Phenotypic Variation in Natural Populations

Abstract

The recognition and subsequent study of genetic variation in conifers began with the observation that trees varied in observable or measurable (phenotypic) traits and that variation was often distributed geographically. Virtually everything we learned about genetic variation (genotypic) in forest trees before the development of allozymes, genetic markers, and genome sequences derived from the study of phenotypic variation among and within natural and domesticated populations of trees. The predominant research approach for such studies has been the common garden trial, which provides an objective means for dissecting observable phenotypic variation into its component effects, genotypic and environmental, as described by the simple equation P = G + E (phenotype = genotype + environment; Fig. 8.1). In forestry, common garden studies in which accessions from multiple natural populations of a single species are evaluated within the same test site are known as provenance trials. Common garden studies that evaluate pedigreed accessions from domesticated populations, usually across a narrower geographic area, are referred to as genetic tests or progeny tests. Here we are concerned only with the study of genetic variation in natural populations, though genetic testing has revealed a great deal about variation in phenotypic traits and the genetic basis of that variation. Historically treatments of this topic have frequently used the misleading term “geographic variation” to describe patterns of phenotypic variation across the landscape. Since we are not interested in variation in geography, but in biological variation shaped by natural selection driven by climatic factors associated with geography, the choice of “phenotypic variation” in the chapter title more accurately reflects the contents discussed here.