Functional trait evolution in South Africa
Natural and sexual selection on floral traits
White (anthocyanin-deficient) and pink (anthocyanin-rich) color morphs of Hibiscus laevis in Southern Louisiana, above, and of Protea repens in South Africa, below
I. Patterns of ecotypic variation and polymorphism in Hibiscus species of the Southeastern and Central United States
Since 2012, my lab has been exploring patterns of within- and cross-species trait variation in several native Hibiscus species that are common in southern Louisiana. This work will eventually include questions about flower color polymorphism, but the current focus is on vegetative traits, adaptation, and resilience to environmental change. Past and present contributors to this project include Megan Varvaro, Liz Bergeron, and Christopher Adams, and a new masters student position will be available in 2015 (see the notice on my home page for more details).
Current Hibiscus research explores whether wetland plant vulnerability to anthropogenic change is associated with particular leaf traits or the capacity for plastic responses within two widespread species. Using common gardens and greenhouse experiments, we are quantifying trait variation, plasticity and enviromental tolerances within H. laevis and H. moscheutos sampled across their distributions. The goal is to identify differentially-resistant ecotypes and refine assessments of species vulnerability to include physiological and evolutionary adaptation. Focal questions are: How do functional traits vary within two wide-spread Hibiscus species, and are among-population differences predominately due to plasticity or genetic variation? How are trait variation and capacity for plasticity related to the home environment (e.g., climate, disturbance history, and soils)? Finally, are some species or ecotypes more capable of handling altered soil and water chemistry caused by saltwater intrusion or artificial fertilizer inputs? Sampling site used in the common garden and greenhouse studies are shown on the map below. The original illustrations that accompany the map show distinguishing characteristics of the leaves and flowers, as well as an enlarged seed.
Upcoming color polymorphism work on Hibiscus will parallel ongoing polymorphism work in Protea (see below). As we measure Hibiscus trait correlations and phenotypic variation as described above, we are also quantifying pigmentation in flowers, leaves, and stems. These data will be directed towards the question of why color polymorphism persists in some species and populations of Hibiscus section Muenchhusia (e.g., H. moscheutos, H. laevis) but is absent or very rare in the wild for other species (H. coccineus, H. aculeatus, H. grandiflorus).
II. Evolutionary maintenance of Protea color polymorphisms from the population to genus-wide levels
The goal of this research is to identify key ecological and evolutionary conditions that promote color polymorphism and monomorphism at broad phylogenetic and landscape scales. My current focus is on the genus Protea, within which 40% of species have co-occurring pink-flowered and white-flowered plants. By comparing traits of pink and white morphs in 10 populations of four species, I demonstrated that white-flowered plants generally produced heavier seeds, more germinable seeds, more flowers per inflorescence, and more nectar (and recieved longer pollinator visits), yet pink-flowered plants were occasionally less susceptible to seed-eating larvae. Increased seed predation thereby offsets the benefits of producing more flowers and higher quality seed for white morphs in some populations, and together these factors promote the maintenance of both morphs in a world where seed predator pressures likely vary over space and time (Carlson and Holsinger 2010, Carlson and Holsinger 2012).
I am currently testing whether these local processes can be 'scaled up' to explain the presence and absence of different color morphs accross the genus Protea. As part of this effort, I have initiated a citizen science project to learn more about where polymorphic versus monomorphic Protea populations occur. Click the link to learn about the Protea flower color survey on iSpot Southern Africa
I. Geographic and phylogenetic patterns in drought-related traits of South African Protea and Pelargonium
Since 2010, I have been involved in a large-scale collaborative project at the University of Connecticut, funded by the NSF DIMENSIONS of Biodiversity program. The goal of this research is to better understand the evolutionary history of drought-related traits in the genera Protea and Pelargonium in South Africa, which should help refine predictions about their likely responses to climate change. My focus, beginning as a postdoctoral researcher, has been to establish a 700-plant common garden of Protea repens and to direct field sampling of Protea species in South Africa. Clicking on the map below will bring you to an updated interactive map of the Protea repens common garden sampling sites. Photos of this and our previous common gardens can be viewed here. At the completion of the genus-wide sampling phase, we have collected genetic material and functional trait data on 58 species from 95 populations. Follow this link for the UCONN Dimensions of Biodiversity Wiki page.
II. Adaptive differentiation and plasticity in Protea sect. Exsertae
From 2007-2010, I was involved in another NSF-funded project investigating the mode of evolution within the white proteas, a monophyletic clade endemic to the Cape region of South Africa. Species in this clade include Protea aurea, P. lacticolor, P. punctata, P. mundii, P. venusta, and P. subvestita. Our research asked if these species diversified predominately through adaptive (habitat specialization) or non-adaptive (geographic isolation and drift) processes. This work was in collaboration with the South African National Biodiversity Institute and Rachel Prunier, and the principal investigator was Kent Holsinger.
Our research methods combined field-based ecological measurements, common garden experiments, molecular data, and phylogenetic analyses. I was based part-time in Cape Town, collecting seeds and data in wild populations from 35 locations across the Western Cape, Eastern Cape, and KwaZulu-Natal and maintaining two common gardens. My focus was on determining if among-population differences in vegetative traits, physiology and extent of plasticity are related to natural selection and local adaptation within the clade. By comparing selection gradients in the wild and between the common gardens, we have evidence that ecologically-based divergent selection maintains some of the differences in SLA, leaf area and growth among white protea populations (Carlson et al. 2011; see also Prunier et al. 2012; Carlson and Holsinger in press)
I. Pollinator-mediate sexual selection on nectar production
My dissertation research focused on the interactions between a Neotropical plant species and its floral enemies and pollinators. Chrysothemis friedrichsthaliana [Gesneriaceae] is a hummingbird-pollinated herb found in lowland rainforests throughout Central America. In one series of experiments, I linked floral rewards, pollinator behavior and plant reproductive success. My objective was to test adaptive hypotheses for male-biased nectar production, i.e., increased nectar production during the male relative to female phase of dichogamous flowers (reviewed in Carlson and Harms 2006). Male-biased nectar production, as is seen in this and > 15 other species, results in variable floral rewards between flowers on a plant, as well as an increased incentive for pollinators to visit male-phase flowers. I demonstrated that in Chrysothemis, male-biased nectar production was partially explained by pollinator-mediated selection on traits that more strongly promote pollen removal as opposed to pollen delivery (i.e., sexual selection). I found limited evidence that selection to reduce geitonogamy could also contribute to the maintenance of the trait (Carlson 2007, 2008).
II. Liquid defense against floral herbivory
I also examined floral herbivory and the functional significance of the water calyx of C. friedrichsthaliana. The enlarged, cup-like calyx holds and secretes liquid, such that immature flowers develop under water. In a calyx draining experiment, I demonstrated that a highly detrimental microlepidopteran herbivore (Alucitidae), was partially deterred by the water-filled calyces, relative to calyces without water. These results suggest that the water calyx of Chrysothemis functions in part as a physical floral defense (Carlson and Harms 2007).