Ecosensitivity of metric characters — a new methodic approach

Abstract

In the literature concerning metric characters of man, the question of the share of genetic and environmental factors in determination of morphological features was approached almost exclusively by studies of heritability. It is commonly accepted by the majority of authors that measures of heritability (such as h²) do directly inform about the degree of genetic determination of characters in question, and consequently, after simple reinterpretation, about the extent to what the characters may be modified by influences from the environment. Such an approach to the problem of genetic/environmental control of human morphology seems unwarranted and rather misleading. As is well known, analyses of phenotypic variance made in order to partition it into components due to certain effects of genes and of environment serve various purposes; the measure of heritability derived from such an analysis is useful in studies of selection, but, as comprising into one numerical value information on both additive genetic variance and total phenotypic variance, is in fact unapplicable for studies concerning ecosensitivity unless supplemented by additional numerical information.

Hence, the authors propose to construct new measures of ecosensitivity and genetic polymorphism of measurable characters. Derivation of these measures is based on partitioning of phenotypic variance into components due to genetic endowment and to environmental influences as well as consideration of average sizes of characters. The measures are directly corresponding to notions of: the degree of sensitivity of a character to environmental conditions (ecosensitivity), genetically determined size (average effect of all genes), and variability of genetic determination (genetic polymorphism). All these notions pertain to a given population in its own environmental conditions. Genetically determined size may be to a satisfactory approximation measured by the average size of a character in a group studied, ecosensitivity is measured by stating to what extent (in what a fraction) the size of the character is modified in response to varying from individual to individual environmental conditions. Genetic polymorphism is simply the variance of genotypes in a group. This last variable is dependent only upon properties of gene pools, the remaining two always result from the interaction between genetic endowment and environment. For formulas concerning derivation of respective measures of ecosensitivity D_e, and genetic polymorphism P_g see the text. Symbols in the formulas are those commonly adopted for denoting “additive” V_a, “dominance” V_d, “interaction” V_i, and “environmental” V_e parts of phenotypic variance V_p, heritability h², and arithmetic mean x.

Values of D_e and P_g were computed for three sets of anthropometric data originating from examinations of Belgians [Susanne, 1971], Brazilians [Da Rocha et al., 1972], and Polish [Belniak, 1971; Skład, 1973; Urbańczyk, 1975]. As may be seen from Figure 1, measures of heritability do not show any significant intergroup correlation, whence measures of ecosensitivity and genetic polymorphism of anthropometric characters are highly concordant for all three groups, enabling thus inter-character comparisons and interpretations in contrast to hardly interpretable data on heritability. Comparing average values of D, computed separately for males and females in each group (tab. 2), it may be concluded that there is no such a thing like "higher male susceptibility to environmental stresses," at least in relation to morphological characters. "Composite" characters, i.e. those that include various types of tissues in considerable proportions in single measurement (e.g. body weight, circumferences), show higher both ecosensitivity and genetic polymorphism than characters expressing almost exclusively the size of skeletal parts (e.g. head measurements). This fact is easily explainable on grounds of simple consideration that most probably more loci and alleles as well as environmental factors are involved in the determination of several differing tissues than of one kind of a tissue.

The entire array of problems concerning technical difficulties of investigations as well as individual differences in ecosensitivity depending on the variability of genotypes (e.g. homo- or heterozygosity), and the like, are only briefly mentioned or left for further discussion. Application of presented here measures would enable to decide upon the selection of particular anthropometric characters for a given type of study as well as to investigate some general problems concerning sources of human biological variability and thus adaptive mechanisms.