The Natural Hybrid Between Coulter and Jeffrey Pines

Abstract

During recent years, biologists have developed an increasing interest in hybrids among trees. This interest has been shared equally by the practical forester and by those biologists interested mainly in the genetics involved and in the evolutionary significance that might be attached to these hybrids. In order to study satisfactorily tree hybrids from both points of view, the extent and type of hybridization that has taken place under natural conditions must be determined. The objective of the present investigation was to determine this for two species of hard pines, Pinus coulteri Lamb. and Pinus jeffreyi Grev. and Balf., which were considered to be distantly related on the basis of morphological characters. Previous to this study, these two pine species, whose ranges overlap only in central and southern California, were known to hybridize naturally, one hybrid tree having been found in the San Bernardino Mountains (this tree died recently) and several having been located in the mountains of southern San Benito County. In addition, a number of artificial first generation crosses and back-crosses have been produced at the Instituteof Forest Genetics at Placerville, California. In order to study this particular natural hybrid, three independent studies were made: one of the Coulter pine parent in all parts of its range, one of the Jeff rey pine parent throughout its range, and the other of the hybrid wherever it was found. Although the primary interest was in the actual problem of the hybridization and how it was taking place, it was also necessary to study the parental populations, since individual trees of the same species, as well as whole populations within a species, may vary. Therefore, it was essential to determine if one was dealing with the results of hybridization, or merely with inherent variation of the parental species.