Rapid population differentiation in a mosaic environment

SummaryReciprocal transplants were made of pairs of contrasting populations of Anthoxanthum odoratum collected from the Park Grass Experiment at Rothamsted.In most cases, plants survived longer, produced more tillers and produced more dry matter when transplanted into their native plots than when transplanted into ecologically contrasting plots.The average half-life of plants transplanted into contrasting plots was 8 months, that of plants in their native plots was about 2 years.Selection coefficients against “alien” populations based on survival after 18 months ranged from 0·09 to 0·77 with a mean value of 0·36. Similar coefficients of selection were calculated for tiller number and plant weight.The coefficients of selection on the various plots were correlated with both the yield of herbage on the plots and the vegetation height on the plots, this is discussed in relation to the observed pattern of differentiation of A. odoratum on the plots.We conclude that selection pressures acting upon A. odoratum in the mosaic of environments that occur on the Park Grass Experiment are large and sufficient to account for the morphological and physiological differences that have been observed between closely adjacent populations of A. odoratum on the plots.

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