Evolutionary Optimum Research Articles

SELECTION ON INCREASED INTRINSIC GROWTH RATES IN COHO SALMON, ONCORHYNCHUS KISUTCH

Substantial evidence from the animal kingdom shows that there is a trade-off between benefits and costs associated with rapid somatic growth. One would therefore expect growth rates under natural conditions to be close to an evolutionary optimum. Nevertheless, natural selection in many salmonid species appears to be toward larger size and earlier emergence from spawning redds, indicating a potential for increased growth rate to evolve. We tested how selection for genetic variants (growth hormone transgenic coho salmon, Oncorhynchus kisutch, with more than doubled daily growth rate potential relative to wild genotypes) depended on predator timing and food abundance during the early period of life (fry stage). In artificial redds, fry of the fast-growing genotypes showed a highly significant developmental shift, emerging from gravel nests approximately two weeks sooner, but with an 18.6% reduced survival, relative to wild-genotype fry. In seminatural streams, fry of the fast-growing genotypes suffered higher predation than those of wild genotypes when predators were present at the time of fry emergence, but this difference was less pronounced when food was scarce. In streams where predators were introduced after emergence, fry survived equally well regardless of food availability. Surviving fry grew faster in habitats provided with more food, and fast-growing genotypes also grew faster than wild genotypes when predators arrived late and food was abundant. Fewer fish migrated downstream past a waterfall when food availability was high and in the presence of predators, and wild-genotype fry were more likely to migrate than fry of the fast-growing genotypes. After being returned to the experimental streams after migration, fast-growing genotypes survived equally well as those of the same genotypes that did not migrate, whereas migrating wild genotypes experienced higher mortality relative to those of the same genotypes that did not migrate. Comparisons of growth rates between siblings retained under hatchery conditions and those from habitats with the fastest growth in the experimental stream revealed that growth rates were similar for wild genotypes in both environments, whereas the fast-growing genotypes in the streams only realized 90% of their growth potential. The present study has shown that a major shift in developmental timing can alter critical early stages affecting survival and can have a significant effect on fitness. Furthermore, ecological conditions such as food abundance and predation pressure can strongly influence the potential for fast-growing variants to survive under natural conditions. The large-scale removal of many predatory species around the world may augment the evolution of increased intrinsic growth rates in some taxa.

NATURAL SELECTION ON EXTRAFLORAL NECTAR PRODUCTION IN CHAMAECRISTA FASCICULATA: THE COSTS AND BENEFITS OF A MUTUALISM TRAIT

Cost-benefit models of the evolution of mutualism predict that the current state of mutualism results from trade-offs between fitness costs of mutualist traits and the fitness benefits of association. We test the assumptions of such models by measuring patterns of natural selection on a mutualist trait, extrafloral nectar production in Chamaecrista fasciculata. Selection was measured on plants from which ants had been excluded (removing the mutualist benefit of the trait), from which all insects had been excluded (removing costs of herbivory in addition to mutualist benefits), and unmanipulated plants (where both costs and benefits were present). Selection analysis based on half-sibling-mean regressions of fitness on the trait revealed no evidence of costs of extrafloral nectar production in the absence of all insects or in the absence of ants. However, examination of the selective surfaces for these treatments suggest that costs of nectar production may exist and are exacerbated by the presence of herbivory. In the presence of ants, natural selection favors high extrafloral nectar production, consistent with a fitness benefit to this mutualist trait in the presence of the mutualist partner. In this study, the interaction of costs and benefits did not produce an evolutionary optimum for the trait within the range of variation observed, suggesting that application of a cost-benefit framework to this trait will benefit from considering the influence of temporal and spatial variation on the quality of costs and benefits.

NATURAL SELECTION ON EXTRAFLORAL NECTAR PRODUCTION IN CHAMAECRISTA FASCICULATA: THE COSTS AND BENEFITS OF A MUTUALISM TRAIT

Cost-benefit models of the evolution of mutualism predict that the current state of mutualism results from trade-offs between fitness costs of mutualist traits and the fitness benefits of association. We test the assumptions of such models by measuring patterns of natural selection on a mutualist trait, extrafloral nectar production in Chamaecrista fasciculata. Selection was measured on plants from which ants had been excluded (removing the mutualist benefit of the trait), from which all insects had been excluded (removing costs of herbivory in addition to mutualist benefits), and unmanipulated plants (where both costs and benefits were present). Selection analysis based on half-sibling-mean regressions of fitness on the trait revealed no evidence of costs of extrafloral nectar production in the absence of all insects or in the absence of ants. However, examination of the selective surfaces for these treatments suggest that costs of nectar production may exist and are exacerbated by the presence of herbivory. In the presence of ants, natural selection favors high extrafloral nectar production, consistent with a fitness benefit to this mutualist trait in the presence of the mutualist partner. In this study, the interaction of costs and benefits did not produce an evolutionary optimum for the trait within the range of variation observed, suggesting that application of a cost-benefit framework to this trait will benefit from considering the influence of temporal and spatial variation on the quality of costs and benefits.

An error limit for the evolution of language.

On the evolutionary trajectory that led to human language there must have been a transition from a fairly limited to an essentially unlimited communication system. The structure of modern human languages reveals at least two steps that are required for such a transition: in all languages (i) a small number of phonemes are used to generate a large number of words; and (ii) a large number of words are used to a produce an unlimited number of sentences. The first (and simpler) step is the topic of the current paper. We study the evolution of communication in the presence of errors and show that this limits the number of objects (or concepts) that can be described by a simple communication system. The evolutionary optimum is achieved by using only a small number of signals to describe a few valuable concepts. Adding more signals does not increase the fitness of a language. This represents an error limit for the evolution of communication. We show that this error limit can be overcome by combining signals (phonemes) into words. The transition from an analogue to a digital system was a necessary step toward the evolution of human language.

Populations can persist in an environment consisting of sink habitats only.

Populations that live in environments with different habitats have to distribute their offspring over these habitats. When population densities go to equilibrium, the evolutionary optimum is an ideal free distribution. Under an ideal free distribution, no offspring should be put into sink habitats. However, when the environmental conditions in a habitat are not constant but fluctuate, allocating offspring to sink habitats can increase the long term growth rate of a population. We demonstrate this principle in a simple model for offspring allocation. As a consequence, it is possible that populations persist in environments that only consist of sink habitats.

What is the optimum size for the genetic alphabet?

An important question in biology is why the genetic alphabet is made of just two base pairs (G.C and A.T). This is particularly interesting because of the recent demonstration [Piccirilli, J. A., Krauch, T., Moroney, S. E. & Benner, S. A. (1990) Nature (London) 343, 33-37] that the alphabet can in principle be larger. It is possible to explain the size of the present genetic alphabet as a frozen character state that was an evolutionary optimum in an RNA world when nucleic acids functioned both for storing genetic information and for expressing information as enzymatically active RNA molecules--i.e., ribozymes. A previous model [Szathmáry, E. (1991) Proc. R. Soc. London Ser. B 245, 91-99] has described the principle of this approach. The present paper confirms and extends these results by showing explicitly the ways in which copying fidelity and metabolic efficiency change with the size of the genetic alphabet.

A polygenic model for the evolution and maintenance of conditional strategies

We develop a genetic model for conditional strategies which places such strategies in the context of phenotypic plasticity. The model, which treats conditional strategies as polygenic threshold traits, indicates that, given requisite genetic variation in reaction norms, conditional strategies will evolve to their optimum level and be maintained by stabilizing selection, provided environmental variation results in a fitness trade-off for the alternative conditional phenotypes. The precise value for the evolutionary optimum is found to depend primarily on the probability density function of the environmental variation that influences the production of the conditional phenotypes and the magnitude of the fitness trade-offs of the conditional phenotypes across such environmental variation. The model is tested by application to three well-studied conditional strategies. In each case the predictions of the model are in good agreement with the results of these studies.

Why Do We Die? Lifespans as an Evolutionary Optimum

Animals display wide diversity in lifespans. Some tantalizing empirical relations relationships have been observed about lifespans. One is that for mammals lifespan increases with brain size, and decreases with size and metabolic rate. The challenge for any theory is to explain these observations. A wear and tear theory of lifespan postulates that the metabolic rate determines the rate of wear and tear of tissues and hence lifespans. However this leaves unanswered the question why organisms with high metabolic rates possess this seemingly undesirable characteristic which reduces their lifespans. This paper proposes a theory in which lifespan and metabolic rates are the optimal response to natural constraints. Evolution chooses for an animal the optimum mix of lifespan and metabolic rate.

Genetic factors in the electrocardiogram and heart rate of twins reared apart and together

Important physiologic mechanisms have been thought not to exhibit large amounts of variability, due in part to the assumption that critical biologic functions will have evolved to an evolutionary optimum. The attainment of this optimum would necessarily eliminate individual differences in these variables. Using a sample of monozygotic and dizygotic twins reared apart since birth or early infancy, 12-lead electrocardiographic recordings and vectorcardiograms were obtained. Values of these variables for monozygotic and dizygotic twins reared together were obtained from other studies. Maximum likelihood tests of genetic and environmental components of variation for PR interval, QRS duration, QT interval and ventricular rate indicated a significant contribution of genetic effects (most heritabilities ranged from 30 to 60%), with a negligible contribution from common familial environmental effects.

Modifiers of mutation rate: Evolutionary optimum with complete selfing

It often has been assumed that, for infinite random mating populations in a constant environment, natural selection will favor genotypes at a neutral modifier locus that minimize the mutation rate. Mathematical modeling of this process confirms this assertion, independent of the selection regime. The same model under conditions of complete selfing can produce, under certain nondegenerate overdominance conditions, an optimum mutation rate below which increased mutation is favored and above which decreased mutation is favored. This occurs with unidirectional mutation models and a class of reversible mutation models with fitness overdominance. This is the first time that such a modifier optimum has been produced analytically for an infinite population in a constant environment.