol and trehalose. This was proven by using C-13-nuclear magnetic resonance spectroscopy. Glycosylglycerol was synthesized and accumulated with increasing salinities, indicating its role as an osmolyte. In contrast, trehalose was present in relatively high concentrations under hyposaline conditions only. Differences in the patterns of growth versus salinity, as well as in those of osmotic acclimation among the M. chthonoplastes isolates, point to the development of different physiological ecotypes within the species.
This case is a good example of development of ecotypes as a response to differing environmental conditions. The world is made up, it seems, by subtle adaptations to existing conditions. Is speciation but a point along the cumulative and continuous range of differentiation?
A segregating hybrid population of Crepis tectorum was established at an outcrop site occupied by a short-stemmed population (ssp. pumila) to determine whether selection still favors a prostrate habit in this dry, exposed, and nutrient-poor habitat. Despite extensive variation in the experimental population, there was no apparent trade-off between plant height and head number. Plant stature was positively correlated with intensity of herbivore damage and showed a negative relationship with the rate of flowering. These associations might translate into selection for reduced height in years when the risk of grazing is high or when the water availability decreases too fast to allow all flowers to produce seeds. Hence, there is some congruence between past and current selection pressures when the phenotypic range is expanded and when performance variables other than head number are considered.
The take home message of this abstract is that you might expect a short phenotype to be favored by one set of conditions (such as exposure and nutrient availability) but when you look closer the actual response might be to some other factor (such as herbivory). It could be herbivory is just a transitory phenonmenon, and perhaps the balance of selective forces constantly changes overtime. The optimal phenotype is probably a compromise between many factors.
As part of an analysis of genetic diversity in endemic taxa of the Athabasca sand dunes in northern Saskatchewan, Canada, genetic variation was examined by starch gel electrophoresis in six populations of the endemic Achillea millefolium ssp. megacephala, and 13 populations of the closely related widespread taxon, A. millefolium ssp. lanulosa. Endemic populations had more alleles per locus, a higher percentage of polymorphic loci, and greater genetic diversity than did populations of the widespread taxon. At polymorphic loci, total gene diversity was comparable in both taxa, although within-population gene diversity was higher in the endemic taxon. Population differentiation (G(ST)) was considerably lower in ssp. megacephala than in ssp. lanulosa, although G(ST) values were reduced when the parameter was calculated separately for geographic subdivisions of the widespread taxon. Our results differ from previous studies in which the endemic is typically depauperate of genetic variation relative to related widespread species. We suggest that obligate sexual reproduction and the absence of long-term asexual reproduction may be one of a number of factors that help populations of ssp. megacephala maintain higher levels of genetic variation on the Athabasca sand dunes.
Genetic variation might be partitioned in many different ways. We typically assume that the widespread taxon is the most genetically diverse (or that the other species arose from it), but this study shows that this is not a safe assumption. I like this because it shows that patterns aren't always simple to predict.
The ability of organisms to produce different phenotypes under different environmental conditions (phenotypic plasticity) has been an object of evolutionary and ecological studies since the neodarwinian synthesis. Yet, until lately, our knowledge in this field was limited to statistical approaches based on the classical tools of quantitative genetics. In recent years, however, a new dialog between organismal biologists and researchers interested in uncovering the mechanistic details of physiological and phenotypic responses has yielded several new insights. Some classic examples of phenotypic plasticity have now been traced to specific alterations in DNA transcription and RNA translation rates, and to changes in patterns of protein expression. Conversely, the explicit use of evolutionary and ecological theory is helping us to put a panoply of molecular data into a coherent historical and organismal perspective.
This study shows why it it so exciting to be a biologist at the end of the 20th century - we are finally developing the tools and sophistication to look at the complexity of life!
Forty populations of Potentilla argentea L. (sensu late) from southern Sweden were investigated to determine the pattern of genetic variation. Since the diploid form of Potentilla argentea has been reported to produce seeds apomictically, the intention was, in particular, to investigate whether the variation pattern indicated a purely asexual mode of reproduction in nature. The investigated populations were chosen to represent the diploid form of Potentilla argentea. Nevertheless, sympatry between diploid and hexaploid forms was found in 14 out of 40 populations sampled. Two samples contained only hexaploid plants, and 24 only diploid plants. The ploidy level was determined by chromosome counting or by morphological analysis of greenhouse grown plants. The mean number of leaves was higher and the size of the largest leaf was greater in hexaploid than in diploid offspring plants grown in the greenhouse. Among the diploids, significant variation was found for both traits between populations as well as within populations. Genetic variation was also detected with the RAPD method. To work, the method required a number of modifications of the standard protocol for DNA-purification, but it then gave clear and reproducible results. The RAPD variation was evenly distributed within and among the investigated diploid populations. The level of variation was comparable to the level found in selfing plants, but the pattern of variation was more as expected from outbreeding plants. The conclusion is drawn that Potentilla argentea may well have a variable breeding system. In nature, it does not, however, behave in a predominantly asexual manner.
A good example of someone testing the assumptions of traditional wisdom. It doesn't really have much to do with ecospecies up front, but it gets to the point that there is variation (and therefor potential complexity) lurking everywhere, even in agamospecies.
Different types of non-genetic variation were compared in order to investigate the relationship between phenotypic plasticity and developmental instability. The following types of variation were estimated for 22 quantitative traits in Iris pumila: variation within the same flower, variation between ramets on the same genet, variation between replicas of the same genotype within a macroenvironment and also between two types of macroenvironmentals, spatial and temporal. The first type of variation is asymmetry between replicated parts, and, therefore, is a measure of developmental instability. The second and third types are also measures of instability but are confounded by microenvironmental variation, and the last two types of variations are measures of phenotypic plasticity. For analysis of spatial variability, I chose two types of environments (open and shaded) in natural habitats of I, pumila. Two successive flowering seasons were used to estimate temporal variability. Asymmetries of different traits were poorly integrated and had a small number of significant correlations with other types of variability. Significant correlations varied in sign. Other types of within-environment variation were better integrated, having a limited number of significant positive correlations with measures of phenotypic plasticity. However, such positive correlations were observed mostly with one kind of plasticity (temporal) and only for some of the less plastic, floral traits. Plasticities of vegetative traits were not correlated with measures of developmental instability. Implications of these results on different models explaining the genetic basis of phenotypic plasticity, as well as on hypotheses concerning differences in plasticity between species and populations, are discussed.
This reminds me of my own research. I would love to know the source of observed variation within my group of organisms. Developmental instability.... where is the line between that and phenotypic plasticity... is it only a matter of human estimation of adaptiveness? However if we could link plasticity to specific alterations in the DNA (methylations etc.) then maybe our level of confidence in separating the instability from the plasticity would be boosted. Assuming that asymmetry between replicated parts is instability is not warranted when leaves may differ on opposite sides of the same plant (as they do in thistles). My hat is off to these authors!
Vaccinium macrorarpon Aiton (Ericaceae) cranberry, a dwarf shrub and a typical taxon in temperate pear bogs, has its central distribution in cool temperate regions in eastern North America. Isolated southern marginal populations are distributed along the Appalachian corridor and on the North Carolina coastal plain. A common garden (Blacksburg, Virginia) was utilized to determine whether marginal cranberry clones exhibit greater phenotypic plasticity than central clones. Three central clones from Massachusetts (MA), Wisconsin (WI), and New York (NY) and three marginal clones from North Carolina (NC), Tennessee (TN), and West Virginia (WV) were tested. A suite of phenotypic traits was measured in response to edaphic variation in the common garden. An analysis of reaction norms took the form of an analysis of covariance to test for significant differences among clones and to estimate regression slopes (plasticity) when compared with environmental (nutrient) variation. There was no regional variation in phenotypic plasticity, but there was significant clonal differentiation for 77% of nonintercorrelated traits. However, in most cases the differences were seemingly random, with little biological importance. Hence little differentiation in relation to population origin was observed among clones. Matrix comparisons were performed using a Mantel test to check for pairwise correlations among the following matrices: geographic distances, trait means, plasticity, and molecular variation assessed by random amplified polymorphic DNA (RAPD) profiling. No correspondence was found among matrices. The recent post-glacial distribution of cranberry may account for the absence of phenotypic and genetic heterogeneity.
This study follows the same line of questioning as Purdu & Bayer, 1966 (above) in that tests the hypothesis that the center of the range will be the area of maximum variability. We see that assumption frequently in its untested state, but then sometimes the assumption is that the periphery will be where the maximum variation will be found. These cranberrys show that neither might be true, or at least that our methodology is not capable of detecting such differences.
Selection on flowers has often been viewed as being particularly strict, constant, and responsible for species differences. Impatiens pallida and I. capensis flowers fit snugly around bees, leading one to expect a close relationship between floral morphology and pollination success. My studies on the amount of pollen removed from androecia and deposited on stigmas in single visits by bumblebees did not confirm this supposition. Trimming off parts of the floral vestibule with scissors and gluing in pleats had very little effect on the amount of pollen that bees moved. In reciprocal transfer experiments, flowers from different populations sometimes differed in the amount of pollen moved, but when the two species were compared in sympatry, pollen removal and deposition differed hardly at all. A comparison of the relationship between pollen movement and floral morphology among 15 populations showed that, although there was great heterogeneity in the amount of pollen moved, the observed differences were independent of floral morphology. None of this supports a belief in strong selection that fine-tunes the mechanical fit between bee and newer; selection for visitation success based on pollinator behaviour may have a much stronger influence on floral characters. (C) 1995 The Linnean Society of London.
What kind of glue do you use to put pleats in a flower? Here the authors shine some light into adaptive story-telling: perhaps it is under conditions of more competition for pollinators that this morphology evolved, or alternatively, perhaps the pollinator-fit is just another of those stories. Thanks to the authors for a questioning the assumption!
The taxonomic status of the littoral prosobranch Littorina neglecta has been investigated using animals from the northeast coast of England. Rough periwinkles were collected from barnacle-covered rock platforms near the northern (Ness Point) and southern (Old Peak) ends of Robin Hood's Bay, and from small and large boulders at the southern end. Shape was analysed in shells from all sites, and the activity levels and heat stability of aspartate aminotransferase (Aat) and alanine aminotransferase (Alat) were determined in samples from Peak Steel and a nearby boulder habitat. Of the shells from Ness Point, 98% could be separated visually, using colour, sculpturing and banding, into three categories (the oviparous L. arcana, and the ovoviviparous L. saxatilis and L. neglecta). The categories were confirmed using discriminant analysis on shell measurements and cross-validation indicates at least 70% accuracy. On Peak Steel L, saxatilis breeds at a small size and its shells are visually quite distinct from those of Peak Steel L. neglecta. In discriminant analysis 84% of the latter classify with the Ness Point L. neglecta. The enzyme studies clearly indicate that the activity level of Aat after 1 min exposure to 56 degrees C is significantly different between Peak Steel L. saxatilis and L. neglecta. In general, it appears that there are different physiological responses in the different groups of animals depending on the zone, microhabitat and taxonomic category. The relationship between the 2 barnacle-dwelling, brooding periwinkles is discussed. It is considered that the existence of two distinct, sympatric ecotypes is unlikely, and that there is a true barnacle-dwelling ecotype of L. saxatilis that is morphologically and biochemically distinct from L. neglecta. The possibility of gene flow between these forms is currently being investigated.
Which of the data catagories should we have the most reliance in: visual differences, statistically detectable differences, reproductive categories, or enzyme stability? The level of ambiguity in this abstract is appealing to me; it gives you a sense of the kind of quandries we taxonomists find ourselves mired in.
One possible response of plant populations to heterogeneous environments is genetic adaptation resulting in the formation of distinct ecotypes. Genetic adaptation to stressful environments may affect both the limits to species boundaries and the potential for response to a changing environment. Reciprocal transplant experiments have frequently been used to describe ecotypic differentiation and to infer the role of natural selection when there is evidence for home site advantage. The demonstration of a home site advantage, however, does not reveal which plant characters are responsible for conferring increased fitness on populations planted in their native site. Here, we combine the classic reciprocal transplant experiment with multivariate regression analysis of selection to ask a series of questions relevant to understanding adaptive genetic differentiation in natural plant populations. Impatiens pallida plants from a mesic floodplain and a dry hillside site were reciprocally transplanted. We initially presumed the hillside to be a stressful site for Impatiens given its sparser population of consistently smaller individuals. This study describes the two environments from the perspective of the plant to ask whether it is stressful. In addition, we investigate genetic differentiation between populations and ask whether the two populations are distinctly adapted to their home sites. To identify traits that may be important for conferring home site advantage, we quantify present-day natural selection in these sites and ask whether the observed selective forces can explain genetic differences. Finally, because phenotypic correlations may play an important role in a population's response to its environment, we investigate relationships among traits to determine the extent to which they are genetically and/or environmentally controlled. The large reduction in total seed production when plants from both populations were grown on the hillside supported our initial bias that this site was stressful to Impatiens. In addition, the higher relative fitness of each population planted in its native site demonstrated that these populations represent distinct ecotypes. Genetic differences between populations were observed for several life history and morphological characters. In particular, plants from the hillside population were smaller and produced cleistogamous flowers earlier than floodplain plants. Selection analysis revealed that, while there is strong selection favoring early flowering on the hillside, there is no advantage to early flowering fnr plants grown on the floodplain. An increased developmental rate, which allows plants to produce seeds before they succumb to drought stress, appears to be the most important mechanism responsible for the greater relative fitness of the hillside population in its native site. While greater total plant leaf area is favored by selection on the floodplain, there is no evidence for selection on this trait on the hillside. Phenotypic covariances among traits differed between sites and populations, resulting in differences in the action of indirect selection. There is evidence that indirect selection on correlated traits is responsible for some of the observed genetic differences.
This study has clearly taken Turesson's methodology of studying ecotypic differentiation to new levels. From the perspective of the abstract, it is impossible to assess whether these authors have succeeded in quantifying the present day selective forces, or determined whether traits are genetically and/or environmentally controlled. Sounds like a tall order, but these authors have definitely tapped into the environment as a source of variability, and, hence, complexity.
Cakile edentula produces upper and lower dimorphic seeds which disperse long and short distances, respectively, from the parent. Canonical discriminant analysis was used to determine the differences between plants from the two seed morphs in both the amount and direction of their overall plastic responses to major environmental factors. In general, plants from the long-distance dispersal seeds were less plastic than those from the short-distance dispersal seeds in response to soil moisture and sand burial. This is probably a result of broader ecological amplitude of the plants from the long-distance dispersal seeds due to their larger seed masses. Differences in the direction of overall plasticity between plants from the two seed morphs are not likely to be adaptive. However, phenotypic selection may affect the relative contribution of plants from the two seed morphs to the next generation.
I seems to me is that there may be more developmental stability in the larger (long distance) seeds, presumably because of resources available to the embryo. I would like to know for sure if these seeds are genetically identical. Are they both the products of outcrossing, or might they differ in being apomictic or selfing? Sounds like a good system to ask some questions about, meanwhile add seed characteristics on the list of sources of variablility.
DNA samples from 980 plants of Avena barbata from 48 ecologically diverse sites in California and Oregon were assayed to determine their genotype for two duplicated loci governing rDNA variants. More than 40 different rDNA genotypes were observed among which 5 made up 96% of our sample in environmentally homogeneous sites; predominant genotypes were less frequent and recombinant genotypes were more frequent in environmentally heterogeneous sites. The spatial distribution of each predominant rDNA genotype was nearly an exact overlay on both macro- and microgeographical scales of a distinctive habitat and also of the distribution of an eight-locus morphological-allozyme variant genotype. In all, seven different habitat-genotype combinations (ecotypes) were distinguishable on the basis of their morphological-allozyme-rDNA genotypes. None of these seven genotypes has been found in ancestral Spanish populations; thus the above predominant multilocus genotypes (ecotypes) of the colonial populations evidently evolved subsequent to the recent introduction (within 150-200 generations) of A. barbata to California. The precise associations of specific alleles and genotypes of the morphological allozyme and rDNA loci with different specifiable habitats leads us to the conclusion that natural selection favoring particular multilocus combinations of alleles in different habitats was the main guiding force in shaping the internal genetic structure of local populations as well as the overall adaptive landscape of A. barbata over California and Oregon.
This abstract represents an enormous effort and it appears that it paid off with the detection of a genetic ecotypes. The predominant rDNA genotypes (the most common?) were those that were most restricted to common, but uniform, habitat! This is an interesting outcome in respect to the question of the expected variability of widespread taxa.
Conventional interpretations of morphological stasis are assessed in terms of energetic costs to organisms in their stressful environments. Assuming that the target of selection of such stress is at the level of energy carriers, this provides a reductionist model to derive the conditions under which morphological stasis is expected. In natural habitats, stress can be widely fluctuating or stable: under extreme conditions these are, respectively, the habitats of ''living fossils'' and of relict species, which both show morphological stasis. When the intensity of stress is relaxed, and, especially, when there is energy from additional resources, the likelihood of diversification increases, since extreme variants are then more likely to survive. Conversely, morphologically complex species would be most at risk during extinction events, so that the diversity of surviving phenotypes would be reduced, giving a tendency towards stasis.
This paper seems to look at what makes for stability rather than complexity. Perhaps stability and simplicity is the flip side of complexity?
The marine gastropod Littorina saxatilis has different ecotypes in shores only a few meters apart. This has both taxonomic and evolutionary implications. Here we report on an extreme type of within-shore dimorphism in shell characters. In the wave-exposed rocky shores in northwestern Spain, we found one form of L. saxatilis in the upper-level barnacle zone. It had a white, ridged shell, with black bands in the grooves. Another form confined to the lower-shore mussel belt had a smooth shell that was either white and tessellated or darkly colored. These two forms cooccured in a narrow midshore zone together with individuals that had combined characters, but were present in low frequencies (11%-29%). We used principal-component analysis of metric shell characters to study variation in shell size and shape. We found that the upper-shore form was larger than the lower-shore form. We also found small but significant differences in shell shape. Experiments in a common laboratory environment suggested the differences in shell ornamentation and color are inherited, but the individuals did not develop the morph-specific characters until a shell height of about 3 mm. The occurrence of mainly two distinct forms may suggest the presence of two species that hybridize. An analysis of five polymorphic enzyme loci in populations of snails from three geographically separated Sites indicated, however, that there was no positive correlation between morphological distances and genetic distances among populations on a geographic scale (tens of kilometers). Thus, we rejected the hypothesis of two species. However, on a microgeographic scale (meters), genetic differentiation between groups with the same form was less than differentiation between forms. This indicated a partial barrier to gene flow between the two forms, and preliminary mate choice data suggested this was caused by nonrandom mating in the midshore zone of overlap.
Specialized lineages, where ever we look! Aren't there any generalists, all-around universal phenotypes anymore? :)
No abstract
Trembling aspen (Populus tremuloides Michx.), a clonal angiosperm, is the most geographically widespread tree in North America. It is generally thought that most extant populations in the western interior of Canada and the United States became established shortly after glacial retreat, but sexual recruitment then effictively ceased owing to inimical climatic conditions. Six populations of trembling aspen were studied in the prairie and montane environments of Waterton Lakes National Park, Alberta. Vegetative tissues were analyzed for electrophoretically detectable variation in 13 enzymes encoded by 14 polymorphic loci and three monomorphic loci. All populations maintained high levels of inter- and intrapopulation diversity (P = 0.891; H = 0.319; A = 2.4). The mean fixation index, F, was -0.102, indicating some deviation from Hardy-Weinberg expectations. Genetic differentiation (F(ST) = 3.0) was apparent in this ecologically diverse, but geographically small-scale, spatial setting. It is suggested that the maintenance of diversity in the absence of frequent modem-day recruitment, and resistance to further geographic differentiation in this spatially heterogeneous environment reflect occasional seedling establishment through "windows of opportunity" and more importantly, the species' clonal morphology. The phalanx growth form and concomitant physiological integration between ramets combine to spread the risk of death and buffer the effects of selection over time and space.
This is an example of why outcrossing can only be considered one end of the spectrum of ways to increase genetic diversity. If plants can't import diversity through sexual processes, they seem to be able to find it within. It seems to me that all of these process will be operative even in outcrossing taxa. The genome is a dynamic structure, ceasely changing.
A 3-year demographic study was conducted to reveal targets of selection on morphology and life history in a population of Crepis tectorum ssp. pumila, a winter annual plant confined to calcareous grasslands (alvars) on the Baltic island of Oland (south Sweden). I calculated the selection differential to describe the change in the mean value of a character due to selection and used multiple regression analyses to partition the direct effect of selection on the trait from indirect responses of selection on other traits. Rosette leaf number, a convenient measure of plant size, was strongly correlated with both viability and fertility (fitness). There was also a strong relationship between fitness and the extent to which the plants expressed traits characterizing this particular taxon. Multiple regression analyses indicated direct selection favouring plants with deeply lobed leaves and a densely branched stem, two distinctive traits of ssp. pumila believed to be adaptive in the alvar habitat. Only stem height was subject to both direct and indirect selection in the wrong direction; taller individuals were more successful than those with a shorter stem, a surprising result considering the inferred advantage of a short stature in the exposed alvar habitat. Selection on other traits assumed to be ecologically important (germination time, flowering time, and seed size) was found to be either absent or variable in direction when other traits were held constant. The failure of plants to survive to the flowering stage in the last two summers indicates strong selection for plants that produce a high percentage of dormant seeds. Overall, the contemporary selection regime as revealed by demographic data was only partly congruent with predictions regarding historical selection pressures based on large-scale patterns of variation (ecotypic differentiation).
This study show us that the characters which"seem" likely to be selected aren't always the same as for what is actually selectively important. This study is a good test of what was happening to a population at one point in time, however, over the course of time patterns of selection will have their own dynamics.
Achillea lanulosa has complex, highly dissected leaves that vary in shape and size along an altitudinal gradient. Plants from a high and an intermediate altitude population were clonally replicated and grown in a controlled environment at warm and cool conditions under bright light. There were genetic differences among populations and among individuals within populations in leaf size and shape. Heritabilities for leaf size and shape characters were moderate. Leaves of the lower altitude population were larger and differed from the higher altitude plants in both coarse and fine shape. Plastic response to temperature of the growth environment paralleled the genetic differentiation between low and high altitude populations. There was no apparent trade-off between genetic control over morphology and the capacity for directional plastic response to the environment. Differences in leaf dissection and size at contrasting altitudes in this species are the result of both genetic divergence among populations and of acclimative responses to local environments.
This is the classical common garden experiment!
An analysis was made of morphological variation in the taxonomically complex Hawaiian genus Wikstroemia. Previous authors had recognized 12 or 26 species. Multivariate analyses indicate that variation is continuous and that natural groups are not distinguishable on the basis of macromorphology. Two micromorphological characters, pollen exine sculpturing and seed coat morphology, were examined for their potential taxonomic utility; however, neither of these characters contributed any useful taxonomic information at the species level. The polyforate, crotonoid pollen grains from populations on the six major islands of Hawaii and from five extra-Hawaiian species are morphologically uniform. In contrast, seed coats showed intra- and interpopulation variability, but the variation patterns are not consistent at the population level or above.
How did the original describers pick 12 or 26 phenotypes? This abstract appeals to me because the authors just blew the old taxonomy out of the water with some good data.
The relation between environmental factors and leaf morphology of Zostera marina L. have long been unclear, primarily because the species is intrinsically variable. The common-garden method was used to determine the genetic, environmental, and interaction components of leaf size variation. Zostera marina consists of several ecotypes with a wide range of phenotypic plasticity. Variation in the morphology of Z. marina was of three types: genetic, accounting for 14% across the localities studied; environmental (phenotypic plasticity along temporal and spatial gradients), accounting for 32%; and interaction between genotype and environmental, accounting for 35%. Five ecotypes were described for the North American Pacific coast: Z. marina L. var. izembekensis Backman, Z. marina L. var. typica Setchell, Z. marina L. var. phillipsii Backman, Z. marina L. var. latifolia Morong, Z. marina L. var. atam Backman. Temporal variation was due to seasonal phenotypic changes in ecotypes. Zostera marina var. izembekensis showed little seasonal morphological changes; Z. marina var. typica demonstrated minor increase in leaf size in spring and summer. Zostera marina var. phillipsii and Z. marina var. latifolia behaved similarly in that leaf size of both increased markedly in spring through early summer. Zostera marina var. phillipsii is adapted to Hood Canal and Puget Sound while Z. marina var. latifolia occupies the outer coast. Zostera marina var. atam exhibits sexual reproduction exclusively and is specifically adapted to the Gulf of California.
This sounds like a classical solution to the problem.... find the actual number of genetically differentiated ecotypes and develop some parameters for the effects of phenotypic plasticity. I would want to know how well this pattern of leaf size variation actually reflects genetic differentiation, but getting to that question is not always as simple as it sounds.