Adaptive Gene Substitutions in Tasmanian Eucalypts: I. Genes Controlling the Development of Glaucousness
Evolution, 9 (1):
1--14 (1955)Abstract
1. The appearance of glaucousness on stems and leaves in Eucalyptus is shown to result from either the deposition of wax on the epidermis or from structural features of the superficial tissues. 2. The melting point of the wax is apparently correlated with other classical taxonomic characteristics. 3. Variation in glaucousness within a species is either sporadic and the result of mutation and occasional hybridization, or it is clinal. 4. The clines in glaucousness are correlated with changes in frost activity, the more glaucous populations occurring in the more frosty localities. Parallel clinal variation has been observed in eight species. 5. Whilst the detailed inheritance of glaucousness has not been worked out, it is clear from transplant experiments and other evidence that the clines in glaucousness represent changes in allelic frequencies at one or more loci controlling the development of wax. 6. The clines are the result of natural selection favoring the survival and reproduction of genotypes capable of developing waxy glaucousness in the more frosty regions of the species range. 7. A comparison is made of the steepness of the clines between genetically isolated species and these intra-specific clines in glaucousness. It appears probable from such comparisons that the forces of selection which create and maintain the clines are of the same order in the two cases. 8. From a consideration of sterility barriers within the genus and other evidence, it appears unlikely that the parallel variation in glaucousness has arisen by any process of introgression. Similar environmental stresses have evoked similar adaptations in a number of related but reproductively isolated species populations.