Genetic structure of natural populations : aspects of the association between genetical and morphological variability patterns

Abstract: Most species comprise a number of more or less reproductively isolated subpopulations. These subpopulations may exhibit striking differences in morphology and ecology, and on the basis of those differences they are often given the rank of subspecies, race, stock, variety, etc. Morphological and ecological traits most often are quantitatively inherited, and to a large extent, are affected by environmental factors. Phenotypic differences, therefore, may give little or no information on genotypic differences. As a consequence, the amount of genetic divergence between populations in many cases is poorly known.The need for such information from an evolutionary and management perspective provided the impetus for this study, which describes the genetical and morphological variability patterns revealed by protein polymorphism and morphological characters. Four species were chosen from three diverse taxonomic groups: Atlantic herring (Clupea harengus), Norway spruce (Picea abies), and Palaemon prawn (P. adspersus and P. squilla). Associations between variability patterns for the two sets of characters were also analyzed to provide information on the causes of morphological variability in natural populations.In Atlantic herring, the amount of genetic differentiation observed was conspicuously small: more than 99% of the total gene diversity was found within populations. This contrasts with previous conclusions that were based on morphological data. The present study indicates that environmental factors have an important influence on the morphological differentiation pattern; they suggest, also, that herring stocks either diverged rather recently or that the amount of gene flow is large enough to prevent genetic differentiation.The patterns for distribution of genetical and morphological variability in Norway spruce, on the other hand, are very similar, indicating that the same evolutionary forces have acted on both sets of characters. A clear geographic component of variation was also observed. Data suggest that historical events during the last glaciation have affected not only the pattern of distribution of allozymic variation in the Norway spruce but probably also the pattern of its morphological traits.The analysis of data collected from progeny of wind-pollinated Norway spruce grown under experimental conditions suggests that inbreeding and inbreeding depression are common phenomena in trials of this species. A significant positive relationship found between allozyme heterozygosity and growth performance (apparent allozyme heterosis) in one population appears to be due to dominance rather than overdominance.Members of nontidal populations of the prawn Palaemon squilla are known to be much smaller than their tidal relatives, and to produce fewer offspring. Using estimates of genetic divergence based on allozyme data, we tested the hypothesis that these deficiencies are an effect of continuous gene flow resulting in poor adaptation. Genetic data did not support this hypothesis: P. squilla ought to have the possibility to adapt to local conditions.The results from the present study modify some previous conceptions about genetical and morphological variability patterns in all species that were investigated.In combination these findings demonstrate the importance of true genetic data for clarifying genetic population structure, understanding the forces that underlie current population structure, and for interpreting ecological and morphological variability patterns among natural populations.