A REEVALUATION OF THE DERIVATION OFQUERCUS MARGARETTAFROMQUERCUS GAMBELII

Abstract

In an earlier issue of this journal hypothesis was presented by one of us (Muller, 1952) to explain the origin of Quercus margaretta, an oak ranging from central and northern Texas eastward to Florida and Virginia, and Q. dr-unimondii, closely related species which occurs at the western extremity of the range of the former in central Texas. According to this hypothesis Q. gambelii, of the Rocky Mountain region, and Q. stellata, of the southeastern quarter of the United States, were the ancestral stocks from which nargaretta and drunmmondii were derived. Stated briefly, the main points were these: (a) During a period of increased precipitation at the close of the Tertiary and in the Pleistocene the spread of Q. Garnbelii stock eastward over the southern plains woLuld have been possible. Simultaneously, the westward extension of Q. stellata from an eastern center could have occurred. two prototypes would have met either on the plains or to the east of that area (op. cit.: 158). (b) ancestral Q. gamnbelii stock migrated along the Gulf Coastal Plain eastward to the Atlantic. (c) . dessication of the entire region has resulted in recession of the prototypes fromi the sonthern Great Plains and consequent isolation of the Gulf Coast populations from those of the Rocky Mountains (op. cit.: 158). (d) western end of the Gulf Coast population, through introgression from Q. stellata, has been modified to formi the population known as Q. drni-oiitondii. The lesser gene flow in the east has left the Q. Gambehii stock relatively tunchanged so that the speciation of this population (as Q. Margaretta) has in large part resulted from its isolation from the ancestral Q. Gamtbelii and only slightly from-i hybridization with Q. stellatt (op. cit.: 159). Current studies (by Tucker) of Qttercuts uindulata, hybrid complex centered in the Rocky Mountain region and undoubtedly involving Q. gaibelii as one parent, have inspired great interest in the extent and time of past migrations of Q. ganibelii stock. Continued study (by Muller) of the relations amnongst Q. stellatca, Q. margaretta, and Q. drunimmondii similarly retain the origins of these species as foci of interest. question of