Hamilton's rule and the causes of social evolution

Hamilton''s rule is a central theorem of inclusive fitness (kin selection) theory and predicts that social behaviour evolves under specific combinations of relatedness, benefit and cost. This review provides evidence for Hamilton''s rule by presenting novel syntheses of results from two kinds of study in diverse taxa, including cooperatively breeding birds and mammals and eusocial insects. These are, first, studies that empirically parametrize Hamilton''s rule in natural populations and, second, comparative phylogenetic analyses of the genetic, life-history and ecological correlates of sociality. Studies parametrizing Hamilton''s rule are not rare and demonstrate quantitatively that (i) altruism (net loss of direct fitness) occurs even when sociality is facultative, (ii) in most cases, altruism is under positive selection via indirect fitness benefits that exceed direct fitness costs and (iii) social behaviour commonly generates indirect benefits by enhancing the productivity or survivorship of kin. Comparative phylogenetic analyses show that cooperative breeding and eusociality are promoted by (i) high relatedness and monogamy and, potentially, by (ii) life-history factors facilitating family structure and high benefits of helping and (iii) ecological factors generating low costs of social behaviour. Overall, the focal studies strongly confirm the predictions of Hamilton''s rule regarding conditions for social evolution and their causes.     

What is this stuff called fitness?

This paper considers a variety of attempts to define fitness in such a way as to defend the theory of evolution by natural selection from the criticism that it is a circular argument. Each of the definitions is shown to be inconsistent with the others. The paper argues that the environment in which an animal evolves can be defined only with respect to the properties of the phenotype of the animal and that it is therefore not illuminating to try to explain the phenotypic properties of the animal in terms of adaptation to an environment that is defined by those very properties. Furthermore, since there is no way that the environment can be defined independently of the presence of the animal there is no way that the quality of an animal can be assessed; and there can be no objective criteria by whichany form of selection can be carried out, therefore there can be no criteria by whichnatural selection can be carried out. It is proposed that fitness is nothing more than the production of offspring, that this is a phenotypic property like all the others, and if it is heritable then the offspring of the parents that produce the most offspring will themselves produce the most offspring, and that in principle it is impossible to account for this in terms of the other phenotypic properties of the fittest animals except by circular argument. Differential rates of reproduction are the causes of evolution and the phenotypic causes are strictly inexplicable.     

Hybrid fitness, adaptation and evolutionary diversification: lessons learned from Louisiana Irises

Estimates of hybrid fitness have been used as either a platform for testing the potential role of natural hybridization in the evolution of species and species complexes or, alternatively, as a rationale for dismissing hybridization events as being of any evolutionary significance. From the time of Darwin's publication of The Origin, through the neo-Darwinian synthesis, to the present day, the observation of variability in hybrid fitness has remained a challenge for some models of speciation. Yet, Darwin and others have reported the elevated fitness of hybrid genotypes under certain environmental conditions. In modern scientific terminology, this observation reflects the fact that hybrid genotypes can demonstrate genotype × environment interactions. In the current review, we illustrate the development of one plant species complex, namely the Louisiana Irises, into a 'model system' for investigating hybrid fitness and the role of genetic exchange in adaptive evolution and diversification. In particular, we will argue that a multitude of approaches, involving both experimental and natural environments, and incorporating both manipulative analyses and surveys of natural populations, are necessary to adequately test for the evolutionary significance of introgressive hybridization. An appreciation of the variability of hybrid fitness leads to the conclusion that certain genetic signatures reflect adaptive evolution. Furthermore, tests of the frequency of allopatric versus sympatric/parapatric divergence (that is, divergence with ongoing gene flow) support hybrid genotypes as a mechanism of evolutionary diversification in numerous species complexes.     

Male parental care, female reproductive success, and extrapair paternity

Birds differ considerably in the degree of male parental care,and it has been suggested that interspecific variation in extrapairpaternity is determined by the relative importance of benefitsto females from male parental care and good genes from extrapairsires. I estimated the relationship between extrapair paternityand the importance of male parental care for female reproductivesuccess mainly based on male removal studies, using a comparativeapproach. The reduction in female reproductive success causedby the absence of a male mate was positively correlated withthe male contribution to feeding offspring. The frequency ofextrapair paternity was negatively related to the reductionin female reproductive success caused by the absence of a mate.This was also the case when potentially confounding variablessuch as developmental mode of offspring and sexual dichromatismwere considered. A high frequency of extrapair paternity occursparticularly in bird species in which males play a minor rolein offspring provisioning and in which attractive males providerelatively little parental care. Bird species with frequentextrapair paternity thus appear to be those in which directfitness benefits from male care are small, females can readilycompensate for the absence of male care, and indirect fitnessbenefits from extrapair sires are important.