What does modularity mean?

Modularity is now generally recognized as a fundamental feature of organisms, one that may have profound consequences for evolution. Modularity has recently become a major focus of research in organismal biology across multiple disciplines including genetics, developmental biology, functional morphology, population and evolutionary biology. While the wealth of new data, and also new theory, has provided exciting and novel insights, the concept of modularity has become increasingly ambiguous. That ambiguity is underlain by diverse intuitions about what modularity means, and the ambiguity is not merely about the meaning of the word-the metrics of modularity are measuring different properties and the methods for delimiting modules delimit them by different, sometimes conflicting criteria. The many definitions, metrics and methods can lead to substantial confusion not just about what modularity means as a word but also about what it means for evolution. Here we review various concepts, using graphical depictions of modules. We then review some of the metrics and methods for analyzing modularity at different levels. To place these in theoretical context, we briefly review theories about the origins and evolutionary consequences of modularity. Finally, we show how mismatches between concepts, metrics and methods can produce theoretical confusion, and how potentially illogical interpretations can be made sensible by a better match between definitions, metrics, and methods.

Evolution of sex-specific pace-of-life syndromes: genetic architecture and physiological mechanisms

Sex differences in life history, physiology, and behavior are nearly ubiquitous across taxa, owing to sex-specific selection that arises from different reproductive strategies of the sexes. The pace-of-life syndrome (POLS) hypothesis predicts that most variation in such traits among individuals, populations, and species falls along a slow-fast pace-of-life continuum. As a result of their different reproductive roles and environment, the sexes also commonly differ in pace-of-life, with important consequences for the evolution of POLS. Here, we outline mechanisms for how males and females can evolve differences in POLS traits and in how such traits can covary differently despite constraints resulting from a shared genome. We review the current knowledge of the genetic basis of POLS traits and suggest candidate genes and pathways for future studies. Pleiotropic effects may govern many of the genetic correlations, but little is still known about the mechanisms involved in trade-offs between current and future reproduction and their integration with behavioral variation. We highlight the importance of metabolic and hormonal pathways in mediating sex differences in POLS traits; however, there is still a shortage of studies that test for sex specificity in molecular effects and their evolutionary causes. Considering whether and how sexual dimorphism evolves in POLS traits provides a more holistic framework to understand how behavioral variation is integrated with life histories and physiology, and we call for studies that focus on examining the sex-specific genetic architecture of this integration.

FEMALE CHOICE AND SEXUAL SELECTION OF MALE WING MELANIN PATTERN IN PIERIS OCCIDENTALIS (LEPIDOPTERA)

Using field experiments, I examined the role of 13 melanin-pattern elements in mate choice by female Pieris occidentalis butterflies. Males that mated successfully differed significantly from unsuccessful males in the extent of marginal forewing melanization but not on the basis of the entire forewing pattern. Deletion of the marginal forewing melanin characters significantly reduced the mating success of manipulated males relative to controls under field conditions. Female choice in P. occidentalis may act to maximize divergence of male color pattern from that of its close relative Pieris protodice. Sexual selection for increased melanization of the marginal dorsal forewing is generated by the strong preference of females for males with such wings and may be constrained by correlations between forewing melanin characters that have different functional roles.

EVOLUTION AND COADAPTATION OF THERMOREGULATORY BEHAVIOR AND WING PIGMENTATION PATTERN IN PIERID BUTTERFLIES

This paper addresses the question of how the relationship between morphological structure and functional performance differs in related groups of organisms. I describe the relationship between a suite of phenotypic characters (behavioral posture and the pattern of wing pigmentation) and one function of these characters (thermoregulatory performance) for two groups of butterflies in the family Pieridae, focusing on how behavior and wing pattern interact to affect specific aspects of thermoregulation. Using both natural and experimentally created variation in wing-melanization patterns, I develop and test a series of predictions about the relations among thermoregulatory posture, melanization pattern, body temperature, and flight activity. Results show that increased melanization in different wing regions has positive, negative, or neutral effects in increasing body temperature of Pieris butterflies. The angle of the wings used during basking alters the relative importance of different modes of heat transfer and thereby determines the contribution of different dorsal wing regions to thermoregulation. Experimentally increased dorsal melanization can either increase or decrease the onset of flight activity and can directly alter thermoregulatory posture. For Pieris, dorsal melanization affects basking and flight, while ventral melanization primarily affects overheating. These results are used to generate a functional map relating melanization pattern to thermoregulatory performance in Pieris. Reflectance-basking posture, white background color, and melanization pattern represent coadapted characters in Pieris that interact to determine thermoregulatory performance. The differences in thermoregulatory posture and background color between pierid butterflies in the subfamilies Pierinae and Coliadinae have led to a reorganization and partial reversal of the thermoregulatory effects of melanization pattern. I suggest that this change in the physical mechanism of thermoregulatory adaption in pierids has qualitatively altered the nature of selection on wing-melanization pattern.

Modularity of a leaf moth-wing pattern and a versatile characteristic of the wing-pattern ground plan

BACKGROUND

One of the most intriguing questions in evolutionary developmental biology is how an insect acquires a mimicry pattern within its body parts. A striking example of pattern mimicry is found in the pattern diversity of moth and butterfly wings, which is thought to evolve from preexisting elements illustrated by the nymphalid ground plan (NGP). Previous studies demonstrated that individuality of the NGP facilitates the decoupling of associated common elements, leading to divergence. In contrast, recent studies on the concept of modularity have argued the importance of a combination of coupling and decoupling of the constituent elements. Here, we examine the modularity of a mimicry wing pattern in a moth and explore an evolvable characteristic of the NGP.

RESULTS

This study examined the wings of the noctuid moth Oraesia excavata, which closely resemble leaves with a leaf venation pattern. Based on a comparative morphological procedure, we found that this leaf pattern was formed by the NGP common elements. Using geometric morphometrics combined with network analysis, we found that each of the modules in the leaf pattern integrates the constituent components of the leaf venation pattern (i.e., the main and lateral veins). Moreover, the detected modules were established by coupling different common elements and decoupling even a single element into different modules. The modules of the O. excavata wing pattern were associated with leaf mimicry, not with the individuality of the NGP common elements. For comparison, we also investigated the modularity of a nonmimetic pattern in the noctuid moth Thyas juno. Quantitative analysis demonstrated that the modules of the T. juno wing pattern regularly corresponded to the individuality of the NGP common elements, unlike those in the O. excavata wing pattern.

CONCLUSIONS

This study provides the first evidence for modularity in a leaf mimicry pattern. The results suggest that the evolution of this pattern involves coupling and decoupling processes to originate these modules, free from the individuality of the NGP system. We propose that this evolution has been facilitated by a versatile characteristic of the NGP, allowing the association of freely modifiable subordinate common elements to make modules.

Concerted evolution and developmental integration in modular butterfly wing patterns

Developing organisms are thought to be modular in organization so that traits in different modules evolve independently whereas traits within a module change in a concerted manner. The eyespot pattern in Bicyclus anynana butterflies provides an ideal system where morphological modularity can be dissected and different levels of genetic integration analyzed. Several lines of evidence show that all eyespots in an individual butterfly are genetically integrated, suggesting that the whole pattern, rather than the separate eyespots, should be considered as a single character. However, despite the strong genetic correlations between the two eyespots on the dorsal forewing of B. anynana, there is great potential for independent changes. Here we use laboratory lines selected in different directions for the size of those eyespots to study correlated responses in the whole eyespot pattern. We show clear changes in eyespot size across all wing surfaces, which depend on eyespot position along the anterior-posterior axis. There are also changes in the number of extra eyespots and in eyespot color composition but no changes in eyespot position relative to wing margin. Our analysis of eyespot pattern modularity is discussed in the light of what is known about the cellular and genetic mechanisms of eyespot formation and the great potential for evolutionary diversification in butterfly wing patterns.

Correlational selection and the evolution of genomic architecture

We review and discuss the importance of correlational selection (selection for optimal character combinations) in natural populations. If two or more traits subject to multivariate selection are heritable, correlational selection builds favourable genetic correlations through the formation of linkage disequilibrium at underlying loci governing the traits. However, linkage disequilibria built up by correlational selection are expected to decay rapidly (ie, within a few generations), unless correlational selection is strong and chronic. We argue that frequency-dependent biotic interactions that have 'Red Queen dynamics' (eg, host-parasite interactions, predator-prey relationships or intraspecific arms races) often fuel chronic correlational selection, which is strong enough to maintain adaptive genetic correlations of the kind we describe. We illustrate these processes and phenomena using empirical examples from various plant and animal systems, including our own recent work on the evolutionary dynamics of a heritable throat colour polymorphism in the side-blotched lizard Uta stansburiana. In particular, male and female colour morphs of side-blotched lizards cycle on five- and two-generation (year) timescales under the force of strong frequency-dependent selection. Each morph refines the other morph in a Red Queen dynamic. Strong correlational selection gradients among life history, immunological and morphological traits shape the genetic correlations of the side-blotched lizard polymorphism. We discuss the broader evolutionary consequences of the buildup of co-adapted trait complexes within species, such as the implications for speciation processes.

Covariance and decoupling of floral and vegetative traits in nine Neotropical plants: a re-evaluation of Berg's correlation-pleiades concept

Nearly forty years ago R. L. Berg proposed that plants with specialized pollination ecology evolve genetic and developmental systems that decouple floral morphology from phenotypic variation in vegetative traits. These species evolve separate floral and vegetative trait clusters, or as she termed them, "correlation pleiades." The predictions of this hypothesis have been generally supported, but only a small sample of temperate-zone herb and grass species has been tested. To further evaluate this hypothesis, especially its applicability to plants of other growth forms, we examined the patterns of phenotypic variation and covariation of floral and vegetative traits in nine species of Neotropical plants. We recognized seven specific predictions of Berg's hypothesis. Our results supported some predictions but not others. Species with specialized pollination systems usually had floral traits decoupled (weak correlation; Canna and Eichornia) or buffered (relationship with shallow proportional slope; Calathea and Canna) from variation in vegetative traits. However, the same trend was also observed in three species with unspecialized pollination systems (Echinodorus, Muntingia, and Wedelia). One species with unspecialized pollination (Croton) and one wind-pollinated species (Cyperus) showed no decoupling or buffering, as predicted. While species with specialized pollination usually showed lower coefficients of variation for floral traits than vegetative traits (as predicted), the same was also true of species with unspecialized or wind pollination (unlike our prediction). Species with specialized pollination showed less variation in floral traits than did species with unspecialized or wind pollination, as predicted. However, the same was true of the corresponding vegetative traits, which was unexpected. Also in contrast to our prediction, plants with specialized pollination systems did not exhibit tighter phenotypic integration of floral characters than did species with generalized pollination systems. We conclude that the patterns of morphological integration among floral traits and between floral and vegetative traits tend to be species specific, not easily predicted from pollination ecology, and generally more complicated than R. L. Berg envisaged.

Selection for independence of floral and vegetative traits: evidence from correlation patterns in five species

Underlying developmental and genetic relationships cause positive correlations among the sizes of a variety of plant traits. Selection for functional independence among traits, however, can reduce these correlations over evolutionary time. In 1960, R.L. Berg hypothesized that the sizes of flowers in insect-pollinated plants should be selected to remain constant regardless of the size of vegetative structures, so that flowers match the sizes of their pollinators for effective pollination. This hypothesis of functional independence of floral sizes from the size of the rest of the plant predicts that correlations between floral and vegetative traits should be reduced relative to correlations within trait groups. We measured correlations in five species of insect-pollinated plants, including four Brassicaceae and Phlox divaricata. Our results support the hypothesis. The correlations among floral traits and the correlations among vegetative traits were significantly greater than the correlations across these two groups of traits in all five species. Keywords: phenotypic correlations, natural selection, functional independence, pollination, floral evolution.