Fisher's fundamental theorem of natural selection

Fisher's Fundamental Theorem of natural selection is one of the most widely cited theories in evolutionary biology. Yet it has been argued that the standard interpretation of the theorem is very different from what Fisher meant to say. What Fisher really meant can be illustrated by looking in a new way at a recent model for the evolution of clutch size. Why Fisher was misunderstood depends, in part, on the contrasting views of evolution promoted by Fisher and Wright.     

Adaptations to sexual selection and sexual conflict: insights from experimental evolution and artificial selection

Artificial selection and experimental evolution document natural selection under controlled conditions. Collectively, these techniques are continuing to provide fresh and important insights into the genetic basis of evolutionary change, and are now being employed to investigate mating behaviour. Here, we focus on how selection techniques can reveal the genetic basis of post-mating adaptations to sexual selection and sexual conflict. Alteration of the operational sex ratio of adult Drosophila over just a few tens of generations can lead to altered ejaculate allocation patterns and the evolution of resistance in females to the costly effects of elevated mating rates. We provide new data to show how male responses to the presence of rivals can evolve. For several traits, the way in which males responded to rivals was opposite in lines selected for male-biased, as opposed to female-biased, adult sex ratio. This shows that the manipulation of the relative intensity of intra- and inter-sexual selection can lead to replicable and repeatable effects on mating systems, and reveals the potential for significant contemporary evolutionary change. Such studies, with important safeguards, have potential utility for understanding sexual selection and sexual conflict across many taxa. We discuss how artificial selection studies combined with genomics will continue to deepen our knowledge of the evolutionary principles first laid down by Darwin 150 years ago.     

Genetic constraints and the evolution of display trait sexual dimorphism by natural and sexual selection

The evolution of sexual dimorphism involves an interaction between sex-specific selection and a breakdown of genetic constraints that arise because the two sexes share a genome. We examined genetic constraints and the effect of sex-specific selection on a suite of sexually dimorphic display traits in Drosophila serrata. Sexual dimorphism varied among nine natural populations covering a substantial portion of the species range. Quantitative genetic analyses showed that intersexual genetic correlations were high because of autosomal genetic variance but that the inclusion of X-linked effects reduced genetic correlations substantially, indicating that sex linkage may be an important mechanism by which intersexual genetic constraints are reduced in this species. We then explored the potential for both natural and sexual selection to influence these traits, using a 12-generation laboratory experiment in which we altered the opportunities for each process as flies adapted to a novel environment. Sexual dimorphism evolved, with natural selection reducing sexual dimorphism, whereas sexual selection tended to increase it overall. To this extent, our results are consistent with the hypothesis that sexual selection favors evolutionary divergence of the sexes. However, sex-specific responses to natural and sexual selection contrasted with the classic model because sexual selection affected females rather than males.     

An optimizing principle of natural selection in evolutionary population genetics

This paper brings together two themes in evolutionary population genetics theory. The first concerns Fisher's Fundamental Theorem of Natural Selection: a recent interpretation of this theorem claims that it is an exact result, relating to the so-called "partial" increase in mean fitness. The second theme concerns the desire to find an optimality principle in genetic evolution. Such a principle is found here: of all gene frequency changes which lead to the same partial increase in mean fitness as the natural selection gene frequency changes, the natural selection values minimize a generalized distance measure between parent and daughter generation gene frequency values.     

Interaction between natural and sexual selection during the evolution of mate recognition

The interaction between natural and sexual selection is central to many theories of how mate choice and reproductive isolation evolve, but their joint effect on the evolution of mate recognition has not, to my knowledge, been investigated in an evolutionary experiment. Natural and sexual selection were manipulated in interspecific hybrid populations of Drosophila to determine their effects on the evolution of a mate recognition system comprised of cuticular hydrocarbons (CHCs). The effect of natural selection in isolation indicated that CHCs were costly for males and females to produce. The effect of sexual selection in isolation indicated that females preferred males with a particular CHC composition. However, the interaction between natural and sexual selection had a greater effect on the evolution of the mate recognition system than either process in isolation. When natural and sexual selection were permitted to operate in combination, male CHCs became exaggerated to a greater extent than in the presence of sexual selection alone, and female CHCs evolved against the direction of natural selection. This experiment demonstrated that the interaction between natural and sexual selection is critical in determining the direction and magnitude of the evolutionary response of the mate recognition system.     

Speciation by natural and sexual selection: models and experiments

A large number of mathematical models have been developed that show how natural and sexual selection can cause prezygotic isolation to evolve. This article attempts to unify this literature by identifying five major elements that determine the outcome of speciation caused by selection: a form of disruptive selection, a form of isolating mechanism (assortment or a mating preference), a way to transmit the force of disruptive selection to the isolating mechanism (direct selection or indirect selection), a genetic basis for increased isolation (a one- or two-allele mechanism), and an initial condition (high or low initial divergence). We show that the geographical context of speciation (allopatry vs. sympatry) can be viewed as a form of assortative mating. These five elements appear to operate largely independently of each other and can be used to make generalizations about when speciation is most likely to happen. This provides a framework for interpreting results from laboratory experiments, which are found to agree generally with theoretical predictions about conditions that are favorable to the evolution of prezygotic isolation.     

Interactions between sexual and natural selection on the evolution of a plumage badge

The evolutionary stability of signals varies due to interactions between sexual and natural selection. A tidal-marsh sparrow, Melospiza georgiana nigrescens, possesses darker pigmentation than an inland-marsh sparrow, M. g. georgiana. Studies of feather-degrading bacteria and convergent evolution among salt-marsh vertebrates suggest this dark coloration is due to environmental selection. Sexually dichromatic swamp sparrow crowns, however, may be additionally under sexual selection. We investigated ties between two plumage patches (rusty cap and black forehead) and two behaviors (male-male aggression and parental care) in the coastal and inland subspecies to test the effect of sexual versus natural selection on badge evolution. Across both subspecies the extent of rusty feathers in the cap patch was correlated positively with parental care and negatively with aggression, and the extent of black feathers in the forehead patch was correlated positively with aggression. Males with larger forehead patches produced more offspring along the coast, while males with larger cap patches did so inland. The date of the first nesting attempt for both subspecies correlated with cap patch extent, suggesting a similar role for female choice. Natural selection likely accounts for darker coastal females. Coastal male head color, however, is darker due to increased selection for larger forehead patches via intrasexual competition, yet it remains largely rusty due to female choice for larger cap patches. Increased sexual dichromatism among coastal plain swamp sparrows thus provides a clear example of the interplay between sexual and natural selection in subspecies divergence.     

From survivors to replicators: evolution by natural selection revisited

For evolution by natural selection to occur it is classically admitted that the three ingredients of variation, difference in fitness and heredity are necessary and sufficient. In this paper, I show using simple individual-based models, that evolution by natural selection can occur in populations of entities in which neither heredity nor reproduction are present. Furthermore, I demonstrate by complexifying these models that both reproduction and heredity are predictable Darwinian products (i.e. complex adaptations) of populations initially lacking these two properties but in which new variation is introduced via mutations. Later on, I show that replicators are not necessary for evolution by natural selection, but rather the ultimate product of such processes of adaptation. Finally, I assess the value of these models in three relevant domains for Darwinian evolution.     

Mating games: cultural evolution and sexual selection

In this paper, we argue that mating games, a concept that denotes cultural practices characterized by a competitive element and an ornamental character, are essential drivers behind the emergence and maintenance of human cultural practices. In order to substantiate this claim, we sketch out the essential role of the game’s players and audience, as well as the ways in which games can mature and turn into relatively stable cultural practices. After outlining the life phase of mating games – their emergence, rise, maturation, and possible eventual decline – we go on to argue that participation in these games (in each phase) does make sense from an adaptationist point of view. The strong version of our theory which proposes that all cultural practices are, or once were, mating games, allows us to derive a set of testable predictions for the fields of archaeology, economics, and psychology.     

The non-existence of a principle of natural selection

The theory of natural selection is a rich systematization of biological knowledge without a first principle. When formulations of a proposed principle of natural selection are examined carefully, each is seen to be exhaustively analyzable into a proposition about sources of fitness and a proposition about consequences of fitness. But whenever the fitness of an organic variety is well defined in a given biological situation, its sources are local contingencies together with the background of laws from disciplines other than the theory of natural selection; and the consequences of fitness for the long range fate of organic varieties are essentially applications of probability theory. Hence there is no role and no need for a principle of the theory of natural selection, and any generalities that may hold in that theory are derivative rather than fundamental.     

On the moulding of senescence by natural selection in sexual and partly sexual populations

Under a wide variety of dynamic environmental conditions, natural selection appears to favor reproductive investment in a sexually produced offspring, carrying only half of the mother’s genes, over the investment in an asexually produced offspring, genetically identical to her. It is maintained that the same environmental conditions must affect the evolutionary cost and benefit of an investment in the prolongation of one’s own life versus an investment in sexual reproduction, in favor of the latter. The effects of different environmental conditions on the division of resources among sexual reproduction, asexual reproduction and prolongation of life are studied.     

On Richerson and Boyd's model of cultural evolution by sexual selection

Richerson and Boyd proposed a model of sexual selection in which both the male trait and the female preference are culturally transmitted. We generalize their model by introducing sex-dependent oblique transmission rates and a fairly comprehensive female preference rule. The model differs markedly from the standard genetic models in that the male trait and the female preference are uncorrelated. Hence, there can be no "sexy son" effect to offset the assumed fertility cost to choosy females. Nevertheless, the cultural processes can support a stable polymorphic equilibrium at which the choosy females are present. Also of interest are the cyclical dynamics observed in the neighborhood of the internal equilibrium.     

Comparative genomics and the study of evolution by natural selection

Genomics profoundly affects most areas of biology, including ecology and evolutionary biology. By examining genome sequences from multiple species, comparative genomics offers new insight into genome evolution and the way natural selection moulds DNA sequence evolution. Functional divergence, as manifested in the accumulation of nonsynonymous substitutions in protein-coding genes, differs among lineages in a manner seemingly related to population size. For example, the ratio of nonsynonymous to synonymous substitution (d_N/d_S) is higher in apes than in rodents, compatible with Ohta's nearly neutral theory of molecular evolution, which suggests that the fixation of slightly deleterious mutations contributes to protein evolution at an extent negatively correlated with effective population size. While this supports the idea that functional evolution is not necessarily adaptive, comparative genomics is uncovering a role for positive Darwinian selection in 10–40% of all genes in different lineages, estimates that are likely to increase when the addition of more genomes gives increased power. Again, population size seems to matter also in this context, with a higher proportion of fixed amino acid changes representing advantageous mutations in large populations. Genes that are particularly prone to be driven by positive selection include those involved with reproduction, immune response, sensory perception and apoptosis. Genetic innovations are also frequently obtained by the gain or loss of complete gene sequences. Moreover, it is increasingly realized, from comparative genomics, that purifying selection conserves much more than just the protein-coding part of the genome, and this points at an important role for regulatory elements in trait evolution. Finally, genome sequencing using outbred or multiple individuals has provided a wealth of polymorphism data that gives information on population history, demography and marker evolution.     

Sexual dichromatism in frogs: natural selection, sexual selection and unexpected diversity

Sexual dichromatism, a form of sexual dimorphism in which males and females differ in colour, is widespread in animals but has been predominantly studied in birds, fishes and butterflies. Moreover, although there are several proposed evolutionary mechanisms for sexual dichromatism in vertebrates, few studies have examined this phenomenon outside the context of sexual selection. Here, we describe unexpectedly high diversity of sexual dichromatism in frogs and create a comparative framework to guide future analyses of the evolution of these sexual colour differences. We review what is known about evolution of colour dimorphism in frogs, highlight alternative mechanisms that may contribute to the evolution of sexual colour differences, and compare them to mechanisms active in other major groups of vertebrates. In frogs, sexual dichromatism can be dynamic (temporary colour change in males) or ontogenetic (permanent colour change in males or females). The degree and the duration of sexual colour differences vary greatly across lineages, and we do not detect phylogenetic signal in the distribution of this trait, therefore frogs provide an opportunity to investigate the roles of natural and sexual selection across multiple independent derivations of sexual dichromatism.     

By-product runaway evolution by adaptive mate choice: A behavioural aspect of sexual selection

Summary From a behavioural perspective on adaptive female choice, I developed a by-product runaway model of adaptive mate choice. The model illustrates the evolution of the tail size of peacocks. I consider the causal mechanisms of adaptive female choice: (1) why (ultimate reasons); (2) how (proximate mechanisms). Assumptions are developed based on these behavioural aspects. For (1) ultimate reasons, I assume that many male losers (low-fitness males) always occur due to genetic and environmental uncertainty (A-1). For (2) proximate mechanisms, I assume that losers tend to differ in the expression of a fitness-sensitive trait (an ultimate target, e.g. body size; A-2), that the fitness-sensitive trait correlates with a secondary sexual trait (a proximate cue, e.g. allometry in body size and tail size; (A-3), and that the cue trait has a genetic basis that is independent of the target trait (e.g. a genetic basis in tail ratio to body size; A-4). The model's results are: persistent female choice by means of a proximate cue (R-1); by-product selection on the independent genetic basis of the cue (R-2); and the non-adaptive or maladaptive runaway evolution of the male proximate cue (R-3). In this model, female mate preferences are non-arbitrary and adaptive, whereas the resulting evolution of male secondary sexual traits is non-adaptive in the sense of survival selection.