A Pocket of Variability in Pinus Rigida.

Steady state gene frequencies around a suming single gene inheritance and no dompocket of differential fitness have been forinance, Teich (1970) demonstrated that mulated by Hanson (1966) in a generalizapopulations of both jack pine and lodgepole tion of the work of Haldane (1948). A pine (Pinus contorta Dougl.) fit Hardypocket of differential fitness would result Weinberg expectations. The elegant analyin a pocket-of-variability, assuming that sis of Sittmann and Tyson (1971) makes it the radius of the area of contrasting fitness highly probable that inheritance is singlewas large in relation to the vagility of the gene and that serotinous-coned trees and organism. Conversely, the absence of a open-coned trees are the respective homopocket-of-variability is predicted whenever zygotes while trees with both types of cones dispersal is broad relative to the size of the are heterozygotes. I t seems likely that the pocket of differential fitness. Around the serotinous cone habit in all pines is basipocket, a cline would be established through cally under the control of a single locus. the interaction of migration and selection. With the long generation time of trees, decThe slope of the cline would depend on the ades may be required for further verificawidth of the band of intermediate fitness, tion. being steepest where the band was absent. In pitch pine some trees bear serotinous The change in gene frequency with distance cones. Pond pine (Pinus serotina LIichx.), from the pocket would be rapid in the band with a nearly allopatric range to the south of intermediate fitness values and approach of pitch pine, is nearly 100% closed-cone limiting values asymptotically in the areas (Fowells, 1965). Pond pine was treated of contrasting fitness. with specific status by Critchfield and Little The serotinous cone habit in pitch pine (1966) but was considered a subspecies of (Pinus rigida Mill.) provided us an exampitch pine by Smouse (1970). On Cape ple of a pocket-of-variability. A serotinous May, New Jersey and on the Delmarva cone is one that remains closed for at least Penninsula a transition type occurs which one year after maturity. The cone scales belongs to neither group, according to are held together with a resin seal and heat Smouse (1970). Little et al. (1967) also is required for prompt opening. Normally, reported that these areas were zones of intemperatures in the tree crown do not reach tergradation. levels sufficient to open serotinous cones. Variation in the frequency of serotinousSerotinous cones do open and shed seed folconed trees has long been a subject of speclowing a fire, so that serotiny seems to be ulation. I t would be reasonable to hyan adaptation to fire, ensuring reproduction pothesize that the variable occurrence of a t the most opportune time in relation to serotinous cones in pitch pine is related to site availability. the influence of pond pine through active T o properly relate patterns of variability hybridization and gene flow, and that the to Hanson's model, characteristics confrequency of pitch pine with closed cones trolled by a single gene should be conwould decline with distance from Cape sidered. For Jack pine (Piaus banksiana May. A second hypothesis is that the disLamb.) there is evidence that the closedtribution of trees with the closed-cone habit cone characteristic may be under simple is related to some underlying selective facgenetic control (Rudolph et al., 1959). Astor (s), possibly fire.