NATURAL SELECTION AMONG HYBRIDS BETWEEN CARDUUS ACANTHOIDES AND C. NUTANS IN ONTARIO

Abstract

Natural hybridization is occurring between two introduced thistles, Carduus acanthoides L. and C. nutans L., in the southern half of Grey County, Ontario. The morphological variation of plants in 37 fields, including a field near Priceville previously studied in 1952, was evaluated by means of a hybrid index in 1957. Carduus nutans was first established in the hybrid area and C. acanthoides was introduced later, probably from the region northwest of Owen Sound, into waste land near Flesherton, where hybridization first occurred. Many large hybrid populations were found in the southern portion of Grey County and most of these hybrids were very similar to C. acanthoides in morphology. It was concluded that there is a numerical supremacy of acanthoides-type hybrid segregates in the hybrid area, and that such hybrids are better adapted for survival under local conditions than C. nutans and will probably supplant the latter species.