Explaining variation in characteristics of individuals is a central goal of evolutionary biology. When variation occurs among individuals within a population, and it is related to fitness (the relative number of offspring contributed to future generations), it is especially interesting because such variation can form the basis of natural selection and evolutionary change. In the same way that individuals within a population may vary with respect to some characteristics, the average characteristics of individuals within a population may also vary among different populations of a single species. Variation among populations of a single species is commonly observed in species that are widely distributed geographically and it provides an opportunity to study how different environments affect the characteristics of organisms.

Geographic variation in phenotypes among populations takes many forms, but a particularly important kind is variation in characteristics called life history traits. Life history traits include characteristics like growth rate, age at maturity, fecundity and life span. The importance of variation in life history traits stems from their close relationship to fitness. For example, fecundity and mortality (e.g., life span) are components of fitness, while growth rate and age at maturity are directly related to fitness. Theoretical and empirical interest in the study of variation in life history traits is therefore not surprising. Paradoxically, in only a few unusual cases, are we able to explain, and even more rarely predict, how different environments affect life history traits (see Roff, 1992). In general, we lack a comprehensive theory that applies to a wide range of organisms and environments. This is largely due to a shortage of experimental and comparative data that would allow tests of existing theory, as well as provide a basis for developing alternative theories (Dunham et al., 1989; Niewiarowski, 1994).

Study of geographic variation in the life history traits of lizards and other ectotherms (animals whose body temperatures vary with environmental temperatures) has provided some insight into the potential mechanisms that link one common kind of environmental variation (temperature) to variation in life history traits (Dunham et al., 1989; Grant and Dunham, 1990; Dunham, 1993). Since the body temperature of lizards depends on the temperature of their environment, and because physiology is temperature sensitive (Huey, 1982), temperature induced variation in physiological parameters like foraging efficiency and success, digestive rate, digestive efficiency, and metabolic rate will have direct consequences for life history traits. How? Since the rate at which individuals grow, mature and reproduce is ultimately a function of how fast they can acquire and then allocate energy harvested from their environments, any factors (e.g., temperature) which affect physiological performance can have a direct and profound effect on life history traits.

Over the past ten years I have been studying sources of variation in life history traits among widely distributed populations of the Eastern Fence Lizard, Sceloporus undulatus. My work has addressed the relative importance of environmental differences in explaining observed life history variation across the species range (Niewiarowski and Roosenburg, 1993; Niewiarowski, 1995). For example, I compared a population from Western Nebraska with one from New Jersey and discovered they are on opposite ends of a spectrum with respect to life history traits. Lizards in Nebraska grow very rapidly, mature at a young age and small size, and die relatively young. On the other hand, lizards from New Jersey grow relatively slowly, reach maturity at nearly twice the age and much larger size, and have relatively long life spans. Could differences between the environments in Nebraska and New Jersey be responsible for the observed differences in life history traits? Results from a reciprocal transplant experiment revealed (Niewiarowski and Roosenburg, 1993) that growth rate of Nebraska lizards was sensitive to differences between the Nebraska and New Jersey environments but that the growth rates of NJ lizards was not (Fig. 1). The transplant experiment together with other experiments (Niewiarowski, 1995) suggested that physiological differences (e.g., metabolic rate, digestive efficiency, etc.) between lizards from these two populations could contribute to observed differences in life history traits. Studies with other species have similarly implicated physiological variation in explaining differences among populations in life history traits (e.g., Beaupre, 1995), but its importance in the fence lizard system in specific, and its general importance in other taxa is poorly understood because variation in physiology among populations has not been widely studied (Garland and Adolph, 1994).

Currently, I am collaborating with several graduate students to study the comparative physiology (especially locomotor performance) of fence lizards from different populations.   Additionally, I have started to experiment with methods of phenotypic engineering to see if  life history traits such as age at maturity can be manipulated.  The comparative physiological research represents a new direction in my work with fence lizards.  Results from transplant experiments and comparative ecological studies suggest that investigation of variation in underlying physiological processes may help identify sources of variation in life history traits.  Phenotypic engineering of age at maturity (in collaboration with Richard Londraville, Mike Angilletta, and Michelle Balk), if successful, would represent a true breakthrough in the study of life history variation.  You can find out more about these ongoing projects by following the links below.

Locomotor and Metabolic Studies
Phenotypic Engineering Age at Maturity

Literature Cited

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Beaupre, S.J.,Dunham, A.E., Overall, K.L. 1993a. Metabolism of a desert lizard: The effects of mass, sex, population of origin, temperature, time of day, and feeding on oxygen consumption of Sceloporus merriami. Physiological Zoology 66:128-147.

Beaupre, S.J., Dunham, A.E., Overall, K.L. 1993b. The effects of consumption rate and temperature on apparent digestibility coefficient, urate production, metabolizable energy coefficient and passage time in canyon lizards (Sceloporus merriami) from two populations. Functional Ecology 7:273-280.

Beyer, E.C. and Spotilla, J.R. 1994. Seasonal variation in metabolic rates and maintenance costs of the eastern fence lizard Sceloporus undulatus. Comparative Biochemistry and Physiology 109A : 1039-1047.

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Grant, B.W., and Dunham, A.E. 1990. Elevational covariation in environmental constraints and life histories of the desert lizard Sceloporus merriami. Ecology 71:1765-1776.

Huey, R.B., Niewiarowski, P.H., Kaufmann, J., and Herron, J.C. (1989). Thermal biology of nocturnal ectotherms: Is sprint performance in Geckos maximal at low body temperatures? Physiological Zoology 62:488-504.

Huey, R.B. 1982. Temperature, physiology and the ecology of reptiles. Pages 25-91 in C. Gans and F.H. Pough, eds. Biology of the reptilia. Vol. 12. Academic Press, New York.

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Niewiarowski, P.H. (1994). Understanding geographic life history variation in lizards. In: Lizard Ecology: Historical and Experimental Perspectives. E. Pianka and L. Vitt (eds.), Princeton University Press, New Jersey. Pp. 31-49.

Niewiarowski, P.H. and Roosenburg W.M. (1993). Reciprocal transplant reveals sources of variation in growth rates of the lizard, Sceloporus undulatus. Ecology 74(7):1992-2002.

Niewiarowski, P.H. and Waldschmidt, S.W. (1992). Variation in metabolic rates of a lizard: Use of SMR in ecological contexts. Functional Ecology 6(1):15-22.

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