Extending and expanding the Darwinian synthesis: the role of complex systems dynamics

Darwinism is defined here as an evolving research tradition based upon the concepts of natural selection acting upon heritable variation articulated via background assumptions about systems dynamics. Darwin's theory of evolution was developed within a context of the background assumptions of Newtonian systems dynamics. The Modern Evolutionary Synthesis, or neo-Darwinism, successfully joined Darwinian selection and Mendelian genetics by developing population genetics informed by background assumptions of Boltzmannian systems dynamics. Currently the Darwinian Research Tradition is changing as it incorporates new information and ideas from molecular biology, paleontology, developmental biology, and systems ecology. This putative expanded and extended synthesis is most perspicuously deployed using background assumptions from complex systems dynamics. Such attempts seek to not only broaden the range of phenomena encompassed by the Darwinian Research Tradition, such as neutral molecular evolution, punctuated equilibrium, as well as developmental biology, and systems ecology more generally, but to also address issues of the emergence of evolutionary novelties as well as of life itself.     

An evolutionary perspective on the systems of adaptive immunity

We propose an evolutionary perspective to classify and characterize the diverse systems of adaptive immunity that have been discovered across all major domains of life. We put forward a new function‐based classification according to the way information is acquired by the immune systems: Darwinian immunity (currently known from, but not necessarily limited to, vertebrates) relies on the Darwinian process of clonal selection to ‘learn’ by cumulative trial‐and‐error feedback; Lamarckian immunity uses templated targeting (guided adaptation) to internalize heritable information on potential threats; finally, shotgun immunity operates through somatic mechanisms of variable targeting without feedback. We argue that the origin of Darwinian (but not Lamarckian or shotgun) immunity represents a radical innovation in the evolution of individuality and complexity, and propose to add it to the list of major evolutionary transitions. While transitions to higher‐level units entail the suppression of selection at lower levels, Darwinian immunity re‐opens cell‐level selection within the multicellular organism, under the control of mechanisms that direct, rather than suppress, cell‐level evolution for the benefit of the individual. From a conceptual point of view, the origin of Darwinian immunity can be regarded as the most radical transition in the history of life, in which evolution by natural selection has literally re‐invented itself. Furthermore, the combination of clonal selection and somatic receptor diversity enabled a transition from limited to practically unlimited capacity to store information about the antigenic environment. The origin of Darwinian immunity therefore comprises both a transition in individuality and the emergence of a new information system – the two hallmarks of major evolutionary transitions. Finally, we present an evolutionary scenario for the origin of Darwinian immunity in vertebrates. We propose a revival of the concept of the ‘Big Bang’ of vertebrate immunity, arguing that its origin involved a ‘difficult’ (i.e. low‐probability) evolutionary transition that might have occurred only once, in a common ancestor of all vertebrates. In contrast to the original concept, we argue that the limiting innovation was not the generation of somatic diversity, but the regulatory circuitry needed for the safe operation of amplifiable immune responses with somatically acquired targeting. Regulatory complexity increased abruptly by genomic duplications at the root of the vertebrate lineage, creating a rare opportunity to establish such circuitry. We discuss the selection forces that might have acted at the origin of the transition, and in the subsequent stepwise evolution leading to the modern immune systems of extant vertebrates.     

INTERACTIVE EFFECTS OF OFFSPRING SIZE AND TIMING OF REPRODUCTION ON OFFSPRING REPRODUCTION: EXPERIMENTAL,MATERNAL, AND QUANTITATIVE GENETIC ASPECTS

We demonstrate that egg size in side-blotched lizards is heritable (parent-offspring regressions) and thus will respond to natural selection. Because our estimate of heritability is derived from free-ranging lizards, it is useful for predicting evolutionary response to selection in wild populations. Moreover, our estimate for the heritability of egg size is not likely to be confounded by nongenetic maternal effects that might arise from egg size per se because we estimate a significant parent-offspring correlation for egg size in the face of dramatic experimental manipulation of yolk volume of the egg. Furthermore, we also demonstrate a significant correlation between egg size of the female parent and clutch size of her offspring. Because this correlation is not related to experimentally induced maternal effects, we suggest that it is indicative of a genetic correlation between egg size and clutch size. We synthesize our results from genetic analyses of the trade-off between egg size and clutch size with previously published experiments that document the mechanistic basis of this trade-off. Experimental manipulation of yolk volume has no effect on offspring reproductive traits such as egg size, clutch size, size at maturity, or oviposition date. However, egg size was related to offspring survival during adult phases of the life history. We partitioned survival of offspring during the adult phase of the life history into (1) survival of offspring from winter emergence to the production of the first clutch (i.e., the vitellogenic phase of the first clutch), and (2) survival of the offspring from the production of the first clutch to the end of the reproductive season. Offspring from the first clutch of the reproductive season in the previous year had higher survival during vitellogenesis of their first clutch if these offspring came from small eggs. We did not observe selection during these prelaying phases of adulthood for offspring from later clutches. However, we did find that later clutch offspring from large eggs had the highest survival over the first season of reproduction. The differences in selection on adult survival arising from maternal effects would reinforce previously documented selection that favors the production of small offspring early in the season and large offspring later in the season—a seasonal shift in maternal provisioning. We also report on a significant parent-offspring correlation in lay date and thus significant heritable variation in lay date. We can rule out the possibility of yolk volume as a confounding maternal effect—experimental manipulation of yolk volume has no effect on lay date of offspring. However, we cannot distinguish between genetic effects (i.e., heritable) and nongenetic maternal effects acting on lay date that arise from the maternal trait lay date per se (or other unidentified maternal traits). Nevertheless, we demonstrate how the timing of female reproduction (e.g., date of oviposition and date of hatching) affect reproductive attributes of offspring. Notably, we find that date of hatching has effects on body size at maturity and fecundity of offspring from later clutches. We did not detect comparable effects of lay date on offspring from the first clutch.     

An evolutionary quantitative genetics model for phenotypic (co)variances under limited dispersal,with an application to socially synergistic traits

Darwinian evolution consists of the gradual transformation of heritable traits due to natural selection and the input of random variation by mutation. Here, we use a quantitative genetics approach to investigate the coevolution of multiple quantitative traits under selection, mutation, and limited dispersal. We track the dynamics of trait means and of variance–covariances between traits that experience frequency‐dependent selection. Assuming a multivariate‐normal trait distribution, we recover classical dynamics of quantitative genetics, as well as stability and evolutionary branching conditions of invasion analyses, except that due to limited dispersal, selection depends on indirect fitness effects and relatedness. In particular, correlational selection that associates different traits within‐individuals depends on the fitness effects of such associations between‐individuals. We find that these kin selection effects can be as relevant as pleiotropy for the evolution of correlation between traits. We illustrate this with an example of the coevolution of two social traits whose association within‐individuals is costly but synergistically beneficial between‐individuals. As dispersal becomes limited and relatedness increases, associations between‐traits between‐individuals become increasingly targeted by correlational selection. Consequently, the trait distribution goes from being bimodal with a negative correlation under panmixia to unimodal with a positive correlation under limited dispersal.     

Evolution of adaptive phenotypic variation patterns by direct selection for evolvability

A basic assumption of the Darwinian theory of evolution is that heritable variation arises randomly. In this context, randomness means that mutations arise irrespective of the current adaptive needs imposed by the environment. It is broadly accepted, however, that phenotypic variation is not uniformly distributed among phenotypic traits, some traits tend to covary, while others vary independently, and again others barely vary at all. Furthermore, it is well established that patterns of trait variation differ among species. Specifically, traits that serve different functions tend to be less correlated, as for instance forelimbs and hind limbs in bats and humans, compared with the limbs of quadrupedal mammals. Recently, a novel class of genetic elements has been identified in mouse gene-mapping studies that modify correlations among quantitative traits. These loci are called relationship loci, or relationship Quantitative Trait Loci (rQTL), and affect trait correlations by changing the expression of the existing genetic variation through gene interaction. Here, we present a population genetic model of how natural selection acts on rQTL. Contrary to the usual neo-Darwinian theory, in this model, new heritable phenotypic variation is produced along the selected dimension in response to directional selection. The results predict that selection on rQTL leads to higher correlations among traits that are simultaneously under directional selection. On the other hand, traits that are not simultaneously under directional selection are predicted to evolve lower correlations. These results and the previously demonstrated existence of rQTL variation, show a mechanism by which natural selection can directly enhance the evolvability of complex organisms along lines of adaptive change.     

GENETIC CONSTRAINTS ON THE INDEPENDENT EVOLUTION OF MALE AND FEMALE REPRODUCTIVE CHARACTERS IN THE TRISTYLOUS PLANT LYTHRUM SALICARIA

Here we test whether the potential exists for the independent evolution of allocation to male, female, and attractive functions within a flower. We employed half-sib and parent-offspring regression methods in the tristylous plant Lythrum salicaria to determine whether there is additive genetic variation for characters important to male and female reproductive success and whether genetic correlations could constrain the independent evolution of male and female function. Although significance levels were not consistent among morph types or between populations, there were significant narrow-sense heritabilities for several traits including stamen mass, pistil mass, perianth mass, petal length, and calyx length. Traits that might be under strong stabilizing selection to promote specific pollen transfer, such as stamen and style lengths, had little heritable variation. In the majority of cases in which heritable variation was present, there were positive genetic correlations among floral traits. A strong positive genetic correlation appeared between stamen and pistil mass in the short-styled morph from one of the populations studied. This suggests that selection might not be able to act independently on biomass allocation to male and female flower parts. No evidence of negative genetic correlations appeared that would suggest trade-offs and that could augment a selection response towards sexual specialization. The observed positive correlations could be explained if we consider the “functional architecture” that underlies the covariance structure. If there is more covariance generated by pleiotropic loci controlling overall flower size than at loci controlling male versus female allocation, it could result in the observed positive covariance. At the phenotypic level, we did find significant negative partial correlations between male and female traits when flower size was controlled, but these trade-offs were among rather than within morphs.     

On the transfer of fitness from the cell to the multicellular organism

The fitness of any evolutionary unit can be understood in terms of its two basic components: fecundity (reproduction) and viability (survival). Trade-offs between these fitness components drive the evolution of life-history traits in extant multicellular organisms. We argue that these trade-offs gain special significance during the transition from unicellular to multicellular life. In particular, the evolution of germ–soma specialization and the emergence of individuality at the cell group (or organism) level are also consequences of trade-offs between the two basic fitness components, or so we argue using a multilevel selection approach. During the origin of multicellularity, we study how the group trade-offs between viability and fecundity are initially determined by the cell level trade-offs, but as the transition proceeds, the fitness trade-offs at the group level depart from those at the cell level. We predict that these trade-offs begin with concave curvature in single-celled organisms but become increasingly convex as group size increases in multicellular organisms. We argue that the increasingly convex curvature of the trade-off function is driven by the cost of reproduction which increases as group size increases. We consider aspects of the biology of the volvocine green algae – which contain both unicellular and multicellular members – to illustrate the principles and conclusions discussed.     

Heritability of parental effort in a passerine bird

Abstract The study of the evolution of parental care is central to our understanding of social systems, sexual selection, and interindividual conflict, yet we know virtually nothing about the genetic architecture of parental care traits in natural populations. In this paper, we use data from a long term field study of a passerine bird, the long-tailed tit ( Aegithalos caudatus ), to examine the heritability of the rate at which parents feed offspring. This measure of effort is positively related to offspring survival, is repeatable within individuals, and does not appear to be confounded by environmental effects. Using both parent-offspring regression, and an animal model approach, with a pedigree derived from ringing data, we show that our measure of effort has a significant heritable component.     

Quantitative genetics of growth and cryptic evolution of body size in an island population

While evolution occurs when selection acts on a heritable trait, empirical studies of natural systems have frequently reported phenotypic stasis under these conditions. We performed quantitative genetic analyses of weight and hindleg length in a free-living population of Soay sheep (Ovis aries) to test whether genetic constraints can explain previously reported stasis in body size despite evidence for strong positive directional selection. Genetic, maternal and environmental covariance structures were estimated across ontogeny using random regression animal models. Heritability increased with age for weight and hindleg length, though both measures of size were highly heritable across ontogeny. Genetic correlations among ages were generally strong and uniformly positive, and the covariance structures were also highly integrated across ontogeny. Consequently, we found no constraint to the evolution of larger size itself. Rather we expect size at all ages to increase in response to positive selection acting at any age. Consistent with expectation, predicted breeding values for age-specific size traits have increased over a twenty-year period, while maternal performance for offspring size has declined. Re-examination of the phenotypic data confirmed that sheep are not getting larger, but also showed that there are significant negative trends in size at all ages. The genetic evolution is therefore cryptic, with the response to selection presumably being masked at the phenotypic level by a plastic response to changing environmental conditions. Density-dependence, coupled with systematically increasing population size, may contribute to declining body size but is insufficient to completely explain it. Our results demonstrate that an increased understanding of the genetic basis of quantitative traits, and of how plasticity and microevolution can occur simultaneously, is necessary for developing predictive models of phenotypic change in nature.     

Cooperation and conflict in the evolution of individuality. IV. Conflict mediation and evolvability in Volvox carteri

The continued well being of evolutionary individuals (units of selection and evolution) depends upon their evolvability, that is their capacity to generate and evolve adaptations at their level of organization, as well as their longer term capacity for diversifying into more complex evolutionary forms. During a transition from a lower- to higher-level individual, such as the transition between unicellular and multicellular organisms, the evolvability of the lower-level (cells) must be restricted, while the evolvability of the new higher-level unit (multicellular organism) must be enhanced. For these reasons, understanding the factors leading to an evolutionary transition should help us to understand the factors underlying the emergence of evolvability of a new evolutionary unit. Cooperation among lower-level units is fundamental to the origin of new functions in the higher-level unit. Cooperation can produce a new more complex evolutionary unit, with the requisite properties of heritable fitness variations, because cooperation trades fitness from a lower-level (the costs of cooperation) to the higher-level (the benefits for the group). For this reason, the evolution of cooperative interactions helps us to understand the origin of new and higher-levels of fitness and organization. As cooperation creates a new level of fitness, it also creates the opportunity for conflict between levels of selection, as deleterious mutants with differing effects at the two levels arise and spread. This conflict can interfere with the evolvability of the higher-level unit, since the lower and higher-levels of selection will often "disagree" on what adaptations are most beneficial to their respective interests. Mediation of this conflict is essential to the emergence of the new evolutionary unit and to its continued evolvability. As an example, we consider the transition from unicellular to multicellular organisms and study the evolution of an early-sequestered germ-line in terms of its role in mediating conflict between the two levels of selection, the cell and the cell group. We apply our theoretical framework to the evolution of germ/soma differentiation in the green algal group Volvocales. In the most complex member of the group, Volvox carteri, the potential conflicts among lower-level cells as to the "right" to reproduce the higher-level individual (i.e. the colony) have been mediated by restricting immortality and totipotency to the germ-line. However, this mediation, and the evolution of an early segregated germ-line, was achieved by suppressing mitotic and differentiation capabilities in all post-embryonic cells. By handicapping the soma in this way, individuality is ensured, but the solution has affected the long-term evolvability of this lineage. We think that although conflict mediation is pivotal to the emergence of individuality at the higher-level, the way in which the mediation is achieved can greatly affect the longer-term evolvability of the lineage.     

Multilevel selection 1: Quantitative genetics of inheritance and response to selection

Interaction among individuals is universal, both in animals and in plants, and substantially affects evolution of natural populations and responses to artificial selection in agriculture. Although quantitative genetics has successfully been applied to many traits, it does not provide a general theory accounting for interaction among individuals and selection acting on multiple levels. Consequently, current quantitative genetic theory fails to explain why some traits do not respond to selection among individuals, but respond greatly to selection among groups. Understanding the full impacts of heritable interactions on the outcomes of selection requires a quantitative genetic framework including all levels of selection and relatedness. Here we present such a framework and provide expressions for the response to selection. Results show that interaction among individuals may create substantial heritable variation, which is hidden to classical analyses. Selection acting on higher levels of organization captures this hidden variation and therefore always yields positive response, whereas individual selection may yield response in the opposite direction. Our work provides testable predictions of response to multilevel selection and reduces to classical theory in the absence of interaction. Statistical methodology provided elsewhere enables empirical application of our work to both natural and domestic populations.     

Maternal investment and male reproductive success in angiosperms: parent-offspring conflict or sexual selection?

It is possible to interpret components of seed development in angiosperms from the perspective of parent-offspring conflict (a special case of kin selection) or sexual selection. Available parent-offspring conflict models predict the evolution of traits determining the outcome of competition among related individuals soliciting maternal resources. In such models, ‘selfishness’ may spread even if it reduces female fecundity and thus population mean fitness may decline. These models are limited, however, because most of them do not simultaneously consider selection among maternal genotypes varying in the tendency to respond to their offspring. Available sexual selection models, in contrast, do consider the joint evolution of polygenic male traits (influencing viability, mating success and fecundity) and female preferences (influencing the mating success of different male phenotypes). These models have shown that male traits may evolve that are non-optimal with respect to viability. Only one recent sexual selection model explicitly incorporates direct fecundity selection upon females; this model concludes that fecundity will be maximized at equilibrium. Hence population mean fitness may decline due to reduced male viability but not due to diminished female fecundity. Available sexual selection models, however, are limited because they do not consider the effects of interactions among relatives. The assumptions and qualitative results of the two types of models are compared and discussed in the context of seed development. Differential allocation of maternal resources among genetically distinct developing seeds may be viewed from the perspective of either. Because the results of the available models of parent-offspring conflict and sexual selection are not wholly consistent and because data confirming the genetic basis of maternal patterns of investment or differential male reproductive success are scant, it is not clear which set of conclusions is most appropriate to apply to plants. To achieve the generality towards which mathematical approaches aspire, new models concerning the evolution of traits influencing resource allocation in plants must incorporate the components of both parent-offspring conflict and sexual selection.     

Selection against late emergence and small offspring in Atlantic salmon (Salmo salar)

Timing of breeding and offspring size are maternal traits that may influence offspring competitive ability, dispersal, foraging, and vulnerability to predation and climatic conditions. To quantify the extent to which these maternal traits may ultimately affect an organism's fitness, we undertook laboratory and field experiments with Atlantic salmon (Salmo salar). To control for confounding effects caused by correlated traits, manipulations of the timing of fertilization combined with intraclutch comparisons were used. In the wild, a total of 1462 juveniles were marked at emergence from gravel nests. Recapture rates suggest that up to 83.5% mortality occurred during the first four months after emergence from the gravel nests, with the majority (67.5%) occurring during the initial period ending 17 days after median emergence. Moreover, the mortality was selective during this initial period, resulting in a significant phenotypic shift toward an earlier date of and an increased length at emergence. However, no significant selection differentials were detected thereafter, indicating that the critical episode of selection had occurred at emergence. Furthermore, standardized selection gradients indicated that selection was more intense on date of than on body size at emergence. Timing of emergence had additional consequences in terms of juvenile body size. Late-emerging juveniles were smaller than early-emerging ones at subsequent samplings, both in the wild and in parallel experiments conducted in seminatural stream channels, and this may affect success at subsequent size-selective episodes, such as winter mortality and reproduction. Finally, our findings also suggest that egg size had fitness consequences independent of the effects of emergence time that directly affected body size at emergence and, in turn, survival and size at later life stages. The causality of the maternal effects observed in the present study supports the hypothesis that selection on juvenile traits may play an important role in the evolution of maternal traits in natural populations.     

Divergent selection and phenotypic plasticity during incipient speciation in Lake Victoria cichlid fish

Divergent selection acting on several different traits that cause multidimensional shifts are supposed to promote speciation, but the outcome of this process is highly dependent on the balance between the strength of selection vs. gene flow. Here, we studied a pair of sister species of Lake Victoria cichlids at a location where they hybridize and tested the hypothesis that divergent selection acting on several traits can maintain phenotypic differentiation despite gene flow. To explore the possible role of selection we tested for correlations between phenotypes and environment and compared phenotypic divergence (P_ST) with that based on neutral markers (F_ST). We found indications for disruptive selection acting on male breeding colour and divergent selection acting on several morphological traits. By performing common garden experiments we also separated the environmental and heritable components of divergence and found evidence for phenotypic plasticity in some morphological traits contributing to species differences.     

On the reorganization of fitness during evolutionary transitions in individuality

The basic problem in an evolutionary transition is to understandhow a group of individuals becomes a new kind of individual,possessing the property of heritable variation in fitness atthe new level of organization. During an evolutionary transition,for example, from single cells to multicellular organisms, thenew higher-level evolutionary unit (multicellular organism)gains its emergent properties by virtue of the interactionsamong lower-level units (cells). We see the formation of cooperativeinteractions among lower-level units as a necessary step inevolutionary transitions; only cooperation transfers fitnessfrom lower levels (costs to group members) to higher levels(benefits to the group). As cooperation creates new levels offitness, it creates the opportunity for conflict between levelsas deleterious mutants arise and spread within the group. Fundamentalto the emergence of a new higher-level unit is the mediationof conflict among lower-level units in favor of the higher-levelunit. The acquisition of heritable variation in fitness at thenew level, via conflict mediation, requires the reorganizationof the basic components of fitness (survival and reproduction)and life-properties (such as immortality and totipotency) aswell as the co-option of lower-level processes for new functionsat the higher level. The way in which the conflicts associatedwith the transition in individuality have been mediated, andfitness and general life-traits have been re-organized, caninfluence the potential for further evolution (i.e., evolvability)of the newly emerged evolutionary individual. We use the volvocaleangreen algal group as a model-system to understand evolutionarytransitions in individuality and to apply and test the theoreticalprinciples presented above. Lastly, we discuss how the differentnotions of individuality stem from the basic properties of fitnessin a multilevel selection context.     

The heritability of inducible defenses in tadpoles

The evolution of plastic traits requires phenotypic trade-offs and heritable traits, yet the latter requirement has received little attention, especially for predator-induced traits. Using a half-sib design, I examined the narrow-sense heritability of predator-induced behaviour, morphology, and life history in larval wood frogs (Rana sylvatica). Many of the traits had significant additive genetic variation in predator (caged Anax longipes) and no-predator environments. Whereas most traits had moderate to high heritability across environments, tail depth exhibited high heritability with predators but low heritability without predators. In addition, several traits had significant heritability for plasticity, suggesting a potential for selection to act on plasticity per se. Genetic correlations confirmed known phenotypic relationships across environments and identified novel relationships within each environment. This appears to be the first investigation of narrow-sense heritabilities for predator-induced traits and confirms that inducible traits previously shown to be under selection also have a genetic basis and should be capable of exhibiting evolutionary responses.     

The heritability of sexually dimorphic traits in the yellow dung fly Scathophaga stercoraria (L.)

A precise method was used for estimating the proportion of heritable variation in two life history parameters of the yellow dung fly, whereby environmental components of variance were minimized. Significant heritable variation for body size was revealed for father to son and mother to daughter relationships. Variation in development time was not significantly heritable. There is a marked sexual dimorphism in body size in this species which is discussed in the light of the observed sex-genotype interaction in heritabilities and low genetic correlation for size between the sexes. It is suggested that opposing pressures of sexual and natural selection and/or genetic pleotropy may be responsible for the maintenance of heritable variation, and the evolution of sexual dimorphism in these two traits.     

Life-history evolution and the origin of multicellularity

The fitness of an evolutionary individual can be understood in terms of its two basic components: survival and reproduction. As embodied in current theory, trade-offs between these fitness components drive the evolution of life-history traits in extant multicellular organisms. Here, we argue that the evolution of germ-soma specialization and the emergence of individuality at a new higher level during the transition from unicellular to multicellular organisms are also consequences of trade-offs between the two components of fitness-survival and reproduction. The models presented here explore fitness trade-offs at both the cell and group levels during the unicellular-multicellular transition. When the two components of fitness negatively covary at the lower level there is an enhanced fitness at the group level equal to the covariance of components at the lower level. We show that the group fitness trade-offs are initially determined by the cell level trade-offs. However, as the transition proceeds to multicellularity, the group level trade-offs depart from the cell level ones, because certain fitness advantages of cell specialization may be realized only by the group. The curvature of the trade-off between fitness components is a basic issue in life-history theory and we predict that this curvature is concave in single-celled organisms but becomes increasingly convex as group size increases in multicellular organisms. We argue that the increasingly convex curvature of the trade-off function is driven by the initial cost of reproduction to survival which increases as group size increases. To illustrate the principles and conclusions of the model, we consider aspects of the biology of the volvocine green algae, which contain both unicellular and multicellular members.     

The speciation of conger eel galectins by rapid adaptive evolution

Many cases of accelerated evolution driven by positive Darwinian selection are identified in the genes of venomous and reproductive proteins. This evolutional phenomenon might have important consequences in their gene-products' functions, such as multiple specific toxins for quick immobilization of the prey and the establishment of barriers to fertilization that might lead to speciation, and in the molecular evolution of novel genes. Recently, we analyzed the molecular evolution of two galectins isolated from the skin mucus of conger eel (Conger myriaster), named congerins I and II, by cDNA cloning and X-ray structural analysis, and we found that they have evolved in the rapid adaptive manner to emergence of a new structure including strand-swapping and a unique new ligand-binding site. In this review article we summarize and discuss the molecular evolution, especially the rapid adaptive evolution, and the structure-function relationships of conger eel galectins.