Trade-offs represent the costs paid in the currency of fitness when a beneficial change in one trait is linked to a detrimental change in another. If there were no trade-offs, then selection would drive all traits correlated with fitness to limits imposed by history and design. However, we find that many life-history traits are maintained well within those limits. Therefore, trade-offs must exist. Trade-offs have played a central role in the development of life-history theory, from Gadgil & Bossert (1970), Charnov & Krebs (1973), Schaffer (1972, 1974a, b) and Bell (1980) on to the present. They have been measured through field observations (e.g. Clutton-Brock, Guinness & Albon, 1982, 1983), through experimental manipulations in laboratory (e.g. Partridge & Farquhar, 1981) and field (e.g. Askenmo, 1979), through -phenotypic correlations in the laboratory (e.g. Bell, 1984a, b) and through genetic correlations (e.g. Rose & Charlesworth, 1981a, b), to mention only a few of the more prominent studies. They have been reviewed by Stearns (1976, 1977), Bell (1980), Charlesworth (1980), Warner (1984), Reznick (1985), Partridge & Harvey (1985, 1988) and most thoroughly by Bell & Koufopanou (1986). In addition, the methods used to measure trade-offs have been the subject of criticism (Tuomi, Hakala & Haukioja, 1983; Partridge, 1987) and controversy (Reznick, Perry & Travis, 1986; Bell, 1986). The most prominent life-history trade-off involves the cost of reproduction. It has two major components, costs paid in survival and costs paid in future reproduction. Two approaches to analysing those costs were suggested by Williams: genetic costs represented by antagonistic pleiotropy (Williams, 1957) and phenotypic costs represented by negative correlations between current reproductive effort and future survival and reproduction (Williams, 1966a, b). A third, physiological approach to trade-offs has been developed by Hirshfield & Tinkle (1974) and Calow (1979), among many others (cf. Townsend & Calow, 1981). In this extensive discussion, a few points have not always received the attention they deserve: (1) That trade-offs can be measured and analysed at the level of the genotype, the phenotype and what lies between (intermediate structure) is well known and uncontroversial but it has not always been emphasized that each of those levels makes an essential contribution to our understanding. It is not a question of either genetic correlations or phenotypic correlations or physiological trade-offs but of how such measurements combine to deliver information about potential evolutionary responses. A study conducted at just one of these levels is likely to be of as little use as the information on the nature of the elephant delivered by one blind man holding its tail. (2) One can draw a useful distinction between intraindividual trade-offs for example, between the reproductive effort made by a female in one season and the probability that she will survive to the next season and intergenerational trade-offs for example, between a female's reproductive effort and the probability that her offspring will survive to the next season. Intraindividual tradeoffs (and only some of them) have received most attention but intergenerational trade-offs, which are arguably just as important, have been relatively ignored. They deserve more attention. (3) The genetic structure of a population, in particular the genetic variance-covariance matrix for a set of important life-history traits, reflects the very recent past, describes the present and predicts the near-term future. There is no logical or direct way to use the current genetic structure of a population to infer the trade-offs that constrained the past approach to the current state even if they occurred as recently as a few tens of generations ago (J. Travis, personal communication). (4) Our understanding of a trade-off can be described as first order (slope known), second order (curvature known) or third order (all details, including interaction effects, known). In a few cases we have reliable information about firstorder effects. In no case known to me do we have reliable information on second-order effects, which are important in the theory (e.g. Schaffer, 1974a). Measurement of third-order effects, however desirable (Pease & Bull, 1988), remains a matter for future research.

/pdf/trade-offs-in-life-history-evolution-2lf62bfyhu.pdf

Trade-offs in life-history evolution

In the Hamadryas baboon, males are substantially larger than females. A troop of baboons is subdivided into a number of ‘one-male groups’, consisting of one adult male and one or more females with their young. The male prevents any of ‘his’ females from moving too far from him. Kummer (1971) performed the following experiment. Two males, A and B, previously unknown to each other, were placed in a large enclosure. Male A was free to move about the enclosure, but male B was shut in a small cage, from which he could observe A but not interfere. A female, unknown to both males, was then placed in the enclosure. Within 20 minutes male A had persuaded the female to accept his ownership. Male B was then released into the open enclosure. Instead of challenging male A , B avoided any contact, accepting A’s ownership.

Evolution and the Theory of Games

Theoretical studies of life history evolution almost unanimously incorporate some form of tradeoff between life history variables, such as reduced parental survival due to increased reproductive effort. These tradeoffs, or costs of reproduction, are fundamental in predicting the optimal life history in any given environment. The importance of such costs in theoretical models has motivated researchers to check that they exist by seeking an association between some component of reproductive effort, such as fecundity, and some form of cost, such as parental survival. Four methods have been used for making these evaluations: 1) Phenotypic correlations, where some index of reproductive effort is correlated with a potential cost, 2) Experimental manipulations, where reproduction is directly manipulated and a cost-related response is measured, 3) Genetic correlations, where formal quantitative genetic designs are used to estimate the genetic correlation between some component of reproductive effort and some component of cost, and 4) Correlated responses to selection, where artificial selection causes correlated changes in reproductive effort and some component of costs. Because the theoretical studies are models of evolution, I argue that a relevant empirical study must deal with genetically based tradeoffs. By this criterion, only the last two methods (which represent a small minority of the reviewed studies) properly address the "costs" dealt with by theoreticians; however, this evaluation is not intended as a litmus test for the merits of a given approach to the problem. I discuss both the important contributions of the first two methods to our understanding of the costs of reproduction and the limitations of the last two methods in assessing "costs".

Costs of reproduction: an evaluation of the empirical evidence

Variation, the fuel that feeds evolutionary change, originates at the levels of both the genotype and the phenotype. Genetically identical organisms reared under different conditions may display quite distinct characteristics. Until recently, the types and sources of such phenotypic variation have been given little consideration in evolutionary theory. But a knowledge of the mechanisms and developmental patterns underlying phenotypic variation is crucial to the understanding of important evolutionary phenomena. Therefore some biologists are predicting an increasing focus on this variation leading to a Renaissance of the Phenotype.

The Evolutionary Significance of Phenotypic PlasticityPhenotypic sources of variation among organisms can be described by developmental switches and reaction norms

Molecular genetic markers have been developed into powerful tools to analyse genetic relationships and genetic diversity. As an extension to the variety of existing techniques using polymorphic DNA markers, the Random Amplified Polymorphic DNA (RAPD) technique may be used in molecular ecology to determine taxonomic identity, assess kinship relationships, analyse mixed genome samples, and create specific probes. Main advantages of the RAPD technology include (i) suitability for work on anonymous genomes, (ii) applicability to problems where only limited quantities of DNA are available, (iii) efficiency and low expense.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-294X.1992.tb00155.x

Applications of random amplified polymorphic DNA (RAPD) in molecular ecology

Over half of all living species of plants and animals are parasitic, which by definition involves intimate association with and unfavourable impact on hosts (Price, 1980). This paper will only consider parasites whose ‘unfavourable impact’ adversely affects the birth and/or mortality rates of their hosts (Anderson, 1978). Most organisms are potential hosts and must deal with the problem of parasitism. The probability of parasitic infection of a host is influenced by both environmental and genetic factors. Traditionally it was assumed that a host was either resistant or susceptible to a particular parasite and therefore the interaction between a parasite and potential host had only two possible outcomes: either the resistant host rebuffed the parasitic attack and remained uninfected or the parasite successfully invaded and significantly reduced the reproductive success of the susceptible host. This approach, however, ignored the intraspecific genetic variation present within both host and parasite populations (Wakelin, 1978). Since the outcome is determined by the interaction of a finite set of host genes and parasite genes, genetic variation in host susceptibility and parasite infectivity (Richards, 1976; Wakelin, 1978) suggests that more than two outcomes are possible. Variation in host and parasite genomes does not begin and end at the susceptibility/infectivity loci. Other genes may also influence the outcome of host–parasite interactions by altering the life-history patterns of hosts and parasites, and lead to a variety of outcomes.

Host life-history variation in response to parasitism

Traditionally, the major biotic determinants of animal community structure were assumed to be competition and predation. Recent theoretical, experimental and field studies indicate that parasitism is also important. The spectrum of effects that parasites have on host individuals is as broad as the range of parasite-host interactions. This review considers evidence for direct effects of parasites on fecundity and survival that influence host demography, and for indirect effects on host physiology and behavior that interface with competition and predation.

https://www.cell.com/trends/ecology-evolution/pdf/0169-5347(91)90071-5.pdf

Parasitism: A cryptic determinant of animal community structure.

The portion of an organism's energy budget that is allocated to reproduction at each age is selected to maximize overall lifetime reproductive success. Thus reproductive effort at each age should be increased until the benefit of current reproduction no longer outweighs future costs. A number of conditions should select for variations in levels of reproductive effort. High levels of reproductive effort are predicted in environments with constant and high juvenile survivorship, in years favorable for juvenile survivorship in variable environments, and when extrinsic adult mortality is high (Hirshfield and Tinkle 1975). High extrinsic mortality in adults may result from physical or biotic factors such as predation or parasitism. Biomphalaria glabrata snails exposed to the trematode parasite, Schistosoma mansoni, may or may not become infected. However, all snails exposed to these parasites appear to compensate for expected future loss in reproductive success by an increase in egg laying soon after exposure. Snails that become infected with schistosomes suffer increased mortality and show reduction or complete cessation of egg production usually beginning 4-6 wk after exposure. Increased reproductive effort early in life for an iteroparous species has been assumed to be associated with a cost to parents in terms of reduced survival and/or future reproduction, yet little data exist to substantiate this claim (Stearns 1976). In this paper, B. glabrata snails are shown to experience an early reproductive increase soon after exposure to S. mansoni whether or not they become parasitized. Our work further explores the consequences of fecundity compensation. Snails which were exposed but not parasitized showed reduced future fecundity as a result of their high early reproductive output.

A Cost of Increased Early Reproductive Effort in the Snail Biomphalaria glabrata

Life history traits expressed by organisms vary due to ecological and evolutionary constraints imposed by their current environmental conditions and genetic heritage. Trematodes often alter the life history of their host snails by inducing parasitic castration. Our understanding of the variables that influence the resulting changes in host growth, fecundity and survivorship is insufficient to confidently predict specific outcomes of novel snail-trematode combinations. In a literature review of the last 30 years, we found 41 publications examining various life history characteristics of trematode-infected snails. These publications reported 113 different field and laboratory experiments involving 30 snail species and 39 trematode species and provided a data set for assessing factors that potentially affect life history outcomes. Analysis of the diverse responses across various snail-trematode systems and experimental conditions teased out general patterns for the expression of host growth, fecundity and survival. These were used to address existing hypotheses and develop several new ones relating the response of snail-trematode interactions to environmental and genetic factors. Finally, we propose directions for future experiments that will better assess the ecological and evolutionary factors influencing snail life history responses to trematode parasitism.

Snail-trematode life history interactions: past trends and future directions.

https://www.cell.com/trends/parasitology/pdf/S1471-4922(03)00279-4.pdf

Dennis J. Minchella

Papers

Host life-history variation in response to parasitism

Parasitism: A cryptic determinant of animal community structure.

A Cost of Increased Early Reproductive Effort in the Snail Biomphalaria glabrata

Snail-trematode life history interactions: past trends and future directions.

Costs of immune defense: an enigma wrapped in an environmental cloak?