Heritability Of Life History Traits Research Articles

A review of the life cycles and life-history adaptations of pelagic tunicates to environmental conditions

AbstractDeibel, D., and Lowen, B. 2012. A review of the life cycles and life-history adaptations of pelagic tunicates to environmental conditions. – ICES Journal of Marine Science, 69: 358–369. Phylogeny, life cycles, and life-history adaptations of pelagic tunicates to temperature and food concentration are reviewed. Using literature data on lifetime egg production and generation time of appendicularians, salps, and doliolids, rmax, the maximum rate of lifetime reproductive fitness, is calculated as a common metric of adaptation to environmental conditions. The rmax values are high for all three groups, ranging from ∼0.1 to 1.9 d−1, so population doubling times range from ∼8 h to 1 week. These high values of rmax are attributable primarily to short generation times, ranging from 2 to 50 d. Clearly, pelagic tunicates are adapted to event-scale (i.e. days to weeks) rather than seasonal-scale changes in environmental conditions. Although they are not closely related phylogenetically, all three groups have a unique life-history adaptation promoting high lifetime fitness. Appendicularians have late oocyte selection, salps are viviparous, and doliolids possess a polymorphic asexual phase. There has been little research on hermaphroditic appendicularians, on large oceanic salps, and on doliolids generally. Research is needed on factors regulating generation time, on the heritability of life-history traits, and on age- and size-specific rates of mortality.

Quantitative genetics of life-history traits in a long-lived wild mammal.

Quantitative genetic studies of life-history traits in wild populations are very rare, yet variance/covariance estimates of these traits are crucial to understanding the evolution of reproductive strategies. We estimated heritabilities (h2) of several life-history traits (longevity, age and mass at primiparity, and reproductive traits) in two bighorn sheep (Ovis canadensis) populations, and both phenotypic (rP) and genetic (rA) correlations between life-history traits in one population. We included adult mass in our analyses because it is related to several life-history traits. We used the mother-daughter regression method and resampling tests based on data from long-term monitoring of marked females. Contrary to the theoretical prediction of low heritability for fitness-related traits, heritability estimates in the Ram Mountain population ranged from 0.02 to 0.81 (mean of 0.52), and several were different from zero. Coefficients of variation tend to support the hypothesis of a higher environmental influence on life-history traits. In contrast, at Sheep River we found low heritabilities of life-history traits. Phenotypic correlations varied between -0.09 and 0.95. Several genetic correlations were strong, particularly for different reproductive traits that are functionally related, and ranged from -0.34 to 1.71. Overall, genetic and phenotypic correlations between the same variables were similar in magnitude and direction. We found no phenotypic or genetic correlations suggesting trade-offs among life-history traits. Bighorn sheep may not form the large, outbred populations at equilibrium that are assumed by both Fisher's fundamental theorem and by theories predicting antagonistic pleiotropy between life-history traits. Alternatively, the absence of negative genetic correlations may result from genetic variation in ability to acquire resources or from novel environmental conditions existing during the study period.

Environmental stress and the expression of genetic variation.

We have started to test the effects of environmental extremes on the expression of genetic variation for traits likely to be under selection in natural populations. We have shown that field heritability may be high for stress response traits in contrast to morphological traits, which tend to show lower levels of heritable variation in nature compared with the laboratory. Selection for increased stress resistance can lead to a number of other evolutionary changes, and these may underlie trade-offs between favourable and stressful environments. Temperature extremes can have a marked influence on the heritability of life history traits. Heritabilities for fecundity can be high when parental flies are reared at low temperatures and under field conditions. The expression of genetic variation for development time is somewhat more complex when temperature extremes are considered. Populations at species margins may be ideal for studying the effects of environmental stress on evolution.

Variability in life history traits of the aphid, Acyrthosiphon pisum (Harris), from sexual and asexual populations.

Many aphid species have shown remarkable adaptability by invading new habitats and agricultural crops, although they are parthenogenetic and might be expected to show limited genetic variation. To determine if the mode of reproduction limits the level of genetic variation in adaptively important traits, we assess variation in 15 life history traits of the pea aphid, Acyrhosiphon pisum (Harris), for five populations sampled along a north-south transect in central North America, and for three traits for three populations from eastern Australia. The traits are developmental times and rates as affected by temperature, body weights as affected by temperature, fecundity, measures of migratory tendency, and photoperiodic responses. The most southerly population from North America is shown to be obligately parthenogenetic, as are the Australian populations, and the four more northerly North American populations are facultatively parthenogenetic with the number of parthenogenetic generations per year increasing from north to south. The broad-sense heritabilities of life history traits varied from 0.36 to 0.71 for nine quantitive traits based on a comparison of within-and between-lineage variances. Using these traits, 7-13 distinct genotypes (i.e. clones) were identified among each of the 18 lines sampled from the North American populations, but the number did not differ significantly among populations. The level of genetic variation differed from trait to trait. For 4 of 12 quantitative traits, the level of variation in the obligately parthenogenetic population from North America was lowest, but significantly lower than all the sexual populations for only 1 trait. The obligately parthenogenetic population had the highest level of genetic variation for two traits, and had intermediate levels for the others. The most northerly population, which was sexual and had relatively few parthenogenetic generations each year, had the lowest level of variation for 5 of 12 traits and the highest level of variation for 2 traits. There was no decline in variability from north to south correlated with the increase in the annual number of parthenogenetic generations. The Australian populations showed no less variation than the North American populations for two of three traits, although the pea aphid was introduced to Australia only 5 years prior to the study, whereas the aphid has been in North America for at least 100 years. The mode of reproduction has not had a substantial impact on the level of genetic variation in life history traits of the pea aphid, but there are population-specific factors that effect the level of variation in certain traits.

Quantitative genetics and fitness: lessons from Drosophila.

This paper examines patterns of heritability and genetic covariance between traits in the genus Drosophila. Traits are divided into the categories, morphology, behaviour, physiology and life history. Early theoretical analyses suggested that life history traits should have heritabilities that are lower than those in other categories. Variable pleiotrophy, environmental variation, mutation and niche variation may, however, maintain high heritabilities. In Drosophila the heritabilities of life history traits are lower than morphological or physiological traits but may exceed 20 per cent. The pattern of variation in the heritability of behavioural traits is similar to that of life history traits. Genetic covariance between morphological traits and between morphological and life history traits are all positive but those between life history traits have variable sign. Negative covariance between traits supports the variable pleiotropy hypothesis but other factors such as environmental heterogeneity, or mutation cannot be excluded.

THE GENETIC BASIS OF ALTITUDINAL VARIATION IN THE WOOD FROG RANA SYLVATICA. I. AN EXPERIMENTAL ANALYSIS OF LIFE HISTORY TRAITS.

Meaningful evolutionary interpretations of life history patterns, including age and size at first reproduction, fecundity schedule, and adult and juvenile survival, require that a distinction be made between genotypic variation and environmentally induced phenotypic variation. This distinction must be made not only between populations or geographic localities but within populations (i.e., the heritability of life history traits). As pointed out in a recent review by Stearns (1977), an extraordinarily large number of studies only document variation in life history patterns both between and within species, but then comment on its adaptive significance. In contrast, few investigations have employed experimental techniques that unambiguously distinguish the genetic and environmental sources of variation and clarify their ecological and evolutionary implications (Clausen et al., 1940, 1948; Hickman, 1975; Berven et al., 1979; Ballinger, 1979; Stearns and Sage, 1980). Ignoring the variation induced by the environment may lead to spurious conclusions of adaptiveness. Environmental influences may mask the underlying genetic differences between populations, which, in some cases, may be counter to the observed phenotypic variation (Levins, 1969; Berven et al., 1979). Specific environmental differences may also have non-additive effects on different genotypes giving rise to significant genotype X environment interactions. Furthermore, plasticity in development, whereby individuals acquire morphological or life history traits that allow them to complete their life cycles in a wide range of biotic and abiotic conditions, may be an important adaptation in its own right (Bradshaw, 1965; Marshall and Jain, 1968). Such ambiguities underscore the importance of properly identifying the nature of the observed variation. The components of the life history of an organism are generally viewed as a collection of coadapted traits which have been molded by natural selection (see review by Stearns, 1976). In this sense, distinctions among populations in life history patterns are thought to be genetically based and to reflect distinctions among environments in ecologically important selective pressures. Larger body size, delayed reproduction and larger clutch and egg sizes, are characteristic of amphibian populations living at high elevations (Pettus and Angleton, 1967; Tilley, 1973; Licht, 1975). However, as ectotherms, amphibian growth and development are directly and strongly affected by such environmental factors as temperature (Smith-Gill and Berven, 1979) and density (Wilbur, 1976, 1977a, 1977b). Some of the clinal variation in life history characteristics may have a genetic basis, but few studies have experimentally established the proportion of the variation that is induced. The absence of such evidence does not justify the assumption that all geographic variation reflects genetic differences. For the past four years I have been studying the variation in life history traits of the wood frog, Rana sylvatica, along an altitudinal gradient from Maryland to western Virginia. Wood frogs are widely distributed in North America ranging from the southern Appalachians to within the Arctic Circle. Considerable variation in morphology (Martof and Humphries, 1959), reproductive characteristics (Herreid and Kinney, 1967; Meeks and Nagel, 1973) and developmental patterns (Her-

The Influence of Temperature on Genetic Interrelationships of Life History Traits in a Population of Drosophila melanogaster: What Tangled Data Sets We Weave

Stearns (1977) suggested that in order to test the various theories of life history evolution population biologists need to collect data under controlled conditions and measure the heritabilities of life history traits. We suggest that in order to perform adequate tests of the assumptions or predictions of life history theory, investigators must perform analyses which allow them to partition the phenotypic variances and covariances of the traits observed into their components. In the case of single traits we should estimate the terms of: