CHANGES IN THE VARIATION PATTERN OF A HYBRID POPULATION OF HELIANTHUS OVER AN EIGHT‐YEAR PERIOD

Recent studies of natural populations of higher plants have pointed consistently to the hypothesis that hybridization between populations having markedly different adaptive properties, particularly if it occurs in a disturbed, changing environment where new ecological niches are likely to be available, is one of the strongest factors bringing about evolutionary change (Stebbins, 1959). Consequently studies of such populations, even if carried out over the relatively short periods of time available to the average investigator, should yield valuable information about the evolution of natural populations. The annual species of Helianthus, or sunflowers, are particularly favorable for this purpose. They are variable, widespread, and weedy, entering often into disturbed habitats. They are cross-fertilizing, due to self-incompatibility. Hybridization between species occurs frequently, and the interspecific hybrids, although highly sterile, nevertheless can produce some offspring, particularly through backcrossing, or introgression. There is some evidence that the variation of some species has been increased by this process. Since all of the annual species have the same chromosome number, n= 17, evolution within the group has not involved polyploidy. Finally, the group has been subjected to a series of careful cytotaxonomic studies by Heiser (1947, 1949, 1951a,b, 1954) from whose work the above information was obtained. The two annual sunflowers of California, Helianthus annuus and H. Bolanderi, occupy different ecological niches. The former is a weed of roadsides, very common in the Central Valley and in Southern California. The commonest subspecies is subsp. lenticularis (Dougl.) Ckll., which was already in the state when the first botanical collections were made, and was used by the Indians for various purposes (Heiser, 1949). Since it does not now occur in natural sites in California, it was very likely introduced originally by the Indians from farther east. Since the advent of the white man, repeated introductions from the eastern states have almost certainly been made. The cultivated sunflower, H. annuus subsp. macrocarpus, frequently escapes to roadsides, and hybrids between the two subspecies are not uncommon (Heiser, 1949, 1954). Helianthus Bolanderi, on the other hand, is a native, endemic species, its natural range extending from southern Oregon to south central California. It consists of two races, one of which is found in the inner coast ranges and Sierra foothills, chiefly on serpentine soil; the other is a ruderal plant which replaces or occurs with H. annuus subsp. lenticularis, in the western part of the Sacramento Valley, particularly in Solano, Yolo, and Colusa counties. Heiser (1949) made artificial hybrids between H. annuus and H. Bolanderi, and compared them directly with the natural hybrid swarms. He found that the F1 hybrids had variable pollen and seed fertility, ranging mostly between 2 and 10 per cent. Meiosis in them is somewhat irregular, with chains of 6 chromosomes being usually found and chains of 8 occasionally. The two species differ, therefore, by several reciprocal translocations. Plants of backcross progenies have fertility values ranging from as low as the F1 hybrids to as high as the parental species. 1 Part of this study was carried out at Brookhaven National Laboratory under the auspices of the U. S. Atomic Energy Commission. 2 Present address: Department of Biological Sciences, San Fernando Valley State College, Northridge, California.