Measuring spatial variation in natural selection using randomly‐sown seeds of Arabidopsis thaliana

Abstract

We describe a simple method of measuring spatial variation in fitness using randomly sown seeds. Given a random initial distribution of seeds, any spatial variation in the final distribution of genotypes must be the result of spatial variation in natural selection. Departures from spatial randomness are tested using a modification of join-count statistics, based on the probability that two randomly chosen plants separated by a given distance carry the same genetic marker. Monte Carlo simulations showed that for the randomly sown seeds design the join-count test had higher power to detect spatial variation in fitness than analysis of variance. The power was relatively independent of the number of genotypes used or the number of microsites sampled. The method was tested in the field using three standard lines of Arabidopsis thuliuna. At the end of the life cycle, adjacent plants were significantly more likely to be the same line than randomly chosen pairs and the spatial dependence extended to distances of nearly 50 cm. Simple experiments like this may be especially useful for "comparative ecological genetics", allowing systematic studies of natural selection among many species or habitats.

Shade-Induced Plasticity and Its Ecological Significance in Wild Populations of Arabidopsis Thaliana

“…Indeed, the importance of small-scale variation in such factors is underscored by the significant block effects found in our field study (Table 4) and even among the spatial blocks in our greenhouse study (Table 2), as well as previous studies Lechowicz 1991, Stratton andBennington 1996). First, directional selection gradients for number of rosette leaves at bolting may differ between years as much as or more than they do between the two contrasting sites.…”

“…At the AS site, however, correlations calculated with block means FLOWERING TIME IN WILD ARABIDOPSIS were strong and positive (1997, r ϭ 0.81; 1998, r ϭ 0.96), indicating that local environmental heterogeneity, if important, probably imposes heterogeneous selection at much finer spatial scales (Stratton and Bennington 1996). This may be due to local environmental heterogeneity, a possibility supported at the SR site where correlations between bolting date and leaf number calculated with block means were weakly negative (1997, r ϭ Ϫ0.14; 1998, r ϭ Ϫ0.13).…”

Shade induced changes in biomechanical petiole properties in the stoloniferous herb Trifolium repens

“…Most natural environments are characterized by fine-grained temporal and spatial variation in the availability of essential resources such as light, water and nutrients thereby exerting different selection pressures on plant development and morphology (Kalisz 1986;Stewart and Schoen 1987;Stratton 1995;Stratton and Bennington 1996). If changes in phenotype and/or developmental pattern confer a fitness advantage adaptive plasticity will evolve (Dudley and Schmitt 1996;Kingsolver 1995).…”

From molecules to phenotypes? – The promise and limits of integrative biology

“…Studies of natural selection are not very useful unless they are repeated not only over several locations, but over a period of time (Kingsolver et al 2001), given the known tendency of selection pressures themselves to vary spatially and temporally , Stratton 1995, Stratton & Bennington 1996. We repeated the selection experiments in two years and in two locations, while simultaneously measuring selection on flowering time and on the genetically correlated character leaf number.…”