Geographic variation in slash pine (Pinus elliottii Engelm.)

When a plant species occurs over a wide geographic range, individuals or populations growing in different localities frequently display differences in one or more traits. This phenotypic variation associated with locality (geographic variation) may he due to environmental or genetic factors, or interactions between them. Environmental differences are a consequence of modifications caused by habitat factors. Genetic variation associated with locality (racial variation), on the other hand, is due to such mechanisms as mutation, natural selection, hybridization, or combinations of these factors. It basically results from the fact that the individuals within populations differ genetically. The genetic heterogeneity between individuals is caused by mutation or hybridization. It is maintained by Intricate mechanisms inherent in most species, enhancing chances of survival of the species in a constantly changing environment. This genetic variation among individuals is the basis for racial variation. If the localities are characterized by different environments, and if some degree of reproductive isolation is present, racial variation will occur. Plants that are genetically most suited to their particular habitat will survive and reproduce In greater numbers than those not so well endowed. Some degree of reproductive isolation is necessary because If interbreeding occurs randomly throughout a species range, natural selection in a given locality would merely result in a change in the mean of the whole species. In forest trees, sufficient isolation is provided by the limited distance of pollen and seed dispersal. Although natural selection Is the most Important cause of racial variation, it is believed that such variation nay also result from chance fluctuations in gene frequencies (genetic drift) leading to fixation of genes. Genetic drift is most apt to occur in small, isolated populations and environmental differences need not he present. Geographic variation occurs in characteristic patterns, depending upon the nature of the forces that caused it. Since climatic factors are often important natural selection forces, and since climate often changes gradually over a species range, the pattern of racial variation frequently is continuous or clinal. However, relatively uniform and discontinuous habitats may cause relatively discrete populations or ecotypes. Likewise, present or past Isolation may cause ecotypes or combinations of both clinal and ecotypic variation. Needless to say, geographic variation In forest trees is common, and it is of great interest to forest land managers and forest scientists. The nature of geographic variation (i.e., the proportion of environmental and genetic components) is important to land managers because if differences in economically important traits are genetic they must use care in selecting sources of seed for forest planting. Likewise, forest geneticists are keenly aware of the possibilities of capitalizing on racial variation in development of superior strains. Taxonomists are interested in patterns of variation in their attempts to classify trees on both the species and subspecies level. The present study was designed mainly to Investigate the nature and patterns of geographic and racial variation for a number of characteristics in slash pine (Pinus elliottii Engelm.), one of the more important commercial trees of the Southeast. Secondary objectives were (l) to search for causes of patterns of variation that sight he found, and (2) to compare the magnitude of variation associated with localities against that associated with individuals within localities.