Fitness Characteristics and Allozyme Heterozygosity in an Artificial Population of the Sable Martes zibellina L.

Associations between some characteristics of fitness (the age of the first reproduction, life span, fertility, and the number of missed pregnancies) and heterozygosity for some allozyme loci have been studied in a population of the sable Martes zibellina L. kept in cages at a farm. Of all fitness characteristic studied, a weak correlation has only been found between heterozygosity and the age of the first reproduction (maturation rate). The age of the first reproduction is positively associated with the life span and fertility. The results obtained are discussed in terms of the relationship between the genetic variation of longevity and developmental rate and prospects of their application to breeding practice.     

Natural selection on age-specific fertilities in human females: comparison of individual-level fitness measures.

Lifetime reproductive success and timing of reproduction are key components of life-history evolution. To understand the evolution of reproductive schedules, it is important to use a measure of fitness that is sensitive both to reproductive quantity and reproductive timing. There is a contradiction between the theory, which mainly focuses on the rate measures of fitness (r and lambda), and empirical studies, which mainly use lifetime reproductive success (LRS), or some of its correlates, as a fitness measure. We measured phenotypic selection on age-specific fertilities in three pre-modern human populations using individually estimated finite rate of increase, er (lambda). We found that lambda and lifetime reproductive success ranked individuals differently according to their fitness: for example, a female giving birth to four children at a young age may actually have a higher fitness than a female giving birth to six children at a greater age. Increase in fertility at the young age classes (15-19 years) was favoured by selection, but the intensity of selection on fertility was higher in the older age classes (20-30 years), where the variance in fertility was highest. Hence, variation in fertility in the older age classes (20-30) was actually responsible for most of the observed variation in fitness among the individuals. Additionally, more than 90% of variation in fitness (lambda) was attributable to individual differences in LRS, whereas only about 5% of all variation in fitness was due to differences in the reproductive schedule. The rate-sensitive fitness measure did not significantly challenge the importance of total fertility as a component of fitness in humans. However, the rate-sensitive measure clearly allowed more accurate estimation of individual fitness, which may be important for answering some more specific questions.     

Optimal life histories and the age-specific costs of reproduction

Increases in reproduction at a given age may carry costs measured as reductions in subsequent survival and/or future fertility. Such costs generate constraints within which natural selection may mould life histories to maximize fitness. In this paper, I derive expressions predicting the age-specific costs of reproduction conditional on the maximization of fitness. Survival costs should, on this hypothesis, vary as the inverse of the reproductive value curve; fertility costs should vary as the ratio of successive terms in the stable age distribution. For many organisms, this means that survival costs should increase markedly with age, while fertility costs should be nearly age-invariant. Data on such age-specific costs is scarce, but that which is available (mainly for humans) agrees with these predictions.     

A tradeoff between reproduction and growth in contemporary Finnish women

Women have been suggested to trade growth in height for reproduction, as an earlier age at menarche and first birth seem to be related to shorter adult stature. Although women likely accrue fitness benefits by maturing and starting reproduction at young age, short adult stature may be selected against by natural and sexual selection later in their life. We studied how age at menarche and first reproduction affected adult height and whether adult height in turn was related to lifetime reproductive success in Finnish women born 1946–1958. Our results show that a delay of 1 year in age at menarche and first reproduction was related to a 0.43- and 0.20-cm increase in adult height, respectively. The sex of the first-born offspring was not related to adult height. Moreover, women gained fitness benefits by starting reproduction early but not by growing tall. These findings among Finnish women are thus compatible with tradeoffs between reproduction and growth, by showing a compromised adult height at the cost of early age at menarche and first birth. However, in these women, natural selection favored those women who traded their stature for young motherhood.     

Soft selection reduces loss of heterozygosity in asexual reproduction

The adaptive value of sexual reproduction is still debated in evolutionary theory. It has been proposed that the advantage of sexual reproduction over asexual reproduction is to promote genetic diversity, to prevent the accumulation of harmful mutations or to preserve heterozygosity. Since these hypothetical advantages depend on the type of asexual reproduction, understanding how selection affects the taxonomic distribution of each type could help us discriminate between existing hypotheses. Here, I argue that soft selection, competition among embryos or offspring in selection arenas prior to the hard selection of the adult phase, reduces loss of heterozygosity in certain types of asexual reproduction. Since loss of heterozygosity leads to the unmasking of recessive deleterious mutations in the progeny of asexual individuals, soft selection facilitates the evolution of these types of asexual reproduction. Using a population genetics model, I calculate how loss of heterozygosity affects fitness for different types of apomixis and automixis, and I show that soft selection significantly reduces loss of heterozygosity, hence increases fitness, in apomixis with suppression of the first meiotic division and in automixis with central fusion, the most common types of asexual reproduction. Therefore, if sexual reproduction evolved to preserve heterozygosity, soft selection should be associated with these types of asexual reproduction. I discuss the evidence for this prediction and how this and other observations on the distribution of different types of asexual reproduction in nature is consistent with the heterozygosity hypothesis.     

The Evolution of Senescence and Post-Reproductive Lifespan in Guppies (Poecilia reticulata)

The study of post-reproductive lifespan has been of interest primarily with regard to the extended post-menopausal lifespan seen in humans. This unusual feature of human demography has been hypothesized to have evolved because of the “grandmother” effect, or the contributions that post-reproductive females make to the fitness of their children and grandchildren. While some correlative analyses of human populations support this hypothesis, few formal, experimental studies have addressed the evolution of post-reproductive lifespan. As part of an ongoing study of life history evolution in guppies, we compared lifespans of individual guppies derived from populations that differ in their extrinsic mortality rates. Some of these populations co-occur with predators that increase mortality rate, whereas other nearby populations above barrier waterfalls are relatively free from predation. Theory predicts that such differences in extrinsic mortality will select for differences in the age at maturity, allocation of resources to reproduction, and patterns of senescence, including reproductive declines. As part of our evaluation of these predictions, we quantified differences among populations in post-reproductive lifespan. We present here the first formal, comparative study of the evolution of post-reproductive lifespan as a component of the evolution of the entire life history. Guppies that evolved with predators and that experienced high extrinsic mortality mature at an earlier age but also have longer lifespans. We divided the lifespan into three non-overlapping components: birth to age at first reproduction, age at first reproduction to age at last reproduction (reproductive lifespan), and age at last reproduction to age at death (post-reproductive lifespan). Guppies from high-predation environments live longer because they have a longer reproductive lifespan, which is the component of the life history that can make a direct contribution to individual fitness. We found no differences among populations in post-reproductive lifespan, which is as predicted since there can be no contribution of this segment of the life history to an individual's fitness. Prior work on the evolution of post-reproductive lifespan has been dominated by speculation and correlative analyses. We show here that this component of the life history is accessible to formal study as part of experiments that quantify the different segments of an individual's life history. Populations of guppies subject to different mortality pressures from predation evolved differences in total lifespan, but not in post-reproductive lifespan. Rather than showing the direct effects of selection characterizing other life-history traits, post-reproductive lifespan in these fish appears to be a random add-on at the end of the life history. These findings support the hypothesis that differences in lifespan evolving in response to selection are confined to the reproductive lifespan, or those segments of the life history that make a direct contribution to fitness. We also show, for the first time, that fish can have reproductive senescence and extended post-reproductive lifespans despite the general observation that they are capable of producing new primary oocytes throughout their lives.     

Genic heterozygosity, chromosomal interchanges and fitness in rye: any relationship?

Relationship between heterozygosity at allozyme loci, chromosomal interchanges and fitness was analyzed in a rye cultivar showing a polymorphism for such rearrangements. Nine allozyme systems (ACO, ACPH, GOT, GPI, LAP, MDH, PER, PGD and PGM) and five components of fitness (number of fertile tillers, total offspring, egg cell fertility, flowers/ear and seeds/ear) were studied. The estimated selection coefficients against interchange heterozygotes ranged from s = 0.12 to s = 0.34. A significant effect of the genic heterozygosity on some fitness components was observed in interchange heterozygotes (tillering and total offspring), in their standard homozygous sibs (flowers/ear and seeds/ear) and in the descendants of the crosses between standard karyotypes (flowers/ear, seeds/ear and egg cell fertility). However, the main effect was linked to genetic background associated to different crosses. Significant differences for Acph-1, Gpi-1, Lap-1, Mdh-1, Mdh-4, Pgd-2 and Pgm-1 loci were also found in some of these crosses although these differences were inconsistent. This suggests that probably the allozyme loci analyzed were not directly contributing to the fitness and that they are linked, in some cases, to different deleterious alleles depending on both cross and locus. This fact could support the local effect hypothesis as explanation although we do not discard the existence of some inbreeding level (general effect hypothesis) since all crosses and loci studied show a overall consistent trend of increased fitness with increased heterozygosity.     

Geographic variation as a result of evolution of the traits with broad and narrow norms of reaction in the moor frog (Rana arvalis)

Females reproductive, size, and age characteristics were studied in isolated local populations of Rana arvalis in the southern and northern parts of its range. The yearlings of the southern populations used to get larger by their first overwintering due to earlier beginning of the breeding season, as compared with the yearlings of the northern population. As a result, "southern" females become sexually mature at the age of two years while the "northern" ones become mature at the age of three years. This causes geographic differences in age composition among two populations, the "southern" reproductive females being younger on average than the "northern" ones. The earlier female maturation in the first case is not compensated by respective rise of the growth rate; to the contrary, the "southern" females grow more slowly during the first two years of their life and appear to be smaller than the "norhern" ones. These reproduction and growth patterns arise supposedly due to paedomorphosis, which causes specific reproductive characteristics, namely decrease in the egg size, increase in the reproductive effort and more strong correlation between female fertility and body size. Local and geographic differences are expressed not in the extent but in the structure of reproductive pattern, as no negative correlation was revealed between female reproductive age and body size in the southern populations. Southern habitats cannot be considered as "unfavourable with respect to body size", so the geographic differences under consideration cannot be explained by optimization of the reproductive strategies at population level. Paedomorphosis appears as a result of the female maturation rate possessing a wider norm of reaction than the growth rate. At the same time, fixation of the specific growth rate narrows norm of reaction of some other characters important for the phenotype reproductive fitness thus predetermining their subsequent evolution.     

Lifetime fitness consequences of early‐life ecological hardship in a wild mammal population

Early‐life ecological conditions have major effects on survival and reproduction. Numerous studies in wild systems show fitness benefits of good quality early‐life ecological conditions (“silver‐spoon” effects). Recently, however, some studies have reported that poor‐quality early‐life ecological conditions are associated with later‐life fitness advantages and that the effect of early‐life conditions can be sex‐specific. Furthermore, few studies have investigated the effect of the variability of early‐life ecological conditions on later‐life fitness. Here, we test how the mean and variability of early‐life ecological conditions affect the longevity and reproduction of males and females using 14 years of data on wild banded mongooses (Mungos mungo). Males that experienced highly variable ecological conditions during development lived longer and had greater lifetime fitness, while those that experienced poor early‐life conditions lived longer but at a cost of reduced fertility. In females, there were no such effects. Our study suggests that exposure to more variable environments in early life can result in lifetime fitness benefits, whereas differences in the mean early‐life conditions experienced mediate a life‐history trade‐off between survival and reproduction. It also demonstrates how early‐life ecological conditions can produce different selection pressures on males and females.     

The evolution of senescence and post-reproductive lifespan in guppies (Poecilia reticulata)

The study of post-reproductive lifespan has been of interest primarily with regard to the extended post-menopausal lifespan seen in humans. This unusual feature of human demography has been hypothesized to have evolved because of the “grandmother” effect, or the contributions that post-reproductive females make to the fitness of their children and grandchildren. While some correlative analyses of human populations support this hypothesis, few formal, experimental studies have addressed the evolution of post-reproductive lifespan. As part of an ongoing study of life history evolution in guppies, we compared lifespans of individual guppies derived from populations that differ in their extrinsic mortality rates. Some of these populations co-occur with predators that increase mortality rate, whereas other nearby populations above barrier waterfalls are relatively free from predation. Theory predicts that such differences in extrinsic mortality will select for differences in the age at maturity, allocation of resources to reproduction, and patterns of senescence, including reproductive declines. As part of our evaluation of these predictions, we quantified differences among populations in post-reproductive lifespan. We present here the first formal, comparative study of the evolution of post-reproductive lifespan as a component of the evolution of the entire life history.

Guppies that evolved with predators and that experienced high extrinsic mortality mature at an earlier age but also have longer lifespans. We divided the lifespan into three non-overlapping components: birth to age at first reproduction, age at first reproduction to age at last reproduction (reproductive lifespan), and age at last reproduction to age at death (post-reproductive lifespan). Guppies from high-predation environments live longer because they have a longer reproductive lifespan, which is the component of the life history that can make a direct contribution to individual fitness. We found no differences among populations in post-reproductive lifespan, which is as predicted since there can be no contribution of this segment of the life history to an individual's fitness.

Prior work on the evolution of post-reproductive lifespan has been dominated by speculation and correlative analyses. We show here that this component of the life history is accessible to formal study as part of experiments that quantify the different segments of an individual's life history. Populations of guppies subject to different mortality pressures from predation evolved differences in total lifespan, but not in post-reproductive lifespan. Rather than showing the direct effects of selection characterizing other life-history traits, post-reproductive lifespan in these fish appears to be a random add-on at the end of the life history. These findings support the hypothesis that differences in lifespan evolving in response to selection are confined to the reproductive lifespan, or those segments of the life history that make a direct contribution to fitness. We also show, for the first time, that fish can have reproductive senescence and extended post-reproductive lifespans despite the general observation that they are capable of producing new primary oocytes throughout their lives.

     

Life history of breeding partners alters age‐related changes of reproductive traits in a natural population of blue tits

Reproductive senescence, an intra‐individual decline in reproductive function with age, is widespread, but proximate factors determining its rate remain largely unknown. Most studies of reproductive senescence focus on females, leaving senescence in male function and its implications for female function largely understudied. We constructed linear mixed models to explore the interactive effects of paternal and maternal age and a life‐history trait (i.e. age at first reproduction) on four fitness components (i.e. laying date, clutch size, number of fledglings and number of recruits) measured in a wild, breeding population of blue tits Cyanistes caeruleus ogliastrae where individual breeding success has been followed for over 30 years (our dataset spanned 29 years). Previous studies have shown that, across female lifespan, laying date decreases and subsequently increases; earlier laying dates result in higher fitness because hatchlings have greater access to a seasonal food source. Our analyses reveal that females that initiate reproduction early in life show a greater delay in laying date with old age. In addition to delayed laying dates, older females lay smaller clutches. However, the magnitude of female age effects was influenced by the age at first reproduction of their breeding partners. Senescence of laying date and clutch size was reduced when females mated with males that reproduced early in life compared to males that delayed reproduction. We confirmed that both laying date and clutch size were significantly correlated with reproductive fitness suggesting that these dynamics early in the breeding cycle can have long‐term consequences. These complex phenotypic interactions shed light on the proximate mechanisms underlying reproductive senescence in nature and highlight the potential importance of cross‐sex age by life‐history interactions.     

The force of selection on the human life cycle

In this article, I present evidence for a robust and quite general force of selection on the human life cycle. The force of selection acts in remarkably invariant ways on human life histories, despite a great abundance of demographic diversity. Human life histories are highly structured, with mortality and fertility changing substantially through the life cycle. This structure necessitates the use of structured population models to understand human life history evolution. Using such structured models, I find that the vital rates to which fitness is most sensitive are prereproductive survival probabilities, particularly the survival of children ages 0 to 4 years. The fact that the preponderance of selection falls on transitions related to recruitment combined with the late age at first reproduction characteristic of the human life cycle creates a fitness bottleneck out of recruitment. Because of this, antagonistic pleiotropy with any trait that detracts from the constituent transitions to recruitment is expected. I explore the predictors of variation in the force of selection on early survival. High fertility increases the selective premium placed on early survivorship, whereas high life expectancy at birth decreases it.     

The effect of injury on whole-plant senescence: an experiment with two root-sprouting Barbarea species

Background and Aims Senescence is the process of losing fitness when growing old, and is shaped by the trade-off between maintenance and reproduction that makes reproduction more unsure and maintenance more costly with age. In repeatedly reproducing plants, reductions in growth and fertility are signs of senescence. Disturbance, however, provides an opportunity to reset the ageing clock and consequently potentially ameliorate senescence.Methods To test the effects of disturbance on traits closely related to fitness and thus to senescence, a long-term garden experiment was established with two short-lived perennial congeners, Barbarea vulgaris and Barbarea stricta, that differ in their ability to resprout after injury. In the experiment, five damage treatments were applied to plants in four different phenophases.Key Results It was found that damage to the plant body significantly prolonged life span in B. vulgaris but decreased whole-life seed production in both species. High concentration of seed production in one growing season characterized short life spans. Both more severe damage and a more advanced phenological phase at the time of damage caused reproduction to be spread over more than one growing season and equalized per-season seed production. In terms of seed quality, average weight of a single seed decreased and seed germination rate increased with age regardless of damage.Conclusions Although disturbance is able to reset the ageing clock of plants, it is so harmful to plant fitness that resprouting serves, at best, only to alleviate slightly the signs of senescence. Thus, in terms of whole-life seed production, injured plants were not more successful than uninjured ones in the two studied species. Indeed, in these species, injury only slightly postponed or decelerated senescence and did not cause effective rejuvenation.     

Rank-Related Fitness Differences and Their Demographic Pathways in Semi-Free-Ranging Rhesus Macaques (<Emphasis Type="Italic">Macaca mulatta</Emphasis>)

Researchers have explored the fitness consequences of female dominance hierarchies in many primate populations, with most studies highlighting differences in age of maturation, fertility, and offspring survival. We use resampling techniques and van Tienderen’s (2000) elasticity path analysis to identify rank-related differences in finite rate of increase (λ) and their demographic correlates among segments of a semi-free-ranging rhesus macaque population. Higher-ranking population segments grew at greater rates for some portions of the 40-yr study period. The female members of these segments achieved these lifetime fitness differences through higher fertility and especially higher adult survival rates. This is the first clear evidence that social rank influences female primate adult survival, and is a crucial fitness component for any long-lived, slow-reproducing animal. Traditional methods of comparing lifespans, and other life history variables, among rank categories fail to identify most of the rank-related differences primarily because they require completed life histories that are available only on a small number of the females known in the population.     

Partial life cycle analysis: a model for pre-breeding census data

Matrix population models have become popular tools in research areas as diverse as population dynamics, life history theory, wildlife management, and conservation biology. Two classes of matrix models are commonly used for demographic analysis of age‐structured populations: age‐structured (Leslie) matrix models, which require age‐specific demographic data, and partial life cycle models, which can be parameterized with partial demographic data. Partial life cycle models are easier to parameterize because data needed to estimate parameters for these models are collected much more easily than those needed to estimate age‐specific demographic parameters. Partial life cycle models also allow evaluation of the sensitivity of population growth rate to changes in ages at first and last reproduction, which cannot be done with age‐structured models. Timing of censuses relative to the birth‐pulse is an important consideration in discrete‐time population models but most existing partial life cycle models do not address this issue, nor do they allow fractional values of variables such as ages at first and last reproduction. Here, we fully develop a partial life cycle model appropriate for situations in which demographic data are collected immediately before the birth‐pulse (pre‐breeding census). Our pre‐breeding census partial life cycle model can be fully parameterized with five variables (age at maturity, age at last reproduction, juvenile survival rate, adult survival rate, and fertility), and it has some important applications even when age‐specific demographic data are available (e.g., perturbation analysis involving ages at first and last reproduction). We have extended the model to allow non‐integer values of ages at first and last reproduction, derived formulae for sensitivity analyses, and presented methods for estimating parameters for our pre‐breeding census partial life cycle model. We applied the age‐structured Leslie matrix model and our pre‐breeding census partial life cycle model to demographic data for several species of mammals. Our results suggest that dynamical properties of the age‐structured model are generally retained in our partial life cycle model, and that our pre‐breeding census partial life cycle model is an excellent proxy for the age‐structured Leslie matrix model.     

Age-specific effect of heterozygosity on survival in alpine marmots, Marmota marmota

The fitness consequences of heterozygosity and the mechanisms underpinning them are still highly controversial. Using capture–mark–recapture models, we investigated the effects of individual heterozygosity, measured at 16 microsatellite markers, on age-dependent survival and access to dominance in a socially monogamous mammalian species, the alpine marmot. We found a positive correlation between standardized multilocus heterozygosity and juvenile survival. However, there was no correlation between standardized multilocus heterozygosity and either survival of older individuals or access to dominance. The disappearance of a significant heterozygosity fitness correlation when individuals older than juveniles are considered is consistent with the prediction that differences in survival among individuals are maximal early in life. The lack of a correlation between heterozygosity and access to dominance may be a consequence of few homozygous individuals attaining the age at which they might reach dominance. Two hypotheses have been proposed to explain heterozygosity-fitness correlations: genome-wide effects reflected by all markers or local effects of specific markers linked to genes that determine fitness. In accordance with genome-wide effects of heterozygosity, we found significant correlations between heterozygosities calculated across single locus or across two sets of eight loci. Thus, the genome-wide heterozygosity effect seems to explain the observed heterozygosity-fitness correlation in the alpine marmot.     

The evolution of fitness in life-history theory

Theory concerning the evolution of life history (the schedule of reproduction and survival) focuses on describing the life history which maximises fitness. Although there is an intuitive link between life history and fitness, there are in fact several measures of the 'black box' concept of fitness. There has been a debate in the bio-mathematical literature on the predictive difference between the two most commonly used measures; intrinsic rate of increase r and net reproductive ratio R0. Although both measures aim to describe fitness, models using one of the measures may predict the opposite of similar models using the other measure, which is clearly undesirable. Here, I review the evolution of these fitness measures over the last four decades, the predictive differences between these measures and the resulting shift of the fitness concept. I focus in particular on some recent developments, which have solved the dilemma of predictive differences between these fitness measures by explicitly acknowledging the game-theoretical nature of life-history evolution.     

Selection on age at first and at last reproduction in the long‐lived Alpine Swift Apus melba

The way an organism spreads its reproduction over time is defined as a life‐history trait, and selection is expected to favour life‐history traits associated with the highest fitness return. We use a long‐term dataset of 277 life histories to investigate the shape and strength of selection acting on the age at first reproduction and at last reproduction in the long‐lived Alpine Swift. Both traits were under strong directional selection, but in opposite directions, with selection favouring birds starting their reproductive career early and being able to reproduce for longer. There was also evidence for stabilising selection acting on both traits, suggesting that individuals should nonetheless refrain from reproducing in their first 2 years of life (i.e. when inexperienced), and that reproducing after 7 years of age had little effect on lifetime fitness, probably due to senescence.     

Early reproductive investment,senescence and lifetime reproductive success in female Asian elephants

The evolutionary theory of senescence posits that as the probability of extrinsic mortality increases with age, selection should favour early‐life over late‐life reproduction. Studies on natural vertebrate populations show early reproduction may impair later‐life performance, but the consequences for lifetime fitness have rarely been determined, and little is known of whether similar patterns apply to mammals which typically live for several decades. We used a longitudinal dataset on Asian elephants (Elephas maximus) to investigate associations between early‐life reproduction and female age‐specific survival, fecundity and offspring survival to independence, as well as lifetime breeding success (lifetime number of calves produced). Females showed low fecundity following sexual maturity, followed by a rapid increase to a peak at age 19 and a subsequent decline. High early life reproductive output (before the peak of performance) was positively associated with subsequent age‐specific fecundity and offspring survival, but significantly impaired a female's own later‐life survival. Despite the negative effects of early reproduction on late‐life survival, early reproduction is under positive selection through a positive association with lifetime breeding success. Our results suggest a trade‐off between early reproduction and later survival which is maintained by strong selection for high early fecundity, and thus support the prediction from life history theory that high investment in reproductive success in early life is favoured by selection through lifetime fitness despite costs to later‐life survival. That maternal survival in elephants depends on previous reproductive investment also has implications for the success of (semi‐)captive breeding programmes of this endangered species.     

The importance of growth and mortality costs in the evolution of the optimal life history

A central assumption of life history theory is that the evolution of the component traits is determined in part by trade-offs between these traits. Whereas the existence of such trade-offs has been well demonstrated, the relative importance of these remains unclear. In this paper we use optimality theory to test the hypothesis that the trade-off between present and future fecundity induced by the costs of continued growth is a sufficient explanation for the optimal age at first reproduction, alpha, and the optimal allocation to reproduction, G, in 38 populations of perch and Arctic char. This hypothesis is rejected for both traits and we conclude that this trade-off, by itself, is an insufficient explanation for the observed values of alpha and G. Similarly, a fitness function that assumes a mortality cost to reproduction but no growth cost cannot account for the observed values of alpha. In contrast, under the assumption that fitness is maximized, the observed life histories can be accounted for by the joint action of trade-offs between growth and reproductive allocation and between mortality and reproductive allocation (Individual Juvenile Mortality model). Although the ability of the growth/mortality model to fit the data does not prove that this is the mechanism driving the evolution of the optimal age at first reproduction and allocation to reproduction, the fit does demonstrate that the hypothesis is consistent with the data and hence cannot at this time be rejected. We also examine two simpler versions of this model, one in which adult mortality is a constant proportion of juvenile mortality [Proportional Juvenile Mortality (PJM) model] and one in which the proportionality is constant within but not necessarily between species [Specific Juvenile Mortality (SSJM) model]. We find that the PJM model is unacceptable but that the SSJM model produces fits suggesting that, within the two species studied, juvenile mortality is proportional to adult mortality but the value differs between the two species.