Heritable Fitness Research Articles

Transitions in individuality.

The evolution of multicellular organisms is the premier example of the integration of lower levels into a single, higher-level individual. Explaining the evolutionary transition from single cells to multicellular organisms is a major challenge for evolutionary theory. We provide an explicit two locus genetic framework for understanding this transition in terms of the increase of cooperation among cells and the regulation of conflict within the emerging organism. Heritability of fitness and individuality at the new level emerge as a result of the evolution of organismal functions that restrict the opportunity for conflict within and ensure cooperation among cells. Conflict leads, through the evolution of adaptations that reduce it, to greater individuality and harmony for the organism.

Haematozoan Parasites, Mating Systems and Colourful Plumages in Songbirds

The Hamilton-Zuk hypothesis of sosigonic selection argues that intensity of mate choice and corresponding ornamentation is positively related to pressure from parasites that are competent to engender positive heritability of fitness. Research into this idea has been controversial, not least for the original group of species studied: nearctic passeriforms. These birds are examined yet again, addressing certain published reservations about previous results: (1) using phylogenetic regression to cater for similarity linked to shared ancestry, (2) investigating only independently scored brightness data, (3) excluding species represented by small samples of birds examined for parasites, as well as (4) those represented by zero haematozoa prevalences, and (5) controlling for sampling effort, body weight and ecological variables. For the data as a whole the association between total brightness and the number of birds parasitised by haematozoa relative to the number of individuals examined (haematozoa relative presence) was not consistently significant. Likewise when front and back brightnesses were considered separately nonsignificance resulted. On controlling for ecological variables, significant positive regressions were found more often with front brightness than with back brightness. Furthermore, brightness and mating system interact significantly with each other in their relations with the parasitism index. Following from this, omission of overt polygynists produced a robustly significant, positive, relationship between front brightness and relative parasite presence. This regression, involving 82 apparently monogamous species, explained only ca 10% of the total variation. The findings support parasite-mediated sexual selection, while at the same time emphasising additional factors in the determination of bird species' brightness and sexual selection intensity

Human female orgasm and mate fluctuating asymmetry

Human, Homo sapiens, female orgasm is not necessary for conception; hence it seems reasonable to hypothesize that orgasm is an adaptation for manipulating the outcome of sperm competition resulting from facultative polyandry. If heritable differences in male viability existed in the evolutionary past, selection could have favoured female adaptations (e.g. orgasm) that biased sperm competition in favour of males possessing heritable fitness indicators. Accumulating evidence suggests that low fluctuating asymmetry is a sexually selected male feature in a variety of species, including humans, possibly because it is a marker of genetic quality. Based on these notions, the proportion of a woman's copulations associated with orgasm is predicted to be associated with her partner's fluctuating asymmetry. A questionnaire study of 86 sexually active heterosexual couples supported this prediction. Women with partners possessing low fluctuating asymmetry and their partners reported significantly more copulatory female orgasms that were reported by women with partners possessing high fluctuating asymmetry and their partners, even with many potential confounding variables controlled. The findings are used to examine hypotheses for female orgasm other than selective sperm retention.

Blood parasites, sexual selection and reproductive success of European kestrels

The Hamilton-Zuk hypothesis (HZH) of sexual selection postulates that parasites could induce heritable fitness differences that are a cue for female mate choice. The Resource Provisioning hypothesi...

Post-copulatory guarding, female choice and the levels of gregarine infections in the field cricket, Gryllus bimaculatus

A recent model in sexual selection has proposed a role for parasites in maintaining heritable fitness variation. Females are envisaged as benefitting from preferentially mating with males that show resistance to infection. The post-copulatory guarding behaviour characteristic of many species of field cricket, has been envisaged as a means by which females assess male health and vigour. This hypothesis was tested in a field cricket, G. bimaculatus, which harbours a protozoan gut parasite. In enclosed arena trials, no direct correlations between female behaviours and levels of infection in males were found. However, there were significant correlations between the intensity of male guarding and the number of parasites found in the gut; infected males guarded more intensely in order to maintain contact with the female. In a second experiment simulating open field conditions, females left heavily parasitized males sooner than mildly or uninfected individuals. These data are discussed in relation to female choice for male health and vigour.

Mate Choice Near or Far

When strong positive heritability of fitness arises due to host-parasite coevolution, consequent sosigonic mate preference undermines monogamy through tendencies to extra-pair copulation. “Low” females bonded to “low” males try to parasitise their partnership by obtaining fertilization, surreptitiously if possible, from “high” males: correspondingly, in the case of birds, “Low” males may parasite by encouraging egg dumping in their nests by “high” females who have allowed copulation. It follows that nests of birds of low status should sometimes show evidence at times of both types of parasitism while nests of high status should show faithful monogamy. Rather differently from the argument in Hamilton and Zuk (1982), showiness in monogamous species is more likely to be related to such extra-pair objectives than to pair—bonding for nesting. Venereal disease makes males cautious about copulating with any female. Although prevented by true monogamy, when monogamy is partial, venereal disease may become the incentive for increasing female sexual advertisement. In extreme cases it may combine with other ecological factors to initiate sex role reversal, in which the female becomes the non-parenting sex of the species. As regards source of the heritability that backs the sosigonic selection assumed in such speculations, reasons are given for preferring a coevolutionary cycling of ancient, preserved , parasite-defense alleles to the alternatives of an abundant stream of good new defense mutations, or a process of elimination of purely deleterious mutations.

The heritability of fitness: some single gene models

Because directional selection exhausts additive-genetic variance, it is frequently claimed that the heritability of fitness should be very close to zero. However, mutation-selection balance generates a certain amount of additive-genetic variance, so that even parent-offspring measures of heritability may be greater than zero at equilibrium. Intra-generation heritability may also be non-zero, providing the potentials for genetic change following environmental change.

SEXUAL SELECTION BY THE HANDICAP MECHANISM.

The handicap mechanism of sexual selection by female choice has been strongly criticized because it does not cause sexual selection to reinforce viability selection and it cannot account for the origin of mating preferences. However, several models indicate that the handicap mechanism can have important effects when operating in conjunction with Fisher's mechanism in polygynous populations. These models have been criticized because they require that fitness remains heritable indefinitely. I develop a simple haploid model of the handicap mechanism based on nonheritable variation in paternal investment, thus eliminating the problem of heritable fitness. This model produces the same evolutonary dynamics as both simple and quantitative genetic models of the handicap mechanism based on heritable fitness. If the parameters are such that Fisherian runaway selection does not occur in the null model (i.e., the polymorphic equilibria, which lie along the "Fisher line," are stable), then the handicap mechanism turns the Fisher line into an evolutionary trajectory upon which all other trajectories converge. This occurs because Fisher's mechanism generates no net selection on female preference when the population is on the Fisher line, so that any additional source of selection (direct or indirect) on female choice causes the population to evolve deterministically along the Fisher line. This change in the evolutionary dynamics has the important consequence of eliminating the potential for rapid population divergence for mating systems via genetic drift along the Fisher line.

Family mean correlations among fitness components in wild radish: controlling for maternal effects on seed weight

Qualitative estimates of genetic associations among life-history and fitness components are reported for 30 sibships (or families) in a common garden population of Raphanus raphanistrum L. (Brassicaceae: wild radish). Genetic associations are detected in a qualitative way when correlation coefficients among family means are highly significant. In this study, correlation coefficients among family means are compared with phenotypic correlation coefficients and to each other to address three questions: (i) Do strong genetic trade-offs among fitness components exist in wild radish under experimental conditions? (ii) Do qualitative estimates of genetic associations differ from phenotypic correlations? and (iii) Since seed weight is strongly phenotypically correlated with many components of plant performance and since families differ in seed weight because of strong maternal environmental (nongenetic) effects, does controlling for seed weight affect the correlations among family means? Many heritable fitness components were positively correlated among families; however, strong positive phenotypic correlation coefficients were much more common than strong positive family-mean correlation coefficients. No strong negative genetic associations were detected. The tendency for phenotypic correlations to be more positive than family mean correlations is consistent with the expectation that environmental variation generates positive correlations among characters, masking genetically based trade-offs. Partitioning out the effect of seed weight had only a small effect on the magnitude of the among-family correlation coefficients.

AN ESTIMATE OF THE RELATIVE OPPORTUNITIES FOR NATURAL AND SEXUAL SELECTION IN A POPULATION OF MILKWEED BEETLES.

In a recent paper Wade and Arnold (1980) provide an elegant framework for the empirical study of sexual selection. They point out that an evolutionary response to sexual selection is dependent on both the intensity of selection and the heritability of fitness. While the heritability of fitness or its components would be very difficult to measure in most field populations, the parameters necessary for an estimation of the intensity of selection can often be obtained. Their paper focuses on measures of the intensity of, or opportunity for, sexual selection on males and shows this intensity to be equal to the variance in number of mates per individual divided by the square of the mean number of mates. Though variation among individuals in mating success is an obvious requirement for the operation of sexual selection, surprisingly few studies have succeeded in documenting the extent of such individual differences in natural populations. While empirical studies of sexual selection have been quite successful in documenting those morphological or behavioral characteristics that determine mating success in a variety of species, actual measures of variance in mating success or the intensity of sexual selection in natural populations are rarer. This is particularly true when mating success is considered over the lifetime of the organism rather than a single breeding season. Considerations of variance in mating success measured over the lifetime of the study organisms must take into account both the intraand intersexual interactions commonly associated with sexual selection and any variation in survivorship during the reproductive period that is usually associated with natural selection. Because the expression of an individual's mating ability is conditional on surviving to a given mating opportunity, a long-lived individual could accrue more mates than a shorter-lived competitor simply because he experiences more opportunities for encountering mates. Thus, the intensity of both natural and sexual selection must be considered when one uses the total number of mates acquired in a lifetime as a measure of fitness. The question then arises as to whether differences between males in their ability to find and court females (mating efficiency) or differences between males in survivorship after onset of reproduction contribute more to the total variance in the fitness of males. In understanding the evolution of life-history or mating strategies it would also be important to know if there are any phenotypic or genetic correlations between these two components of variance; such correlations have been identified by Arnold and Wade (unpubl.) as cointensities or the covariance between fitnesses at sequential episodes of selection. In order to estimate variance among individuals in number of mates one must be able to recognize individuals, identify their whereabouts during the mating season or seasons, and identify mating events. These criteria are most often met in vertebrate mating systems and accurate measures of mating success have been made for several such systems (e.g., Howard, 1979; Payne, 1979; Downhower and Brown, 1980). Few studies follow individuals through more than one breeding season, however, and thus measures of variance in lifetime mating success or fitness are virtually non-existent. Studies of invertebrates rarely examine

The lek paradox is not resolved

In a population at equilibrium in a stable environment under natural selection there can be very little heritability of fitness. In some species, for example, those with leks, females exhibit strong preferences for certain characteristics in their mates, though all they obtain are his genes and these cannot contribute much to the fitness of their offspring. In paternal species, where there are often clear economic benefits in obtaining good mates, mate choice mechanisms do not seem so highly developed. This is the lek paradox. We examine three forces, mutational load, migrational load, and two kinds of linkage disequilibrium, which might generate enough heritability to maintain a mating convention in a nonpaternal species, and we evaluate the strength of the selective forces they generate. Only under rather extreme conditions can these forces be of more than negligible strength.

Sexual selection, natural selection and quality advertisement

In Darwin's and Fisher's theory of sexual selection, females prefer ornamented males; the evolution of larger ornaments is limited by increased mortality, lor example through predation. An adornment of given size should often raise mortality more in low than in high quality phenotypes. Possible consequences for the evolution and optimal size of ornaments are here examined with mathematical models. Fisher suggested that an ormamem may evolve if it initially improves male survival. Female preference then spreads for adorned males. Their consequent mating advantage furthers propagation of the preferred trail. An alternative, the ‘handicap mechanism’, suggests that only those males best able to survive can do so with a large, handicapping ornament. Choosing adorned males, females might therefore bear offspring with high general survivorship, but sons also inherit the handicapping ornament. When it reduces survival more in low than in high quality phenotypes, the handicap mechanism (in conjunction with the Fisherian mating advantage) becomes powerful with lower heritability of fitness than previously supposed. It still requires that fitness has some heritability, for which there is indirect evidence, but field measurements are lacking. In Darwin's and Fisher's theory, both mating advantage and mortality disadvantage increase with ornament size, and balance at its optimal development. If a given adornment reduces survival most in low quality phenotypes, the optimum increases with phenotypic quality. Ornament size then may provide a measure of fitness, and can be used in mate choice. A male can usually not gain fitness by developing larger adornments than other males of the same quality. Ornaments can probably also evoke through direct competition between males, without female preferences for adorned males. Field experiments are needed to clarify this and other aspects of sexual selection. Sexual ornaments form part of the reproductive effort. Certain predictions from life-history theory should therefore apply. One is that ornaments will increase in size over the first few reproductive seasons; this is the case in many species.