Natural selection and genetic drift are important evolutionary forces in determining genetic and phenotypic differentiation in plant populations. Ye et al. study population differentiation at five quantitative traits (QST) in Psilopeganum sinense (Rutaceae), an endangered species with fragmented distribution and small population sizes, and compare this with differentiation at putatively neutral microsatellite markers (FST). The results show that QST < FST and thus provide no evidence of local adaptation in well-differentiated populations at a relatively large spatial scale, and instead suggest a role of stabilizing selection and drift leading to phenotypic differentiation among small populations.
In invasive alien species, evolutionary change can occur rapidly. Although there are several mechanisms that are believed to inhibit adaptation, e.g. low genetic variability, local adaptation of invasive alien plants to distinct habitat types has been demonstrated repeatedly. It has been suggested that invasive alien species can be used as model organisms for studying ecological and evolutionary processes in real time.
A prominent invasive alien plant species that has colonized distinct habitats in Europe over a long time period is Impatiens glandulifera. In the invaded range, this species frequently occurs in near-natural habitats, primarily in riparian habitats, fenland, mesotrophic grassland and deciduous woodland. Impatiens glandulifera is a suitable species to study local adaptation to different habitats because it is an outcrossing annual with potentially fast evolution.
A recent paper in Annals of Botany combines field and greenhouse experiments to study Impatiens glandulifera (Himalayan balsam), which has invaded distinct habitats in Central Europe that show remarkable differences in shade, soil acidity and competition. The authors find no indication of local adaptation in this study. Their results suggest that the species is coping with environmental variation by means of high phenotypic plasticity following a ‘jack-and-master’ strategy, by which it is able to maintain high fitness under a wide range of environmental conditions, but performs particularly well in favourable habitats.
Pahl, A. T., Kollmann, J., Mayer, A., & Haider, S. (2013) No evidence for local adaptation in an invasive alien plant: field and greenhouse experiments tracing a colonization sequence. Annals of botany, 112(9), 1921-1930.
Background: Local adaptation enables plant species to persist under different environmental conditions. Evolutionary change can occur rapidly in invasive annual species and has been shown to lead to local adaptation. However, the patterns and mechanisms of local adaptation in invasive species along colonization sequences are not yet understood. Thus, in this study the alien annual Impatiens glandulifera was used to investigate local adaptation to distinct habitats that have been consecutively invaded in central Europe.
Methods: A reciprocal transplant experiment was performed using 15 populations from alluvial deciduous forests, fallow meadows and coniferous upland forests, and a greenhouse experiment was performed in which plants from these habitats were grown under treatments reflecting the main habitat differentiators (shade, soil acidity, competition).
Results: Biomass production, specific leaf area, plant height and relative growth rate differed between habitats in the field experiment and between treatments in the greenhouse, but not between seed origins. Overall, there was no indication of local adaptation in either experiment.
Conclusions: Since I. glandulifera is a successful invader in many habitats without showing local adaptation, it is suggested that the species is coping with environmental variation by means of high phenotypic plasticity. The species seems to follow a ‘jack-and-master’ strategy, i.e. it is able to maintain high fitness under a wide range of environmental conditions, but performs particularly well in favourable habitats. Therefore, the proposed colonization sequence is likely to be based primarily on changes in propagule pressure. It is concluded that invasive alien plants can become dominant in distinct habitats without local adaptation.
Pollinator-driven speciation week continues, with Floral adaptation to local pollinator guilds in a terrestrial orchid, a paper by Sun, Gross and Schiestl. The star is the orchid Gymnadenia odoratissima, and orchid found in lowland temperate Europe and up in the mountains. It flowers between June and mid-August. Unlike the orchids yesterday, these flowers offer a food reward as well as a strong scent.
The study examined a number of lowland and mountain orchids in Switzerland. A team of patient observers with nets watched to see what visited the orchids in all the locations, during the day and night. In the lowland orchids they found visitors to b butterflies/moths and beetles. In the mountains there were also Diptera, flies. There wasn’t much overlap between the species of pollinators.
The next stage was to start transferring populations of orchids. The obvious way to move the orchids was to change their altitude, to move the lowland plants to the mountains and bring the highland plants down. Sun et al. also moved some samples along the same altitude. This helped provide another check on how the orchids coped with being moved. What they found was that the mountain orchids were fairly successful in the lowlands, but the lowland plants did comparatively badly in the mountains.
However, the transferred mountain orchids did much worse in the lowlands than if they’d stayed put. Sun et al. point out one of the things the lowlands lack are Empidid flies. Fly pollination is an important factor for plants at higher altitudes. The lowland plants weren’t much less successful when they moved. It suggests that the mountain plants have adapted their flowers to take advantage of the flies in a way that the lowland plants haven’t. For example the chemicals in the scents of the flowers are different between the lowland and mountain plants. The mountain flowers also tended to be paler, and this is attractive to moths, who are more common in the uplands than the lowlands. There are more butterflies in the lowlands, who pollinate by day and so darker flowers may be comparatively more attractive here.
What the experiments show is that the changes in floral display aren’t simply products of altitude. It seems that G. odoratissima is altering its flowers in reaction to the local pollinator guilds. You can pick up this paper from Annals of Botany.
Timing of flowering is an adaptive trait regulated by environmental cues and has been intensively studied in annual plants, but in perennials it is currently not well characterized. Quilot-Turion et al. apply cold treatments to two locally adapted populations of the perennial Arabidopsis lyrata from central Europe and Scandinavia and study responses to vernalization. They find that the population from central Europe flowers more rapidly and invests more in inflorescence growth than the Scandinavian population, which has longer leaves and higher leaf rosettes. QTL mapping in the F2 population reveals genomic regions governing these traits and some QTL × vernalization interactions. The results indicate that the two populations have diverged in their plastic responses to vernalization in traits closely connected to fitness through changes in many genomic areas.
Early life-history stages of cacti can benefit from the facilitative effects of nurse plants that reduce solar radiation and water stress. Miranda-Jácome et al. conduct a reciprocal transplant experiment, coupled with the artificial manipulation of sun/shade conditions, to test for the effects of local adaptation on germination, seedling survival and growth of the columnar cactus Pilosocereus leucocephalus. They find that significant local adaptation is mainly detected under full sunlight conditions, indicating that sun/shade acts as a selective agent in water-limited environments. Facilitation provided by nurse plants in these environments can attenuate the patterns of local adaptation among plants benefiting from nurse plant effects.
Responses of high-mountain plant species to global change are highly influenced by the genetic background of the species, including genetic diversity or gene flow between populations. García-Fernández et al. study the genetic patterns of Silene ciliata (Caryophyllaceae), a high-mountain cushion plant that shows local adaptation to altitude, by examining populations along two altitudinal gradients on separate mountains in central Spain. They find that genetic diversity is similar in all populations, and although substantial gene flow is found both along altitudinal gradients and horizontally within each elevation belt, greater values are obtained along altitudinal gradients.
Early life history stages are among the most critical phases in the life cycle of plants. De Frenne et al. assess the response of plant regeneration from seed of two forest understorey plants (Anemone nemorosa and Milium effusum) to variations in temperature along a latitudinal gradient. They find decreasing seedling emergence and fitness towards the northern edge of the distribution range of the species, as well as stronger growth responses to temperature increases in northern than in southern seedlings of the grass M. effusum. They conclude that wide intraspecific variation in plant regeneration in response to warming may differentially alter future plant dynamics across broad spatial scales.
Population differentiation in a variable environment is related to the selection pressures that plants experience. Ward et al. compare differences in growth- and defence-related traits in two isolated populations of Acacia raddiana trees from sites at either end of an extreme environmental gradient in the Negev desert. They find no evidence of trade-offs between physical and chemical defences and plant growth parameters; rather, there appears to be positive correlations between plant size and defence parameters. The great variation in several traits in both populations may result in a diverse potential for responding to selection pressures in different environments.
Mertensia fusiformis (Boraginaceae) is a spring-flowering perennial showing pronounced intraspecific variation in style length and stigma–anther separation. Forrest et al. show that population-level variation in flowering time, driven by patchiness in timing of snowmelt, causes different populations to experience different temperature regimes during flowering and functionally distinct suites of pollinators. The interaction between plant and pollinator phenology in particular appears to be involved in maintaining style-length variation in this species.