PHYSIOLOGICAL VARIATION IN ANIMALS
Authors: PROSSER C. LADD 
Published: January 2008

Numerous examples are cited of physiological characters which, like morphological characters, vary genotypically, ontogenetically or phenotypically. Physiological variation is useful in describing interspecific relations, intraspecific variation and the limits of ecological range.

Physiological variation of a character permits the establishment of populations at environmental limits and thus predisposes a race for genetic fixation of a character (Baldwin effect). Such variation is primary in providing one mechanism of reproductive isolation. Secondary physiological variation functions in habitat selection, particularly after reproductive isolation.

Criteria for physiological variation are given by measurement of internal state in relation to environmental stress. In regulators a criterion is the point of failure of homeostatic mechanisms; in adjusters a criterion is the limit of tolerance after maximum acclimation. Both regulation and adjustment are adaptive to environmental change; in both types, phenotypic variation must be distinguished from genotypic variation.

Examples of physiological variation in osmotic adaptation, in different populations of the same species, are given for species of Paramecium, Euplotes, Asterias, Gammarus, Callinectes, Anopheles, Onchorhynchus and Gasterosteus. Genetically different races have been established for Gasterosteus aculeatus and Euplotes vassus.

Ionic balance has been little studied as a basis for variation of populations but specific requirements for some elements, such as copper and calcium, may have led to local differences.

Stress of temperature extremes has induced many physiological variants. Closely related large homoiotherms differ in insulating capacity, smaller species differ also in metabolic response, particularly to cold. Poikilotherms differ in tolerance limits after acclimation, in ‘selected’ temperatures, in metabolic level according to their temperature history, in Q10, in critical temperatures for reproduction and development. Physiological races with respect to temperature tolerance or selection are cited for species of Notropis, Micropterus, Gambusia, Pandalus and Carabus. Examples of metabolic differences in populations are found in aquatic but not terrestrial poikilotherms. Examples of races with respect to optimal temperature of development are found in species of Paramecium, Crassostrea, Urosalpinx, Daphnia, Drosophila, Lymantria and Rana.

Criteria of variation with respect to oxygen are the half‐saturation value of transport pigments, the critical oxygen concentration, metabolic level, tolerance of products of anaerobiosis, auxiliary breathing mechanisms and stimulation of pigment development by hypoxia. Correlations of these functions with history of oxygen availability are cited, but data are inadequate to establish genetic races.

Selection of specific food plants depends on secondary products, not primary food; many subspecies and races are known by food plant selection, especially among bugs, gall flies and butterflies. Preference can be transferred from one plant to another by forced‐feeding habituation; a food habit can be transmitted by behaviour from one generation to the next, or it may be fixed by genetic differences. Many genetic strains resistant to natural and artificial poisons are known.

Light is an isolating mechanism for cave animals, for races differing in photo‐taxes, for races and related species differing in pigmentation and in establishing photoperiodic cycles.

Variation in tolerance of type of substratum, of stream and of wind velocity isolates in some species.

Examples of isolation by reproductive behaviour are given in crickets, Drosophila, birds and other groups. Selection of host by parasites and symbionts is a subtle isolating mechanism, probably based largely on chemical specificity. Serological incompatibility is a secondary mechanism in maintaining genetic intersterility.

Physiological variation is usually accompanied by detectable morphological variation which may or may not be adaptive. Reproductive isolation may exist with slight physiological or morphological variation or conversely interbreeding may occur among very different populations.

Need for physiological study of intraspecific variation according to history and distribution of natural populations is stressed.