The evolution of anisogamy: the adaptive significance of damage, repair and mortality

Classic theory on the evolution of anisogamy focuses on the trade-off between gamete productivity and provisioning and mechanisms associated with post-zygotic survival. In this article, the role of mortality acting on both zygotes and gametes is explored as a factor influencing the evolution of different sized gametes. In particular, variable mortality through differential survival or metabolic damage is shown to affect the persistence of isogamy, the evolution of more than two sexes and the evolution of anisogamy. Evolutionary stable isogamous states are shown to be locally unstable and disruptive selection can induce the evolution of anisogamy. Analysis of both the isogamous and anisogamous ESS points reveals that the persistence of either of these conditions is not always assured. The implications of variable survival on the evolution of anisogamy are discussed.     

The vertical distribution of phytoplankton in stratified water columns

What determines the vertical distribution of phytoplankton in different aquatic environments remains an open question. To address this question, we develop a model to explore how phytoplankton respond through growth and movement to opposing resource gradients and different mixing conditions. We assume stratification creates a well-mixed surface layer on top of a poorly mixed deep layer and nutrients are supplied from multiple depth-dependent sources. Intraspecific competition leads to a unique strategic equilibrium for phytoplankton, which allows us to classify the distinct vertical distributions that can exist. Biomass can occur as a benthic layer (BL), a deep chlorophyll maximum (DCM), or in the mixed layer (ML), or as a combination of BL+ML or DCM+ML. The ML biomass can be limited by nutrients, light, or both. We predict how the vertical distribution, relative resource limitation, and biomass of phytoplankton will change across environmental gradients. We parameterized our model to represent potentially light and phosphorus limited freshwater lakes, but the model is applicable to a broad range of vertically stratified systems. Increasing nutrient input from the sediments or to the mixed layer increases light limitation, shifts phytoplankton towards the surface, and increases total biomass. Increasing background light attenuation increases light limitation, shifts the phytoplankton towards the surface, and generally decreases total biomass. Increasing mixed layer depth increases, decreases, or has no effect on light limitation and total biomass. Our model is able to replicate the diverse vertical distributions observed in nature and explain what underlying mechanisms drive these distributions.     

Landscape dynamics and evolution of colonizer syndromes: interactions between reproductive effortand dispersal in a metapopulation

The evolutionary consequences of changes in landscape dynamics for the evolution of life history syndromes are studied using a metapopulation model. We consider in turn the long-term effects of a change in the local disturbance rate, in the maximal local population persistence, in habitat productivity, and in habitat fragmentation. We examine the consequences of selective interactions between dispersal and reproductive effort by comparing the outcome of joint evolution to a situation where the species has lost the potential to evolve either its reproductive effort or its dispersal rate. We relax the classical assumption that any occupied site in the metapopulation reaches its carrying capacity immediately after recolonization. Our main conclusions are the following: (1) genetic diversity modifies the range of landscape parameters for which the metapopulation is viable, but it alters very little the qualitative evolutionary trends observed for each trait within this range. Although they are both part of a competition/colonization axis, reproductive effort and dispersal are not substitutable traits: their evolution reflects more directly the change in the landscape dynamics, than a selective interaction among them. (2) no general syndrome of covariation between reproductive effort and dispersal can be predicted: the pattern of association between the two traits depends on the type of change in landscape dynamics and on the saturation level. We review empirical evidence on colonizer syndromes and suggest lines for further empirical work. This revised version was published online in July 2006 with corrections to the Cover Date.     

Alternative oviposition behaviours in three New Zealand corduliid dragonflies: their adaptive significance and implications for male mating tactics

Oviposition behaviours of female, and mate acquisition and defence behaviours of male, 'Procordulia' grayi, Procordulia smithii and Hemicordulia australiae (three phylogenetically close corduliid species) are contrasted. Twelve distinct methods of oviposition occur involving different motor patterns. These different oviposition behaviours place ova into different microhabitats. Each species has a distinct repertoire of oviposition methods, with only one of the 12 methods occurring in more than one species. The oviposition behaviours differ in their susceptibility to male interference. The implications for male sperm displacement tactics and for the development of conditional male strategies are discussed. Male mating behaviour varies with the geometry of the breeding site, in a manner consistent with an ideal free distribution model where females vary in their 'value' through differential susceptibility to takeover by other males. It is shown that differential susceptibility of females to takeover would stabilize the observed mixed mating strategy among male 'P.' grayi and P. smithii .     

Evolution in heterogeneous environments: Effects of migration on habitat specialization

Summary Richard Levins introduced fitness sets as a tool for investigating evolution within heterogeneous environments. Evolutionary game theory permits a synthesis and generalization of this approach by considering the evolutionary response of organisms to any scale of habitat heterogeneity. As scales of heterogeneity increase from fine to coarse, the evolutionary stable strategy (ESS) switches from a single generalist species to several species that become increasingly specialized on distinct habitats. Depending upon the organisms' ecology, the switch from one to two species may occur at high migration rates (relatively fine-grained environment), or may only occur at very low migration rates (coarse-grained environment). At the ESS, the evolutionary context of a species is the entire landscape, while its ecological context may be a single habitat.Evolution towards the ESS can be represented with adaptive landscapes. In the absence of frequency-dependence, shifting from a single strategy ESS to a two strategy ESS poses the problem of evolving across valleys in the adaptive surface to occupy new peaks (hence, Sewell Wright's shifting balance theory). Frequency-dependent processes facilitate evolution across valleys. If a system with a two strategy ESS is constrained to possess a single strategy, the population may actually evolve a strategy that minimizes fitness. Because the population now rests at the bottom of a valley, evolution by natural selection can drive populations to occupy both peaks.     

Reproductive tactics and fertilization success of mature male Miyabe charr,Salvelinus malma miyabei

Synopsis Spawning behaviour, fertilization success and reproductive ecology of stream-resident and lake-run male Miyabe charr, Salvelinus malma miyabei, were studied under natural and artificial conditions. In Shikaribetsu Lake, Japan, the ratio of the two types of males is approximately 1:1, estimated by weir data. The majority of stream-resident males mature sexually at 2⁺ years of age, while the lake-run males begin to mature at 4⁺. In contrast to the large-bodied lake-run males, which paired with females, the smaller stream-resident males attempted to fertilize eggs by ‘streaking’ into the nest. We demonstrated through electrophoretic techniques that stream-resident males were able to fertilize eggs by streaking. Under artificial conditions, the estimated maximum proportion of eggs fertilized by a single stream-resident male was about 16.8. The evolutionary stability of the charr's dual mating system may be explained by Gross's theory for alternative life-history evolution, although further testing is necessary.     

Evolution of life history parameters in animals with indeterminate growth,particularly fish

Summary Most evolutionary life history theory is developed in terms of the allocation of resources to the competing ends of growth, reproduction, and survivorship. In this paper we show that certain dimensionless numbers may be used to describe the relationship between growth, maturation, and adult mortality; our theory aims to predict these numbers and we are led to aggregate some basic features of life histories, rather than explicitly considering the allocation of a limited resource to different components of fitness. The phenomenology developed here has the convenient property that only parameters describing the shapes of two assumed trade-offs among life history traits appear in the solution of the resulting optimisation problem. Comparative inter- and intraspecific data on fish, lizard, snake and shrimp populations suggest that this approach may help explain some common patterns in the life histories of animals with indeterminate growth.     

Alternative mating tactics in male white-faced dragonflies (Leucorrhinia intacta): plasticity of tactical options and consequences for reproductive success

Summary Alternative tactics used by males to obtain mates usually are associated with genetic and/or phenotypic differences between the behavioral morphs. This system of white-faced dragonfly (Leucorrhinia intacta) alternatives is characterized by plasticity of tactical options for individual males. Males may act either as territorials, and defend small perch-centered territories on the study pond, or they act as transients, spending most of their time in vegetation surrounding the pond and sallying out at intervals in search of mates. The two tactics remain in constant proportions over a broad range of densities, so transients do not result only from a filling in of suitable territorial sites. Males adopt tactics independently from day to day, with no significant influences of phenotypic variation, priority of arrival at the breeding site, or prior success in a role. We interpret this system as based on conditional, frequency-dependent choice of alternatives by a population of males not differing significantly in their abilities to employ one tactic or the other, but we cannot exclude entirely the possibility of mixed strategies. Average daily mating success is equal for territorial and transient males, supporting predictions of mixed and conditional ESS hypotheses. Males of each tactic obtain matings daily in proportion to their representation in the population for most data samples. Deviations from the expected mating success provide no information of use in selecting one or the other tactic on subsequent days. We suggest this system of alternatives represents a conditional mating strategy, in which males adopt tactics based on the availability of perches relative to oviposition substrate and on interactions within and between tactics that are influenced by the relative frequencies of territorial and transient males.