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Specimens of the variable strand ragweeds of the Pacific Coasts of North and South America have commonly been divided among two or more specific or subspecific taxa. The results of this study indicate that they comprise a single heteromorphic and chemically variable species, Ambrosia chamissonis (Less.) Greene. Morphological and chemical variants within the complex show no overall correlation, and variation within and between populations is such as to suggest specific continuity. Less variable populations south of California's Monterey Peninsula appear to be derived from limited introductions from the north, and to owe their relative uniformity to genetic drift and development from limited progenitor stock. Genetic drift linked to self-pollination is evident within populations and within progeny groups developed from naturally produced seed, and is here termed ‘ethological drift’. Two varieties are recognized within the species: var. cuneifolia is found near and north of the mouth of the Columbia River. and is characterized by relatively large fruiting involucres with flattened spines and a tendency for unlobed leaves and pilose pubescence; var. chamissonis , with smaller involucres and basally inflated or terete spines, is found throughout the remainder of the range, and includes two intergrading forms—forma chamissonis with unlobed leaves and forma bipinnatisecta with pinnately decompound leaves. |
