Situations are known where sterility barriers have been detectedwhich not only are unmarked by any morphological distinction, butactually intersect with lines of demarcation defined by morphologicaland ecological properties. Excellent examples have been described in thegenus Clarkia by Lewis (1953). In the collective species C. deflexa, somemajor morphologically distinguishable groups are interfertile, whilst inthe morphologically uniform race polyantha populations of differentprovenance show intersterility due to chromosome structural differences.
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