Evolution Of Reproductive Effort Research Articles

A Conceptual Framework for Clutch-Size Evolution in Songbirds

Causes of evolved differences in clutch size among songbird species remain debated. I propose a new conceptual framework that integrates aspects of traditional life-history theory while including novel elements to explain evolution of clutch size among songbirds. I review evidence that selection by nest predation on length of time that offspring develop in the nest creates a gradient in offspring characteristics at nest leaving (fledging), including flight mobility, spatial dispersion, and self-feeding rate. I postulate that this gradient has consequences for offspring mortality rates and parental energy expenditure per offspring. These consequences then determine how reproductive effort is partitioned among offspring, while reproductive effort evolves from age-specific mortality effects. Using data from a long-term site in Arizona, as well as from the literature, I provide support for hypothesized relationships. Nestling development period consistently explains fledgling mortality, energy expenditure per offspring, and clutch size while accounting for reproductive effort (i.e., total energy expenditure) to thereby support the framework. Tests in this article are not definitive, but they document previously unrecognized relationships and address diverse traits (developmental strategies, parental care strategies, energy requirements per offspring, evolution of reproductive effort, clutch size) that justify further investigations of hypotheses proposed here.

Evolution of reproductive effort in viscous populations: the importance of population dynamics

I present two ecological models for the evolution of reproductive effort in viscous populations with empty sites. In contrast with previous studies, I show that limited dispersal needs not have a positive effect on the evolutionarily stable allocation of resources to fecundity versus survival. Rather, depending on the feedback between the trait and the population dynamics, population viscosity may have no effect or even lead to a decrease in the evolutionarily stable reproductive effort when individuals can degrade their environment during their lifetime. I show that the different evolutionary outcomes can be explained by the asymmetry in the level of kin competition resulting from investing into juveniles or into adults.

A Case Study in Experimental Evolution: Reproductive Effort and Induced Responses in Drosophila melanogaster

Abstract To understand the causes of tradeoffs, particularly those involved in ageing, to test the reproductive effort model of life history evolution, and to measure the rate of evolution of induced life history responses, we have been performing an artificial selection experiment on Drosophila melanogaster since November 1993. It consists of four treatments, each with three replicates.The first treatment consists of constant high adult mortality; the second treatment of constant low adult mortality. In all four treatments, propyl acetate is present in the medium for 4 weeks, then absent for 4 weeks. In the third treatment, propyl acetate in the larval medium is associated with high adult mortality. In the fourth treatment, propyl acetate is associated with low adult mortality. To ensure that the effects of treatments are due to differences in mortality and not in density, adult and larval density are kept the same in all treatments.In the first two treatments, the evolution of reproductive effort and age and size at maturity has been rapid and has confirmed the predictions of the reproductive effort model. The relationships between age and size at eclosion and between size at eclosion and fecundity have also evolved. Because the only difference between the first two treatments was the level of adult mortality, this establishes differences in mortality rates as a sufficient cause for the evolution of age and size at eclosion and for the age‐distribution of fecundity rates.In the second two treatments, the evolution of induced responses has been slow, and responses are not yet consistent. The reasons for the slower evolution of induced responses suggest two new theoretical questions.

On the consequences of pre- and postbreeding costs in the evolution of reproductive effort tactics

Abstract:Models of reproductive effort have generally concentrated on the evolution of total reproductive effort, without considering the option for specific components of effort to evolve. Further, most analyses have assumed postbreeding costs of reproduction. We present an optimization model that evaluates the option for components of effort with different temporal distributions of survival costs, affecting either adult survival before offspring independence (prebreeding cost) or survival after offspring independence (postbreeding costs). We assume that total absolute effort is fixed, and that the two components therefore, are constrained in their variation. The results show that, in most cases, optimal effort tactics that imply prebreeding costs must have lower marginal effects on survival than tactics that imply postbreeding costs. Our analysis also shows that the marginal benefits on effective fecundity of effort components may influence the optimal effort tactics. An age-structured version of the mo...

THE EVOLUTION OF REPRODUCTIVE EFFORT IN SQUAMATE REPTILES: COSTS, TRADE-OFFS, AND ASSUMPTIONS RECONSIDERED.

We evaluated Shine and Schwarzkopf's (SS) model of the evolution of reproductive effort (RE) in squamate reptiles. They suggested that fecundity trade-offs were unimportant in the evolution of RE in most squamate reptiles and that only survival trade-offs needed to be considered. However, we show that by assuming no variation in offspring size exists, and that adult mortality is episodic, the results of the SS model are not general. By extension, we argue that conclusions drawn about factors important in the evolution of RE in squamate reptiles are premature. Using a modified version of the SS model, we demonstrate that variation in the form of trade-offs relating offspring size and survival substantially affect relationships among clutch size, relative clutch mass, and lifetime reproductive success. We also demonstrate that the way in which adult mortality is simulated drastically affects conclusions about the potential fecundity trade-offs experienced by populations of squamate reptiles. Finally, we suggest that a complete understanding of the evolution of RE will come from theory that incorporates trade-offs between offspring size and quality, as well as other system-specific constraints on the allocation of energy to growth, maintenance, storage, and reproduction.

The Evolution of Reproductive Effort in Squamate Reptiles: Costs, Trade- Offs, and Assumptions Reconsidered

The Evolution of Reproductive Effort in Squamate Reptiles: Costs, Trade- Offs, and Assumptions Reconsidered

THE EVOLUTION OF REPRODUCTIVE EFFORT IN LIZARDS AND SNAKES.

Life history theory suggests that the optimal evolved level of reproductive effort (RE) for an organism depends upon the degree to which additional current reproductive investment reduces future reproductive output. Future reproduction can be decreased in two ways, through (i) decreases in the organism's survival rate, and/or (ii) decreases in the organism's growth (and hence subsequent fecundity). The latter tradeoff-that is, the "potential fecundity cost"-should affect the evolution of RE only in species with relatively high survival rate, a relatively high rate of fecundity increase with body size, or a relatively high reproductive frequency per annum. Unless these conditions are met, the probable benefit in future fecundity obtained from decreasing present reproductive output is too low for natural selection to favor any reduction in RE below the maximum physiologically possible. Published data on survival rate, reproductive frequency and relative clutch mass (RCM) suggest that many lizard species fall well below the level at which natural selection can be expected to influence RE through such "potential fecundity" tradeoffs. Hence, the relative allocation of resources between growth and reproduction is unlikely to be directly optimized by natural selection in these animals. Instead, energy allocation should influence the evolution of RE only indirectly, via effects on an organism's probability of survival during reproduction. Survival costs of reproduction may be the most important evolutionary determinants of RE in many reptiles, and information on the nature and extent of such costs is needed before valid measures of reptilian RE can be constructed.

The Evolution of Reproductive Effort in Lizards and Snakes

The Evolution of Reproductive Effort in Lizards and Snakes

Clutch size, fecundity and parent-offspring conflict

Selection often acts in different ways on genes expressed in parents and offspring leading to parent—offspring conflict. The effect of parent—offspring conflict on the evolution of reproductive strategies is explored. Models are constructed using kin-selection techniques and it is argued that these are frequently more useful than techniques from classical population genetics. Parent and offspring optima are compared in models of (1) the trade-off between the number and size of offspring, (2) clutch size and (3) the evolution of reproductive effort with age structure. Parent—offspring conflict over clutch size is examined in more detail. Models of sibling competition are reviewed and it is suggested that the reduction in parental fitness caused by sibling competition may lead to selection on clutch size. The possibility that the parent may be selected to produce a hierarchy of sizes of young in order to reduce sibling conflict is investigated. The preliminary results give little support for this hypothesis. An extreme form of sibling conflict, siblicide, is also discussed. In some cases, the kin-selection approach fails in the analysis of siblicide and classical population genetic models are required. The paper concludes that parent—offspring conflict is a potentially significant, and often overlooked, factor influencing the evolution of reproductive strategies. Birds in their little nests agree And ’tis a shameful sight, When children of one family Fall out, and chide, and fight Isaac Watts,Love between Brothers & Sisters, 1721. Birds in their little nests agree With Chinamen, but not with me. Hilaire Belloc,On Food.

The Structure of Guppy Life Histories: The Tradeoff between Growth and Reproduction

This study reports on two experiments characterizing the relationship between growth and reproduction in guppies. The first experiment compares the growth of reproducing and nonreproducing siblings raised on controlled levels of food availability. Nonreproducing females have significantly more energy in somatic tissues. The bulk of this energy is in fat tissues. Standard length and "protein" do not differ significantly. The extra energy in nonreproducers is therefore devoted predominantly to energy storage, as opposed to somatic growth. A comparison of total production, however, reveals that most of the energy "saved" by not reproducing does not appear in somatic tissues. The cause of this loss is unknown. The second experiment compares the somatic growth of different stocks of Trinidadian guppies which had previously been demonstrated to devote different quantities of energy to reproduction (Reznick 1980). Increases in the amount of energy devoted to reproduction were matched by decreases in the energy devoted to somatic tissues. Furthermore, all of the stocks have equal total production. These fish are therefore equally efficient in converting food into biomass but differ in how this biomass is allocated to growth and reproduction. A key assumption of the theory predicting the impact of certain forms of selection on life history evolution is that an increase in the resources devoted to current reproduction reduces the future capacity to reproduce. Because fecundity is directly proportional to size in guppies, the reported tradeoff between growth and reproduction satisfies this assumption. However, such theoretical treatments concentrate solely on the evolution of reproductive effort. Because growth and reproduction are complementary, it is equally plausible, especially for the guppies used in these experiments, that growth rate evolved, rather than reproductive effort.

Natural selection and the evolution of reproductive effort.

Reproductive effort is defined as that proportion of the total energy budget of an organism that is devoted to reproductive processes. Reproductive effort at a given age within a species will be selected to maximize reproductive value at that age. Reproductive effort is not directly affected by changes in juvenile survivorship, nor necessarily reduced by an increase in adult survivorship. Selection for high levels of reproductive effort should occur when extrinsic adult mortality is high, in environments with constant juvenile survivorship, and in good years for juvenile survivorship in a variable environment, provided that the quality of the year is predictable by adults. Data necessary to measure reproductive effort and to understand how selection results in different levels of effort between individuals and species are discussed. We make several predictions about the effect of increased resource availability on reproductive effort. The empirical bases for testing these predictions are presently inadequate, and we consider data on energy budgets of organisms in nature to be essential for such test. We also conclude that variance in life table parameters must be known in detail to understand the selective bases of levels of reproductive effort.