Evolution Of Life-history Patterns Research Articles

Growth rates in a captive population of Tonkean macaques.

Measuring variations in body mass is necessary to gain a deeper understanding of the evolution of life-history patterns, and it provides information on the timing of sexual maturity and the development of sexual dimorphism. In this study, we collected longitudinal data on body mass from infancy to adulthood in a captive population of Tonkean macaques (Macaca tonkeana). Tests to evaluate whether social group, maternal age, and dominance rank influenced growth rates showed that they had no significant effect. We investigated the timing and magnitude of breaking points in the growth paths of males and females, and checked whether these breaking points could correspond to specific reproductive and morphological developmental events. We found that male and female Tonkean macaques have roughly equivalent body masses until around the age of four, when males go through an adolescent growth spurt and females continue to grow at a constant rate. Males not only grow faster than females, but they also continue to grow for nearly one and a half years after females have attained their full body mass. Growth rate differences account for approximately two-thirds of the body mass sexual dimorphism; only the remaining third results from continued male growth beyond the age where full body mass is reached in females. We also discovered remarkable correspondences between the timing of testicular enlargement and the adolescent growth spurt in males, and between dental development and slowdown breaking points in both sexes.

Life‐History Evolution in Guppies. VII. The Comparative Ecology of High‐ and Low‐Predation Environments

Prior research has demonstrated a strong association between the species of predators that co-occur with guppies and the evolution of guppy life histories. The evolution of these differences in life histories has been attributed to the higher mortality rates experienced by guppies in high-predation environments. Here, we evaluate whether there might be indirect effects of predation on the evolution of life-history patterns and whether there are environmental differences that are correlated with predation. To do so, we quantified features of the physical and chemical environment and the population biology of guppies from seven high- and low-predation localities. We found that high-predation environments tend to be larger streams with higher light levels and higher primary productivity, which should enhance food availability for guppies. We also found that guppy populations from high-predation environments have many more small individuals and fewer large individuals than those from low-predation environments, which is caused by their higher birth rates and death rates. Because of these differences in size distribution, guppies from high-predation environments have only one-fourth of the biomass per unit area, which should also enhance food availability for guppies in these localities. Guppies from high-predation sites allocate more resources to reproduction, grow faster, and attain larger asymptotic sizes, all of which are consistent with higher levels of resource availability. We conclude that guppies from high-predation environments experience higher levels of resource availability in part because of correlated differences in the environment (light levels, primary productivity) and in part as an indirect consequence of predation (death rates and biomass density). These differences in resource availability can, in turn, augment the effect of predator-induced mortality as factors that shape the evolution of guppy life-history patterns. We found no differences in the invertebrate communities from high- and low-predation localities, so we conclude that there do not appear to be multitrophic, indirect effects associated with these differences in predation.

Male Reproductive Competition and Sex-Specific Growth Patterns in Bluegill

A comparative study of populations of bluegill Lepomis macrochirus from 12 southern Wisconsin lakes was conducted to examine the social and reproductive factors associated with variation in growth rates, and to determine whether changes in growth rates and the relative abundance of alternative male mating types could be attributed, in part, to adaptive changes in bluegill life history parameters. Actively spawning colonies were encircled with 46-m-long seines to collect nesting parental males, reproductive females, and precocial males (cuckolders). Scale samples were used to back calculate size and estimate age of first maturation. Morphometric image analysis was used to quantify the degree of sexual dimorphism within populations. Adult parental males were more likely to exhibit reduced growth rates after maturation than females. In addition, adult parental males from lakes containing stunted populations matured at younger ages than males from lakes with populations that were not stunted. This shift in the age of maturation was correlated with a reduction in sexual dimorphism and the intensity of male competition for nesting sites. The abundance of alternative male mating types was inversely related to the intensity of male sexual selection, with cuckolder abundance reaching nearly 60% of reproductive males within colonies from the most stunted populations. These changes in sex-specific life history patterns are in accord with current theory on the evolution of life history patterns. At least two factors may be responsible for the observed shifts. Increased availability of nesting habitat in shallow littoral lakes may reduce competition among males by allowing less competitive males to acquire territories. Alternatively, sex-specific angling practices that remove large males from nesting colonies may allow younger, less dominant males to move into colonies and successfully spawn. These findings suggest that removal of nesting males will have the greatest effect on male growth in lakes with small amounts of pelagic habitat and where bluegill nest in numerous small colonies.

GENETIC AND MATERNAL EFFECTS ON SEEDLING CHARACTERS OF SOLIDAGO ALTISSIMA (COMPOSITAE)

The relative contributions of genetic and maternal factors to phenotypic variation were studied in seedlings of the clonal herb Solidago altissima L. Pairwise crosses were made to establish 19 full‐sibships. Because crosses were reciprocal, each parent served as seed parent to half its offspring and as pollen parent to the other half. For 18 characters, genetic variation was estimated from the between sibship variance and maternal variation from the variance between maternal groups within sibships. Early in the growing season, maternal effects exerted strong influence over variation in leaf and stem dimensions, while genetic influences were weak or absent. At the end of the season genetic influences on leaf and stem dimensions, relative biomass allocation between stem and leaves, and the propensity to flower were significant but maternal effects were not. However, both genetic and maternal influences were found on the number and lengths of new rhizomes.Phenotypic, genetic, and maternal correlation matrices were examined through principal component analysis for trade‐offs among vegetative growth, clonal expansion and floral reproduction which could constrain the evolution of life history patterns. At the phenotypic level, seedlings with strong vegetative growth produced more and longer rhizomes, and were more likely to flower. Genetically, correlations among these functions were weak or absent. Strong maternal contributions to seedling vegetative growth correlated positively with the number and length of rhizomes. Thus, no trade‐offs acting during the seedling stage were found. However, there were indications of a genetically influenced trade‐off between the number of rhizomes produced in the seedling year with the size of the ramets produced in the second year.

Genetic and Maternal Effects on Seedling Characters of Solidago altissima (Compositae)

The relative contributions of genetic and maternal factors to phenotypic variation were studied in seedlings of the clonal herb Solidago altissima L. Pairwise crosses were made to establish 19 full-sibships. Because crosses were reciprocal, each parent served as seed parent to half its offspring and as pollen parent to the other half. For 18 characters, genetic variation was estimated from the between sibship variance and maternal variation from the variance between maternal groups within sibships. Early in the growing season, maternal effects exerted strong influence over variation in leaf and stem dimensions, while genetic influences were weak or absent. At the end of the season genetic influences on leaf and stem dimensions, relative biomass allocation between stem and leaves, and the propensity to flower were significant but maternal effects were not. However, both genetic and maternal influences were found on the number and lengths of new rhizomes. Phenotypic, genetic, and maternal correlation matrices were examined through principal component analysis for trade-offs among vegetative growth, clonal expansion and floral reproduction which could constrain the evolution of life history patterns. At the phenotypic level, seedlings with strong vegetative growth produced more and longer rhizomes, and were more likely to flower. Genetically, correlations among these functions were weak or absent. Strong maternal contributions to seedling vegetative growth correlated positively with the number and length of rhizomes. Thus, no trade-offs acting during the seedling stage were found. However, there were indications of a genetically influenced trade-off between the number of rhizomes produced in the seedling year with the size of the ramets produced in the second year.

The Genetic and Environmental Control of Reproductive Timing in a Short-Lived Monocarpic Species Daucus Carota (Umbelliferae)

(1) Offspring of annual, biennial and triennial Daucus carota were grown under three nutrient regimes in a growth chamber to measure the effects of nutrient supply and maternal age of flowering on offspring size and growth rate and the effects of all four variables on year of flowering. (2) Offspring rosette size and recent growth rate were both good predictors of year of flowering. An increase in size but a decrease in relative growth at the end of the summer were associated with an increased probability of flowering in the next season. The results are consistent with the Wilbur—Collins model, which suggests that both size and recent growth rate influence reproductive timing and that individuals track resources by delaying the year of reproduction if resources abound or by accelerating reproduction if resources become limiting. This model, although proposed for amphibians, may describe the general relationship between growth and reproductive timing in monocarpic plants. (3) Maternal age and nutrient supply influenced rosette size, relative growth rate, and flowering time. Annual maternal plants and high nutrients produced both the largest offspring and the greatest number of annuals. Also, the maternal contribution acted directly upon the year of flowering as well as indirectly through size and recent growth. Both genetic variability and habitat heterogeneity explain the variation in year of flowering in natural populations. (4) The results provide evidence that the early-rosette growth rate and the length of the pre-reproductive period are negatively correlated, as predicted by theories about the evolution of life-history patterns. Rapid growth may help annuals to persist in habitats which generally favour biennials. (5) Response to nutrient supply did not vary among maternal age groups as might be expected in colonizing species. This suggests that phenotypic plasticity in response to nutrient supply evolves independently of year of reproduction.

Variance Models in the Study of Life Histories

We propose a general model that estimates fitness from the joint effects of mean and variance. In this hierarchical model the contribution that each individual trait makes to net fitness proceeds in a stepwise fashion from the individual trait level to the fitness component and cohort fitness levels and, finally, to net fitness. We describe useful mathematical functions that incorporate both mean and variance values at each level. Empirical examples (1) demonstrate that variance, in addition to mean, can be an important determinant of fitness and (2) show how both can be used to estimate fitness, in particular in research addressing the evolution of life history patterns.

Life History Phenomena in Two Populations of the Lizard Sceloporus undulatus in Southwestern New Mexico

Fecundity, survivorship and population characteristics were determined for the lizards Sceloporus undulatus consobrinus and S. u. tristichus in southwestern New Mexico. Sceloporus u. consobrinus lays at least four clutches per year with a mean clutch size of 9.9 eggs. Sceloporus u. tristichus lays at least two clutches per year with a mean clutch size of 7.2 eggs. The deviations about the regressions of clutch size on snout-vent length were significantly different between the two subspecies. In S. u. tristichus, eggs are heavier, survivorship higher, predation (as estimated by tail loss frequency) lower, mean adult size smaller, and the population is more dense than in S. u. consobrinus. The S. u. tristichus population is probably closer to K (environmental carrying capacity) than that of S. u. consobrinus. The life history and population characteristics of other populations of the S. undulatus group were compared in light of current concepts of rand K-selection, and these concepts were evaluated. INTRODUCTION Intraspecific variability in the life history characteristics of age at maturity, juvenile survivorship, adult life expectancy, clutch size and age-specific fecundity was described for the lizard Sceloporus undulatus by Tinkle and Ballinger (1972) and Tinkle (1972). Tinkle compared population and life history phenomena in seven populations of S. undulatus to gain insight into the evolution of life history differences among lizards. The purposes of the present study were: (1) to add information concerning life history and population differences in two more populations of S. undulatus, and (2) to evaluate current theory, particularly the role of resource availability, concerning evolution of life history patterns and population phenomena in light of the empirically obtained data. MATERIALS AND METHODS The two subspecies studied were S. undulatus consobrinus and S. u. tristichus. Detailed taxonomic descriptions of these two subspecies are given in Cole (1963). Males of both subspecies have a blue patch on each side of the belly and the throat. Female S. u. consobrinus lack blue belly patches, whereas female S. u. tristichus have blue belly patches which tend to be lighter than those of the male. S. u. consobrinus is the larger of the two subspecies and males tend to be smaller than females in both. Both subspecies were studied where they seemed most abundant in southwestern New Mexico. S. u. consobrinus was studied in a desert1 Present address: Department of Environmental Health, Kettering Laboratory, University of Cincinnati Medical Center, Cincinnati, Ohio 45267.