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Natural selection and adaptive potential in plant populations

Stefan Anderssons current research centres on the following topics:

Natural selection

Natural selection and local adaptation have been favourite subjects since my doctoral project on ecotypic variation in the annual plant Crepis tectorum. This interest has continued to the present day. In the past years, I have been involved in several studies that contrast levels of population differentiation in putatively neutral marker genes with the corresponding data for potentially adaptive phenotypic traits to examine whether past selection pressures have been diversifying, unifying, or too weak to have any influence on the phenotypic variation. This and other methods have been used to test for local adaptation in a number of species, especially in relation to human-induced increases in soil acidity but also in relation to changes in landscape structure (habitat fragmentation). In a current project, we evaluate the potential for parallel ecotype formation in the hemiparasitic plant Rhinanthus serotinus.

Floral reductions in selfing plant lineages

The evolution of small, selfing flowers from large, animal-pollinated flowers is one of the most common evolutionary trends in the plant kingdom. Although much attention has focused on the selective advantage of self-pollination, there have been few attempts to explore the selective and genetic mechanisms underlying the floral reduction so prevalent in selfing lineages. In the past years, I have used a combination of experimental approaches to address this issue in two distantly related species,Crepis tectorum(Asteraceae) and Nigella degenii(Ranunculaceae). The results so far indicate that floral reductions can be a side-effect of reductions in nonfloral characters, that resource allocation costs contribute to floral reductions by imposing negative selection on flower size, and that floral reductions may be facilitated by the inevitable buildup of inbreeding associated with self-pollination.

The evolution of floral color polymorphisms

Flower-color polymorpisms have been a topic of long-standing interest for plant evolutionary biologists. Together with a collegue (Tove Jorgensen, University of East Anglia, UK), I have examined the selective and genetic mechanisms underlying the evolution and maintenance of a striking pollen-color dimorphisms within Nigella degenii. Results from field surveys, genetic analyses, stress-manipulation experiments and comparative studies of population structure indicate that several selection pressures, operating at both vegetative and reproductive stages, contribute to the maintenance of these pollen-color dimorphisms.

Quantitative genetics and conservation biology

In the past years, I have been involved in several studies on genetic erosion — the loss of genetic variation resulting from genetic drift in small, fragmented populations. Most of these studies have included analyses of phenotypic characters that are likely to be the targets of selection in natural populations. Contrary to the results of marker-gene analyses, our results for rare, threatened plants in Sweden and other areas provide little evidence for loss of adaptive potential in ecologically relevant characters with a polygenic basis.

To provide greater confidence when inferring genetic effects of small population size, we have extended our analyses to experimental systems or natural populations with a known history of fragmentation. Data from artificially bottlenecked populations ofNigella degenii and wild populations of Brassica cretica show that genetic drift causes idiosyncratic, trait-specific changes in the genetic variation rather than a consistent, proportional decline in all measures of variation. Our results also indicate that the structuring of quantitative genetic variation can be relatively insensitive to landscape fragmentation, at least over the timescales considered by most conservation biologists.

Analyses of climate response and phenological mismatch using herbarium data

Phenological data have emerged as effective tools for studying the impact of climate change on demographic processes. However, long-term field records, spanning the past 100 years of global warming, are lacking for many species. Phenological records from collections in biological museums include samples from a large number of species and locations, and often extend back more than 100 years, providing data for a much longer period than conventional data sets.

I am in the process of collecting and analysing phenological data from herbarium specimens with special emphasis on abundantly collected, spring-flowering species for which the phenological phase of each individual can be quantified. Data from c. 90 species, representing about 17000 collections from all parts of South Sweden are being analysed to estimate the rate of long-term change in flowering phenology and the year-to-year relationship between flowering time and spring temperature.

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