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In this work we consider fixation of an allele in a population. Fixation is key to understanding the way long-term evolutionary change occurs at the gene and molecular levels. Two basic aspects of fixation are: (i) the chance it occurs and (ii) the way the gene frequency progresses to fixation. We present exact results for both aspects of fixation for the Wright–Fisher model. We give the exact fixation probability for some different schemes of frequency-dependent selection. We also give the corresponding exact stochastic difference equation that generates frequency trajectories which ultimately fix. Exactness of the results means selection need not be weak. There are possible applications of this work to data analysis, modelling, and tests of approximations. The methodology employed illustrates that knowledge of the fixation probability, for all initial frequencies, fully characterises the dynamics of the Wright–Fisher model. The stochastic equations for fixing trajectories allow insight into the way fixation occurs. They provide the alternative picture that fixation is driven by the injection of one carrier of the fixing allele into the population each generation. The stochastic equations allow explicit calculation of some properties of fixing trajectories and their efficient simulation. The results are illustrated and tested with simulations. [ABSTRACT FROM AUTHOR]

Color polymorphism in natural populations can manifest as a striking patchwork of phenotypes in space, with neighboring populations characterized by dramatic differences in morph composition. These geographic mosaics can be challenging to explain in the absence of localized selection because they are unlikely to result from simple isolation-by-distance or clinal variation in selective regimes. To identify processes that can lead to the formation of geographic mosaics, we developed a simulation-based model to explore the influence of predator perspective, selection, migration, and genetic linkage of color loci on allele frequencies in polymorphic populations over space and time. Using simulated populations inspired by the biology of Heliconius long wing butterflies, Cepaea land snails, Oophaga poison frogs, and Sonora ground snakes, we found that the relative sizes of predator and prey home ranges can produce large differences in morph composition between neighboring populations under both positive and negative frequency-dependent selection. We also demonstrated the importance of the interaction of predator perspective with the type of frequency dependence and localized directional selection across migration and selection intensities. Our results show that regional-scale predation can promote the formation of phenotypic mosaics in prey species, without the need to invoke spatial variation in selective regimes. We suggest that predator behavior can play an important and underappreciated role in the formation and maintenance of geographicmosaics in polymorphic species. [ABSTRACT FROM AUTHOR]

In avian brood parasitism, both the host and the parasite are expected to develop various conflicting adaptations; hosts develop a defense against parasitism, such as an ability to recognize and reject parasitic eggs that look unlike their own, while parasites evolve egg mimicry to counter this host defense. Hosts may further evolve to generate various egg phenotypes that are not mimicked by parasites. Difference in egg phenotype critically affects the successful reproduction of hosts and parasites. Recent studies have shown that clear polymorphism in egg phenotype is observed in several host-parasite interactions, which suggests that egg polymorphism may be a more universal phenomenon than previously thought. We examined the mechanism for maintaining egg polymorphism in the rufescent prinia ( Prinia rufescens) that is parasitized by the plaintive cuckoo ( Cacomantis merulinus) from a theoretical viewpoint based on a mathematical model. The prinia has four distinct egg phenotypes: immaculate white, immaculate blue, white with spots, and blue with spots. Only two egg phenotypes, white with spots and blue with spots, are found in the cuckoo population. We show that the observed prinia and cuckoo phenotypes cannot be at an equilibrium and that egg polymorphism can be maintained either at stationary equilibrium or with dynamic, frequency oscillations, depending on the mutation rates of the background color and spottiness. Long-term monitoring of the prinia-cuckoo interaction over a wide geographic range is needed to test the results of the model analyses. [ABSTRACT FROM AUTHOR]

We studied differentiation and geneflow patterns between enantiomorphic door-snail species in two hybrid zones in the Bucegi Mountains (Romania) to investigate the effects of intrinsic barriers (complications in copulation) and extrinsic selection by environmental factors. A mitochondrial gene tree confirmed the historical separation of the examined populations into the dextral Alopia livida and the sinistral Alopia straminicollis in accordance with the morphological classification, but also indicated gene flow between the species. By contrast, a network based on amplified fragment length polymorphisms ( AFLP) markers revealed local groups of populations as units independent of their species affiliation. Admixture analyses based on AFLP data showed that the genomes of most individuals in the hybrid zones are composed of parts of the genomes of both parental taxa. The introgression patterns of a notable fraction of the examined markers deviated from neutral introgression. However, the patterns of most non-neutral markers were not concordant between the two hybrid zones. There was also no concordance between non-neutral markers in the two genomic clines and markers that were correlated with environmental variables or markers that were correlated with the proportion of dextral individuals in the populations. Neither extrinsic selection by environmental factors nor intrinsic barriers resulting from positive frequency-dependent selection of the prevailing coiling direction were sufficient to maintain the distinctness of A. straminicollis and A. livida. Despite being historically separated units, we conclude that these taxa now merge where they come into contact. [ABSTRACT FROM AUTHOR]

ABSTRACT Sex-determining mechanisms are broadly categorised as being based on either genetic or environmental factors. Vertebrate sex determination exhibits remarkable diversity but displays distinct phylogenetic patterns. While all eutherian mammals possess XY male heterogamety and female heterogamety ( ZW) is ubiquitous in birds, poikilothermic vertebrates (fish, amphibians and reptiles) exhibit multiple genetic sex-determination ( GSD) systems as well as environmental sex determination ( ESD). Temperature is the factor controlling ESD in reptiles and temperature-dependent sex determination ( TSD) in reptiles has become a focal point in the study of this phenomenon. Current patterns of climate change may cause detrimental skews in the population sex ratios of reptiles exhibiting TSD. Understanding the patterns of variation, both within and among populations and linking such patterns with the selection processes they are associated with, is the central challenge of research aimed at predicting the capacity of populations to adapt to novel conditions. Here we present a conceptual model that innovates by defining an individual reaction norm for sex determination as a range of incubation temperatures. By deconstructing individual reaction norms for TSD and revealing their underlying interacting elements, we offer a conceptual solution that explains how variation among individual reaction norms can be inferred from the pattern of population reaction norms. The model also links environmental variation with the different patterns of TSD and describes the processes from which they may arise. Specific climate scenarios are singled out as eco-evolutionary traps that may lead to demographic extinction or a transition to either male or female heterogametic GSD. We describe how the conceptual principles can be applied to interpret TSD data and to explain the adaptive capacity of TSD to climate change as well as its limits and the potential applications for conservation and management programs. [ABSTRACT FROM AUTHOR]

Many ecological communities are enormously diverse. Variation in environmental conditions over time and space provides opportunities for temporal and spatial storage effects to operate, potentially promoting species coexistence and biodiversity. While several studies have provided empirical evidence supporting the significance of the temporal storage effect for coexistence, empirical tests of the role of the spatial storage effect are rare. In particular, we know little about how the spatial storage effect contributes to biodiversity over evolutionary timescales. Here, we report the first experimental study on the role of the spatial storage effect in the maintenance of biodiversity in evolving metacommunities, using the bacterium Pseudomonas fluorescens SBW25 as a laboratory model of adaptive radiation. We found that intercommunity spatial heterogeneity promoted phenotypic diversity of P. fluorescens in the presence of dispersal among local communities, by allowing the spatial storage effect to operate. Mechanistically, greater niche differences among P. fluorescens phenotypes arose in metacommunities with intercommunity spatial heterogeneity, facilitating negative frequency-dependent selection, and thus, the coexistence among P. fluorescens phenotypes. These results highlight the importance of the spatial storage effect for biodiversity over evolutionary timescales. [ABSTRACT FROM AUTHOR]

In the “nest site lottery” mechanism, newborns form a pool of candidates and randomly drawn candidates replace the dead adults in their nest sites. However, the selection process has only been analyzed under the assumption of an equilibrium population size. In this study, we extend this model to cases where the population size is not at an equilibrium, which yields a simplified (but fully mechanistic) biphasic population growth model, where the suppression of growth is driven only by the availability of free nest sites for newborns. This new model is free of the inconsistency found in the classical single phase models (such as the logistic model), where the number of recruited newborns can exceed the number of free nest sites. We analyzed the stability of the stationary density surfaces and the selection mechanisms for individual strategies described by different vital rates, which are implied by the new model. [ABSTRACT FROM AUTHOR]

Balancing selection describes any form of natural selection, which results in the persistence of multiple variants of a trait at intermediate frequencies within populations. By offering up a snapshot of multiple co-occurring functional variants and their interactions, systems under balancing selection can reveal the evolutionary mechanisms favouring the emergence and persistence of adaptive variation in natural populations. We here focus on the mechanisms by which several functional variants for a given trait can arise, a process typically requiring multiple epistatic mutations. We highlight how balancing selection can favour specific features in the genetic architecture and review the evolutionary and molecular mechanisms shaping this architecture. First, balancing selection affects the number of loci underlying differentiated traits and their respective effects. Control by one or few loci favours the persistence of differentiated functional variants by limiting intergenic recombination, or its impact, and may sometimes lead to the evolution of supergenes. Chromosomal rearrangements, particularly inversions, preventing adaptive combinations from being dissociated are increasingly being noted as features of such systems. Similarly, due to the frequency of heterozygotes maintained by balancing selection, dominance may be a key property of adaptive variants. High heterozygosity and limited recombination also influence associated genetic load, as linked recessive deleterious mutations may be sheltered. The capture of deleterious elements in a locus under balancing selection may reinforce polymorphism by further promoting heterozygotes. Finally, according to recent genomewide scans, balanced polymorphism might be more pervasive than generally thought. We stress the need for both functional and ecological studies to characterize the evolutionary mechanisms operating in these systems. [ABSTRACT FROM AUTHOR]

Species exhibiting colour polymorphism are thought to have an ecological advantage at the landscape scale, because spatial segregation of alternatively adapted ecotypes into diverse habitats can increase the species' niche breadth and thus confer greater geographic range size. However, morph frequencies are also influenced by intrapopulational processes such as frequency- or density-dependent social interactions. To identify how social feedback may affect clinal variation in morph frequencies, we investigated reciprocal interactions between morph-specific thermal tolerance, local climatic conditions and social environments, in the context of a colour-morph frequency cline associated with a recent range expansion in blue-tailed damselflies ( Ischnura elegans) in Sweden. Cold tolerances of gynochromes (female-like female morph) were positively correlated with local gynochrome frequencies, suggesting a positive frequency-dependent fitness benefit. In contrast, androchrome (male-mimic female morph) cold tolerances were improved following recent exposure to cold weather, suggesting a beneficial environmental acclimation effect. Thus, according to an environment-matching hypothesis for clinal variation, androchrome frequencies should therefore increase towards the (cooler) range limit. In contrast to this prediction, gynochrome frequencies increased at the expanding range limit, consistent with a positive frequency-dependent social feedback that is beneficial when invading novel climates. Our results suggest that when phenotypes or fitnesses are affected by interactions with conspecifics, beneficial social effects on environmental tolerances may (i) facilitate range shifts, and (ii) reverse or counteract typical patterns of intraspecific interactions and environment-matching clines observed in stable populations observed over broader geographic scales. [ABSTRACT FROM AUTHOR]

The most recent data on the system of cmp ( competitiveness) genes that determine the nodulation competitiveness of rhizobial strains, i.e., the ability to compete for nodule formation in leguminous plants, is analyzed. Three genetic approaches for the construction of economically valuable strains of rhizobia are proposed: the amplification of positive regulators of competitiveness, the inactivation of the negative regulators of this trait, and the introduction of efficient competitiveness factors into strains capable of active nitrogen fixation. [ABSTRACT FROM AUTHOR]

Reproductive phenology is a crucial life-history trait that evolves in response to external environmental conditions and frequency- and density-dependent interactions within species. Broadcast spawners-which represent a large fraction of aquatic biodiversity- evolve phenologies that balance strong density-dependent fertilization success against abiotic environmental conditions that are required for successful reproduction. The overall balance between these processes may be particularly complex in dioecious species, where selection on reproductive timing potentially differs between the sexes. Here, we develop a population genetic model of reproductive phenology in a dioecious broadcast spawning species and show that environmental variability and density-dependent fertilization dynamics naturally give rise to profound sex differences in selection on gamete release strategies. The frequency-dependent nature of sperm competition generates sexually antagonistic selection on reproductive timing and facilitates the maintenance of genetic variation in phenological traits. Selection in females favors monomorphic spawning phenologies that maximize net fertilization success and offspring survival across environmental conditions, whereas selection in males often favors polymorphic phenologies that are primarily shaped by sperm competition. Our model helps explain several well-documented empirical observations in aquatic species, including high intraspecific variance of reproductive phenologies, sex-specific spawning phenologies, and spawning during environmentally suboptimal times. [ABSTRACT FROM AUTHOR]

A group's composition is important for its success. Colonies of the spider Anelosimus studiosus appear to have responded to this pressure by evolving the ability to maintain mixtures of docile versus aggressive individuals that help colonies avoid extinction. Here we demonstrate that colony extinction events unite the optimal group composition of all colony constituents, regardless of phenotype, with that of the colony as a whole. This is because colony extinction events explain the majority of individual mortality events in A. studiosus . Through within- and across-habitat colony manipulations, we further determined that reduction in reproductive output by individuals bearing overabundant phenotypes underlies the ability of colonies to adaptively regulate their compositions. When we experimentally created colonies with an overabundance of the docile or aggressive phenotype, individuals bearing the overabundant phenotype exhibited reduced reproductive output, which helped to move colony compositions back towards their site-specific optima. Colonies displaced from their native sites continued to recreate the patterns of reproductive output that characterized their site of origin, suggesting a genetic component to this trait. Individuals thus appear to adaptively cull their reproductive output depending on their phenotype and the composition of their colony. There is also considerable parent–offspring colony resemblance in the extent to which colonies can or do track their ideal compositions. This conveys a kind of collective heritability to this trait. Together, while group selection appears to be the principal driver of ongoing selection on group composition in A. studiosus , patterns of selection among individuals within groups appear to promote colonies' ability to track their ideal mixtures. [ABSTRACT FROM AUTHOR]

Humans divide themselves up into separate cultures, which is a unique and ubiquitous characteristic of our species. Kinship norms are one of the defining features of such societies. Here we show how norms of marital residence can evolve as a frequency-dependent strategy, using real-world cases from southwestern China and an evolutionary game model. The process of kinship change has occurred in the past and is also occurring now in southwestern China. Our data and models show how transitions between residence types can occur both as response to changing costs and benefits of co-residence with kin, and also due to the initial frequency of the strategies adopted by others in the population: patrilocal societies can become matrilocal, and neolocal societies can become duolocal. This illustrates how frequency-dependent selection plays a role both in the maintenance of group-level cultural diversity and in cultural extinction.

Littorina fabalis is an intertidal snail commonly living on the brown algae Fucus vesiculosus and showing frequent shell-color polymorphisms in the wild. The evolutionary mechanism underlying this polymorphism is currently unknown. Shell color variation was studied in mated and non-mated specimens of this species from different microareas in one locality from NW Spain, in order to estimate sexual selection and assortative mating that may (still) be operating in this population. The analyses across microareas allowed us to investigate frequency-dependent selection and assortative mating components, mechanisms that could maintain the polymorphism. The presence of shell scars caused by crab attacks, an environmental variable not related with sexual selection or assortative mating, was used as experimental control. This study provides new evidence of significant disassortative mating and some degree of sexual selection against some shell colors, supporting the results found 21 years ago in a similar study, i.e. in the same species and locality. The similarity of these estimates during the studied period suggests that this experimental approach is consistent and valid to be extended to other populations and organisms. In addition, sexual selection and assortative mating estimates did not change across microareas differing in shell color frequencies, suggesting than the polymorphism can not be maintained by a frequency-dependent (sexual selection-based) mechanism. Our main hypothesis is that negative assortative mating could contribute to the maintenance of the polymorphism, perhaps by males showing distinct female color preferences when searching for mates [Current Zoology 58 (3): 460–471, 2012].

Viruses infecting marine prokaryotes have a large impact on the diversity and dynamics of their hosts. Model systems suggest that viral infection is frequency dependent and constrained by the virus-host encounter rate. However, it is unclear whether frequency-dependent infection is pervasive among the abundant prokaryotic populations with different temporal dynamics. To address this question, we performed a comparison of prokaryotic and viral communities using 16S rRNA amplicon and virome sequencing based on samples collected monthly for 2 years at a Japanese coastal site, Osaka Bay. Concurrent seasonal shifts observed in prokaryotic and viral community dynamics indicated that the abundance of viruses correlated with that of their predicted host phyla (or classes). Cooccurrence network analysis between abundant prokaryotes and viruses revealed 6,423 cooccurring pairs, suggesting a tight coupling of host and viral abundances and their "one-to-many" correspondence. Although stable dominant species, such as SAR11, showed few cooccurring viruses, a fast succession of their viruses suggests that viruses infecting these populations changed continuously. Our results suggest that frequency-dependent viral infection prevails in coastal marine prokaryotes regardless of host taxa and temporal dynamics. IMPORTANCE There is little room for doubt that viral infection is prevalent among abundant marine prokaryotes regardless of their taxa or growth strategy. However, comprehensive evaluations of viral infections in natural prokaryotic communities are still technically difficult. In this study, we examined viral infection in abundant prokaryotes by monitoring the monthly dynamics of prokaryotic and viral communities at a eutrophic coastal site, Osaka Bay. We compared the community dynamics of viruses with those of their putative hosts based on genome-based in silico host prediction. We observed frequent cooccurrence among the predicted virus-host pairs, suggesting that viral infection is prevalent in abundant prokaryotes regardless of their taxa or temporal dynamics. This likely indicates that frequent lysis of the abundant prokaryotes via viral infection has a considerable contribution to the biogeochemical cycling and maintenance of prokaryotic community diversity.

The fitness of an existing phenotype and of a potential mutant should generally depend on the frequencies of other existing phenotypes. Adaptive evolution driven by such frequency-dependent fitness functions can be analyzed effectively using adaptive dynamics theory, assuming rare mutation and asexual reproduction. When possible mutations are restricted to certain directions due to developmental, physiological, or physical constraints, the resulting adaptive evolution may be restricted to subspaces (constraint surfaces) with fewer dimensionalities than the original trait spaces. To analyze such dynamics along constraint surfaces efficiently, we develop a Lagrange multiplier method in the framework of adaptive dynamics theory. On constraint surfaces of arbitrary dimensionalities described with equality constraints, our method efficiently finds local evolutionarily stable strategies, convergence stable points, and evolutionary branching points. We also derive the conditions for the existence of evolutionary branching points on constraint surfaces when the shapes of the surfaces can be chosen freely. [ABSTRACT FROM AUTHOR]

Intraspecific competition influences population and community dynamics and occurs via two mechanisms. Exploitative competition is an indirect effect that occurs through use of a shared resource and depends on resource availability. Interference competition occurs by obstructing access to a resource and may not depend on resource availability. Our study tested whether the strength of interference competition changes with protozoa population density. We grew experimental microcosms of protozoa and bacteria under different combinations of protozoan density and basal resource availability. We then solved a dynamic predator-prey model for parameters of the functional response using population growth rates measured in our experiment. As population density increased, competition shifted from exploitation to interference, and competition was less dependent on resource levels. Surprisingly, the effect of resources was weakest when competition was the most intense. We found that at low population densities, competition was largely exploitative and resource availability had a large effect on population growth rates, but the effect of resources was much weaker at high densities. This shift in competitive mechanism could have implications for interspecific competition, trophic interactions, community diversity, and natural selection. We also tested whether this shift in the mechanism of competition with protozoa density affected the structure of the bacterial prey community. We found that both resources and protozoa density affected the structure of the bacterial prey community, suggesting that competitive mechanism may also affect trophic interactions. [ABSTRACT FROM AUTHOR]

Premise of research. During biological invasion, the genetic diversity of populations may be reduced by founder events and genetic drift. The floral polymorphism tristyly provides an exceptional opportunity to investigate the influence of stochastic forces on the maintenance of genetic polymorphism, because small population size leads to a characteristic signature of morph loss from populations. Here, we investigate the relations between population size and morph-frequency variation in invasive populations of tristylous Lythrum salicaria in Ontario, Canada. We also compare our results to a similar survey conducted 25 years ago in the same region. Methodology. We surveyed the size and morph ratios of 114 L. salicaria populations in 2013. We calculated the relations between population size and style morph absence, population size and style morph evenness, and the number of populations lacking particular style morphs. For comparison of the patterns of morphfrequency variation between surveys, we used a sample of populations (1988/1989: 51 populations; 2013: 101 populations) with similar size distributions. Pivotal results. Our survey confirmed that smaller populations were more likely to lack a style morph than larger populations and that morph ratios were less even in smaller populations. In dimorphic populations, the short-styled morph (S-morph) was absent most often, and the long-styled morph (L-morph) was least often missing, a pattern consistent with the stochastic theory of asymmetrical morph loss for tristylous species. There were no significant differences between the 1988/1989 and 2013 surveys in the frequency of populations missing style morphs or the relations between population size and style morph evenness. Conclusions. Despite an increase in abundance of L. salicaria in Ontario during the past 25 years, genetic drift and founder events still play a dominant role in governing patterns of morph-frequency variation. [ABSTRACT FROM AUTHOR]

Laying a mimetic egg is important for a brood parasite to succeed in defeating the defenses of a host that can recognize and reject nonmimetic foreign eggs. Several recent studies suggest that common cuckoos (Cuculus canorus) do not lay eggs randomly in their regular hosts' nests but select host individuals after inspecting their eggs to maximize the degree of mimicry. The generality of this strategy among brood parasites is, however, not currently known. Here, we studied the matching in egg appearance between plaintive cuckoos (Cacomantis merulinus) and their common tailorbird (Orthotomus sutorius) hosts, both of which have evolved dimorphic blue and white egg phenotypes with brownish spots, presumably as a result of frequency-dependent selection. Experimental parasitism with eggs of the opposite phenotype to that of the host showed that tailorbirds were very sensitive to the appearance of foreign eggs. Tailorbirds ejected all eggs that differed from the eggs of its own morph while accepting almost all eggs belonging to their corresponding morph. Hence, cuckoos would benefit greatly by selecting host nests with egg phenotypes similar to their own. However, cuckoo eggs only matched the host egg morph in approximately half of the cases, which was consistent with the expected frequency from random egg laying. Therefore, we provide evidence that plaintive cuckoos lay eggs randomly with respect to host egg morph and that they do not selectively choose host nests to maximize egg mimicry and acceptance. [ABSTRACT FROM AUTHOR]

Intraspecific variation in behavior has largely been neglected by community ecologists to date. Recently, however, a number of studies demonstrated that intraspecific variation can have profound consequences for species interactions and thereby alter community dynamics in solitary organisms. Here, we study how intraspecific variation in collective behaviors (also called 'collective personalities") affects the outcome of resource contests between 2 co-occurring species of ants, Temnothorax longispinosus and T. curvispinosus. Our results revealed that intraspecific variation in colonies' exploration behavior and aggressiveness predicts the outcome of interspecific contests for food and nest sites. How exploratory behavior affected the outcome of the foraging and nest site contests depended on the species: exploratory behavior seemed to enhance performance in foraging contests in T. curvispinosus but diminish it in T. longispinosus. More interestingly, whether a species was successful in nest site contests depended not only on its collective personality but also on the personality of its opponent colony. Temnothorax longispinosus experienced greater success if its opponent colony had similar exploratory tendencies to its own. In contrast, T. curvispinosus performed best when its opponent colony exhibited a contrasting exploratory tendency to its own. Our data demonstrate that intraspecific variation in colony behavior can have consequences for contest outcomes and ensure that no one species or behavioral strategy consistently experiences superior success. This, in turn, might help to maintain variation in collective behavior in multiple interacting populations and prevent competitive exclusion of one of the species. [ABSTRACT FROM AUTHOR]

Negative frequency-dependent selection ( FDS), where rare genotypes are favoured by selection, is commonly invoked as a mechanism explaining the maintenance of genetic variation in plant defences. However, empirical tests of FDS in plant-herbivore interactions are lacking. We evaluated whether the oviposition preference of the specialist herbivore Lema daturaphila is a mechanism through which this herbivore can exert FDS on its host plant Datura stramonium. The frequency of contrasting resistance-tolerance strategies was manipulated within experimental plots, and the plants were exposed to a similar initial density of their natural herbivore. Herbivore oviposition preference and final density, as well as plant damage and seed production, were estimated. Overall, we found that the high-resistant-low-tolerant genotypes produced four times more seeds when common than when rare, whereas the high-tolerant-low-resistant genotypes achieved twice its fitness when rare than when common. This pattern was the result of differential oviposition preferences. In addition, when the high-resistant-low-tolerant genotypes were common, there was a three-fold decreased in herbivore final density which led to a decrease in damage level by 10%. Thus, in our experiment positive FDS seems to favour resistance over tolerance. We discuss how this result would change if the extent of herbivore local adaptation and damage modify the pattern of positive FDS acting on resistance and the optimal allocation to tolerance. [ABSTRACT FROM AUTHOR]

Background and Aims The balance between stochastic forces and negative frequency-dependent selection largely determines style morph frequencies in heterostylous populations. Investigation of morph frequencies at geographical range limits can provide insights into the forces maintaining the floral polymorphism, and the factors causing biased morph ratios. Here, we investigate style morph frequencies in populations at the south-western European range limit of tristylous Lythrum salicaria, to explore the role of demographic and geographical factors influencing morph ratios in its native range. Methods We measured morph composition and evenness, and the size of 96 populations, along a north to south latitudinal transect from Galicia to Andalucia, Iberian Peninsula, traversing a steep climatic gradient. To examine the potential influence of morph-specific fitness components on morph ratios, we examined reproductive traits in 19 populations. Key Results Most populations of L. salicaria were trimorphic (94-79 %), the majority exhibiting 1 : 1 : 1 morph ratios (68-75 %). Populations with biased morph ratios had a deficiency of the short-styled morph. Population size and morph evenness were positively associated with latitude, with smaller populations and those with less even morph ratios occurring towards the south. Greater variance in morph evenness was evident at the southern range margin. There were no consistent differences in components of reproductive fitness among style morphs, but southern populations produced less fruit and seed than more northerly populations. Conclusions Our results demonstrate the influence of finite population size on morph frequencies in L. salicaria. However, they also illustrate the resilience of Iberian populations to the factors causing deviations from isoplethy and morph loss, especially at the southern range limit where populations are smaller. The maintenance of tristyly in small populations of L. salicaria may be aided by the genetic connectivity of populations in agricultural landscapes resulting from gene flow through pollen and seed dispersal. [ABSTRACT FROM AUTHOR]

The modern synthesis of evolutionary biology unified Darwin's natural selection with Mendelian genetics, but at the same time it created the dilemma of genetic load. Lewontin and Hubby's (1966) and Harris's (1966) characterization of genetic variation in natural populations increased the apparent burden of this load. Neutrality or near neutrality of genetic variation was one mechanism proposed for the revealed excessive genetic variation. Bruce Wallace coined the term "soft selection" to describe an alternative way for natural selection to operate that was consistent with observed variation. He envisioned nature as presenting ecological vacancies that could be filled by diverse genotypes. Survival and successful reproduction was a combined function of population density, genotype, and genotype frequencies, rather than a fixed value of the relative fitness of each genotype. My goal in this review is to explore the importance of soft selection in the real world. My motive and that of my colleagues as described here is not to explain what maintains genetic variation in natural populations, but rather to understand the factors that shape how organisms adapt to natural environments. We characterize how feedbacks between ecology and evolution shape both evolution and ecology. These feedbacks are mediated by density- and frequency-dependent selection, the mechanisms that underlie soft selection. Here, I report on ourprogress in characterizing these types of selection with a combination of a consideration of the published literature and the results from my collaborators' and my research on natural populations of guppies. [ABSTRACT FROM AUTHOR]

Heterodichogamy is defined as the presence of two flower morphs that exhibit male and female functions at different times among individuals within a population, and is regarded as an adaptation to promote outbreeding through enhanced intermorph pollination. In highly fragmented populations in which the morph frequency is biased, heterodichogamy may hamper population growth by reducing seed sets of the more numerous morph, and enhancing seed sets of the less numerous morph. In such situations, we hypothesize that individual plants experience greater seed sets if the opposite sexual morphs are nearby, and that individuals of a less numerous sexual morph have greater seed sets. After confirming heterodichogamy by observing flowering behavior and phenology, we tested these two hypotheses in a highly fragmented population of Machilus thunbergii, a putative heterodichogamous evergreen laurel tree. Our observations confirmed that M. thunbergii is heterodichogamous, consisting of two types of protogynous and bisexual flowers: a morning female ( MF)-afternoon male morph and a morning male ( MM)-afternoon female morph at the individual level. Sexual expression of the two morphs was highly synchronized and reciprocal. Investigation of seed-set rates revealed greater rates of both morphs if the opposite morph was nearby. The less numerous sexual morph ( MF) showed a greater seed-set rate than the more numerous sexual morph ( MM). [ABSTRACT FROM AUTHOR]