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In Iceland the genus Betula is represented by B. rtana L., which has little morphological variation, and a variable series of shrub and tree birches. A comparison of these variable populations in the Skjaldfannardalur area, Isafjardardjup, north-west Iceland with British populations of B. pubescens Ehrh. (2W = 56) and B. nana (2n = 28) using histograms of hybrid indices has been made and it is concluded that the variation is due to introgressive hybridization between B. pubescens and B. nana. The introgressed B. pubescens populations have a lower fertility than those of B. nana.

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