GENETICS OF MIMICRY IN THE TIGER SWALLOWTAIL BUTTERFLIES,PAPILIO GLAUCUS AND P. CANADENSIS (LEPIDOPTERA: PAPILIONIDAE)

The tiger swallowtail butterfly, Papilio glaucus, exhibits a female-limited polymorphism for Batesian mimicry; the Canadian tiger swallowtail, Papilio canadensis, lacks the mimetic (dark) form entirely. The species hybridize to a limited extent where their ranges overlap. Field collections and censuses indicate that mimetic females occur throughout the range of P. glaucus but at lowest frequencies in populations at the latitudinal edges of its geographic range such as the southernmost part of Florida and along the entire northern edge of its distribution from Massachusetts to Minnesota. Frequencies of mimetic females have remained relatively stable over time. Inheritance of the mimetic form is controlled primarily by two interacting sex-linked loci. The typical matrilineal pattern of inheritance in P. glaucus can be explained by polymorphism at a Y-linked locus, b. Analysis of P. glaucus × P. canadensis crosses has also revealed an X-linked locus, s, which controls the expression of the mimetic phenotype. The P. canadensis allele, s^can, suppresses the mimetic phenotype in hybrid and backcross females. Results from more than 12 yr of rearing tiger swallowtails, including interspecies hybrids, indicate that the absence of mimetic P. canadensis females is due to both a high frequency of the “suppressing” allele s^can and low frequency of the black-pigment-determining b + allele. The frequency of s^can (or other suppressing alleles of s) in P. glaucus populations outside the hybrid zone is low. Some males heterozygous at the s locus and some suppressed mimetic females occur within the hybrid zone. A simple genetic model predicts the frequency of daughters that differ in phenotype from their mothers.     

PREDATOR PERCEPTION OF BATESIAN MIMICRY AND CONSPICUOUSNESS IN A SALAMANDER

In Batesian mimicry a palatable mimic deceives predators by resembling an unpalatable model. The evolution of Batesian mimicry relies on the visual capabilities of the potential predators, as prey detection provides the selective force driving evolutionary change. We compared the visual capabilities of several potential predators to test predictions stemming from the hypothesis of Batesian mimicry between two salamanders: the model species Notophthalmus viridescens, and polymorphic mimic, Plethodon cinereus. First, we found mimicry to be restricted to coloration, but not brightness. Second, only bird predators appeared able to discriminate between the colors of models and nonmimic P. cinereus. Third, estimates of salamander conspicuousness were background dependent, corresponding to predictions only for backgrounds against which salamanders are most active. These results support the hypothesis that birds influence the evolution of Batesian mimicry in P. cinereus, as they are the only group examined capable of differentiating N. viridescens and nonmimetic P. cinereus. Additionally, patterns of conspicuousness suggest that selection from predators may drive the evolution of conspicuousness in this system. This study confirms the expectation that the visual abilities of predators may influence the evolution of Batesian mimicry, but the role of conspicuousness may be more complex than previously thought.     

INTRASEXUAL SELECTION CONSTRAINS THE EVOLUTION OF THE DORSAL COLOR PATTERN OF MALE BLACK SWALLOWTAIL BUTTERFLIES,PAPILIO POLYXENES

Males of the eastern black swallowtail (Papilio polyxenes asterius Stoll) with typical coloration were more successful in intrasexual competition for mating territories than were males altered to have female-like mimetic coloration. Sibling males were matched for wingspan and emergence date and released as pairs, one with its pattern altered and one a control that was marked but with unaltered appearance. Significantly fewer altered males were resighted one or more days after release compared with control males (33% vs. 76%, 1990; 46% vs. 83%, 1993). Altered males were less able to establish and maintain themselves in preferred territories. The inability of released, altered males to establish a territory appears related to significantly longer male-male encounters. Encounters involving at least one participant with altered appearance averaged 66 s compared with 24 s if neither male was altered. However, altering the coloration of P. polyxenes males that already had established themselves in a territory had little effect. After courtships of similar duration (≈ 40 s), released virgin females were equally likely to mate with either altered or control males. This suggests that male-male intrasexual selection is of greater importance than female mate choice in maintaining a non-mimetic dorsal coloration in male P. polyxenes.     

MITOCHONDRIAL DNA VARIATION WITHIN AND BETWEEN SPECIES OF THE PAPILIO MACHAON GROUP OF SWALLOWTAIL BUTTERFLIES

Species limits and phylogenetic relationships in the Papilio machaon species group are potentially confounded by a complex pattern of Pleistocene range fragmentation, hybridization, and ecological race formation. Mitochondrial DNA (mtDNA) restriction-site analysis has been used to define genetic affinities and genetic population structure within this species group. The distribution of mtDNA haplotypes generally confirms prior phylogenetic hypotheses and species delineations, but there is poor correspondence between ecological races and mtDNA haplotypes. The amount and distribution of mtDNA sequence variation within species vary among species, reflecting differences in current patterns of gene flow and/or historical population structure. In spite of wing pattern characters that ally them with P. polyxenes, both P. joanae and P. brevicauda have mtDNA that is closely related to that of P. machaon. We suggest that P. joanae and P. brevicauda are of hybrid origin.     

WING SHAPE VARIATION ASSOCIATED WITH MIMICRY IN BUTTERFLIES

Mimetic resemblance in unpalatable butterflies has been studied by evolutionary biologists for over a century, but has largely focused on the convergence in wing color patterns. In Heliconius numata, discrete color‐pattern morphs closely resemble comimics in the distantly related genus Melinaea. We examine the possibility that the shape of the butterfly wing also shows adaptive convergence. First, simple measures of forewing dimensions were taken of individuals in a cross between H. numata morphs, and showed quantitative differences between two of the segregating morphs, f. elegans and f. silvana. Second, landmark‐based geometric morphometric and elliptical Fourier outline analyses were used to more fully characterize these shape differences. Extension of these techniques to specimens from natural populations suggested that, although many of the coexisting morphs could not be discriminated by shape, the differences we identified between f. elegans and f. silvana hold in the wild. Interestingly, despite extensive overlap, the shape variation between these two morphs is paralleled in their respective Melinaea comimics. Our study therefore suggests that wing‐shape variation is associated with mimetic resemblance, and raises the intriguing possibility that the supergene responsible for controlling the major switch in color pattern between morphs also contributes to wing shape differences in H. numata.     

PREFERENCE HIERARCHIES AND THE ORIGIN OF GEOGRAPHIC SPECIALIZATION IN HOST USE IN SWALLOWTAIL BUTTERFLIES

Four allopatric populations of the widely distributed western anise swallowtail butterfly, Papilio zelicaon, use different plant genera as hosts, but simultaneous choice experiments showed that these populations have diverged only slightly in oviposition preference. Of the four populations—two from southeastern Washington State, one from coastal southwestern Washington State, and one from central California—three use hosts that are not available to any of the others. Although variation for the degree of preference for particular plant species occurred within and among populations, all four populations ranked hosts in the same overall order. Monophagy on a local, low-ranking host outside the range of high-ranking hosts did not necessarily lead to the loss of preference for those high-ranking hosts, thereby indicating that the high-ranking hosts would still be accepted, and in some cases even preferred, if a population encountered them again. Hence, the overall preference hierarchy among P. zelicaon populations appears to be evolutionarily conservative. Analyses of differences among families within the California population indicated that increased preference for some hosts is inversely correlated, whereas preference for other hosts may be uncorrelated. Positive correlations may also occur but were not observed among the plant species tested. Overall, the results indicate local monophagy on different plant species in P. zelicaon has not involved major reorganizations in the preference hierarchy of ovipositing females, even in populations that may have fed on a low-ranking host for many generations. Instead, small increases in preference for local hosts have occurred within an evolutionarily conservative preference hierarchy.     

IMMUNE EVASION AND THE EVOLUTION OF MOLECULAR MIMICRY IN PARASITES

Parasites that are molecular mimics express proteins which resemble host proteins. This resemblance facilitates immune evasion because the immune molecules with the specificity to react with the parasite also cross‐react with the host's own proteins, and these lymphocytes are rare. Given this advantage, why are not most parasites molecular mimics? Here we explore potential factors that can select against molecular mimicry in parasites and thereby limit its occurrence. We consider two hypotheses: (1) molecular mimics are more likely to induce autoimmunity in their hosts, and hosts with autoimmunity generate fewer new infections (the “costly autoimmunity hypothesis”); and (2) molecular mimicry compromises protein functioning, lowering the within‐host replication rate and leading to fewer new infections (the “mimicry trade‐off hypothesis”). Our analysis shows that although both hypotheses may select against molecular mimicry in parasites, unique hallmarks of protein expression identify whether selection is due to the costly autoimmunity hypothesis or the mimicry trade‐off hypothesis. We show that understanding the relevant selective forces is necessary to predict how different medical interventions will affect the proportion of hosts that experience the different infection types, and that if parasite evolution is ignored, interventions aimed at reducing infection‐induced autoimmunity may ultimately fail.     

PARALLEL RACE FORMATION AND THE EVOLUTION OF MIMICRY IN HELICONIUS BUTTERFLIES: A PHYLOGENETIC HYPOTHESIS FROM MITOCHONDRIAL DNA SEQUENCES

Mimicry has been a fundamental focus of research since the birth of evolutionary biology yet rarely has been studied from a phylogenetic perspective beyond the simple recognition that mimics are not similar due to common descent. The difficulty of finding characters to discern relationships among closely related and convergent taxa has challenged systematists for more than a century. The phenotypic diversity of wing pattens among mimetic Heliconius adds an additional twist to the problem, because single species contain more than a dozen radically different-looking geographical races even though the mimetic advantage is theoretically highest when all individuals within and between species appear the same. Mitochondrial DNA (mtDNA) offers an independent way to address these issues. In this study, Cytochrome Oxidase I and II sequences from multiple, parallel races of Heliconius erato and Heliconius melpomene are examined, to estimate intraspecific phylogeny and gauge sequence divergence and ages of clades among races within each species. Although phenotypes of sympatric races exhibit remarkable concordance between the two species, the mitochondrial cladograms show that the species have not shared a common evolutionary history. H. erato exhibits a basal split between trans- and cis-Andean groups of races, whereas H. melpomene originates in the Guiana Shield. Diverse races in either species appear to have evolved within the last 200,000 yr, and convergent phenotypes have evolved independently within as well as between species. These results contradict prior theories of the evolution of mimicry based on analysis of wing-pattern genetics.     

THE MICRO AND MACRO IN BODY SIZE EVOLUTION

The diversity of body sizes of organisms has traditionally been explained in terms of microevolutionary processes: natural selection owing to differential fitness of individual organisms, or to macroevolutionary processes: species selection owing to the differential proliferation of phylogenetic lineages. Data for terrestrial mammals and birds indicate that even on a logarithmic scale frequency distributions of body mass among species are significantly skewed towards larger sizes. We used simulation models to evaluate the extent to which macro- and microevolutionary processes are sufficient to explain these distributions. Simulations of a purely cladogenetic process with no bias in extinction or speciation rates for different body sizes did not produce skewed log body mass distributions. Simulations that included size-biased extinction rates, especially those that incorporated anagenetic size change within species between speciation and extinction events, regularly produced skewed distributions. We conclude that although cladogenetic processes probably play a significant role in body size evolution, there must also be a significant anagenetic component. The regular variation in the form of mammalian body size distributions among different-sized islands and continents suggests that environmental conditions, operating through both macro- and microevolutionary processes, determine to a large extent the diversification of body sizes within faunas. Macroevolution is not decoupled from microevolution.     

EVOLUTION OF MIMICRY PATTERNS IN METRIORRHYNCHUS (COLEOPTERA: LYCIDAE): THE HISTORY OF DISPERSAL AND SPECIATION IN SOUTHEAST ASIA

The concept of Müllerian mimicry suggests convergent evolution to an intermediate pattern and does not predict polymorphism in mimicry rings. We examined the evolution of mimicry patterns and the order of divergence of various factors, including the role of aposematic patterns in speciation, in a clade of net-winged beetles with a robust phylogeny that suggests that they dispersed from the Australian to Asian plate. We found strong evidence for the evolution of mimicry via advergence in Metriorrhynchus because older patterns are represented in the Oriental region within more than 100 species of lycids from several lineages. Advergence was likely the cause of the observed intraspecific polymorphism in contrast to the predicted universal monomorphism. Polymorphism was found in populations of two species in Sumatra and Borneo and in populations fine-tuned to subtle variants in various habitats. The advergence is likely to be based on the small population sizes of immigrants. The differences in population sizes result in much higher benefits for dispersing species than native populations. Speciation was trigged by the divergence in aposematic coloration, and the genetic differences accumulated slowly during incomplete isolation. We assumed that the differentiation in genitalia through sexual selection ultimately reinforced speciation initiated by the shift between mimicry patterns.     

TRENDS IN THE EVOLUTION OF THE EUGLENID PELLICLE

Abstract Trends in the evolution of the euglenid pellicle were described using phylogenetic methods on 18S rDNA, morphological, and combined data from 25 mostly phototrophic taxa. The tree topology from a total‐evidence analysis formed a template for a synthetic tree that took into account conflicting results derived from the partitioned datasets. Pellicle character states that can only be observed with the assistance of transmission and scanning electron microscopy were phylogenetically mapped onto the synthetic tree to test a set of previously established homology statements (inferences made independently from a cladogram). The results permitted us to more confidently infer the ancestral‐derived polarities of character state transformations and provided a framework for understanding the key cytoskeletal innovations associated with the evolution of phototrophic euglenids. We specifically addressed the character evolution of (1) the maximum number of pellicle strips around the cell periphery; (2) the patterns of terminating strips near the cell posterior end; (3) the substructural morphology of pellicle strips; (4) the morphology of the cell posterior tip; and (5) patterns of pellicle pores on the cell surface.     

THE SITES OF CELLULOSE SYNTHESIS IN ALGAE: DIVERSITY AND EVOLUTION OF CELLULOSE-SYNTHESIZING ENZYME COMPLEXES

Information on the sites of cellulose synthesis and the diversity and evolution of cellulose-synthesizing enzyme complexes (terminal complexes) in algae is reviewed. There is now ample evidence that cellulose synthesis occurs at the plasma membrane-bound cellulose synthase, with the exception of some algae that produce cellulosic scales in the Golgi apparatus. Freeze-fracture studies of the supramolecular organization of the plasma membrane support the view that the rosettes (a six-subunit complex) in higher plants and both the rosettes and the linear terminal complexes (TCs) in algae are the structures that synthesize cellulose and secrete cellulose microfibrils. In the Zygnemataceae, each single rosette forms a 5-nm or 3-nm single “elementary” microfibril (primary wall), whereas rosettes arranged in rows of hexagonal arrays synthesize criss-crossed bands of parallel cellulose microfibrils (secondary wall). In Spirogyra, it is proposed that each of the six subunits of a rosette might synthesize six β-1,4-glucan chains that cocrystallize into a 36-glucan chain “elementary” microfibril, as is the case in higher plants. One typical feature of the linear terminal complexes in red algae is the periodic arrangement of the particle rows transverse to the longitudinal axis of the TCs. In bangiophyte red algae and in Vaucheria hamata, cellulose microfibrils are thin, ribbon-shaped structures, 1–1.5 nm thick and 5–70 nm wide; details of their synthesis are reviewed. Terminal complexes appear to be made in the endoplasmic reticulum and are transferred to Golgi cisternae, where the cellulose synthases are activated and may be transported to the plasma membrane. In algae with linear TCs, deposition follows a precise pattern directed by the movement and the orientation of the TCs (membrane flow). A principal underlying theme is that the architecture of cellulose microfibrils (size, shape, crystallinity, and intramicrofibrillar associations) is directly related to the geometry of TCs. The effects of inhibitors on the structure of cellulose-synthetizing complexes and the relationship between the deposition of the cellulose microfibrils with cortical microtubules and with the membrane-embedded TCs is reviewed In Porphyra yezoensis, the frequency and distribution of TCs reflect polar tip growth in the apical shoot cell.The evolution of TCs in algae is reviewed. The evidence gathered to date illustrates the utility of terminal complex organization in addressing plant phylogenetic relationships.     

EVOLUTION OF HOST-PARASITE DIVERSITY

Hosts and parasites often have extensive genetic diversity for resistance and virulence (host range). Qualitative diversity occurs when the success of attack is an all-or-nothing response that varies according to the genotypes of the host and parasite. Quantitative diversity occurs when the success of attack is a graded response that depends on additive genetic variation in the host and parasite. Community diversity occurs when parasites vary in the success with which they can attack different host species, leading to a mixture of specialists and generalists. I developed a series of models that classify components of host-parasite interactions according to whether they cause stabilizing or disruptive selection for resistance and virulence. Stabilizing selection reduces diversity by favoring a single optimal phenotype. Disruptive selection creates diversity by favoring a mixture of widely separated phenotypes. The evolution of maximal resistance and virulence are opposed by one of three forces: metabolic costs, frequency dependence, or negative genetic correlations among beneficial traits. The models predict that qualitatively inherited resistance and virulence traits typically cause greater diversity than quantitatively inherited traits. However, each natural system is composed of many stabilizing factors that reduce diversity and disruptive factors that promote diversity. I advocate a style of modeling in which families of related assumptions are compared by their equilibrium properties, and general conclusions from equilibrium properties are tested by complete dynamical analysis. The comparison among models highlights the need for empirical studies that compare levels of diversity among related host-parasite systems.     

NEUTRAL NULL MODELS FOR DIVERSITY IN SERIAL TRANSFER EVOLUTION EXPERIMENTS

Evolution experiments with microorganisms coupled with genome‐wide sequencing now allow for the systematic study of population genetic processes under a wide range of conditions. In learning about these processes in natural, sexual populations, neutral models that describe the behavior of diversity and divergence summaries have played a pivotal role. It is therefore natural to ask whether neutral models, suitably modified, could be useful in the context of evolution experiments. Here, we introduce coalescent models for polymorphism and divergence under the most common experimental evolution assay, a serial transfer experiment. This relatively simple setting allows us to address several issues that could affect diversity patterns in evolution experiments, whether selection is operating or not: the transient behavior of neutral polymorphism in an experiment beginning from a single clone, the effects of randomness in the timing of cell division and noisiness in population size in the dilution stage. In our analyses and discussion, we emphasize the implications for experiments aimed at measuring diversity patterns and making inferences about population genetic processes based on these measurements.     

ALLOMETRIC CONSTRAINTS AND THE EVOLUTION OF ALLOMETRY

Morphological traits often covary within and among species according to simple power laws referred to as allometry. Such allometric relationships may result from common growth regulation, and this has given rise to the hypothesis that allometric exponents may have low evolvability and constrain trait evolution. We formalize hypotheses for how allometry may constrain morphological trait evolution across taxa, and test these using more than 300 empirical estimates of static (within‐species) allometric relations of animal morphological traits. Although we find evidence for evolutionary changes in allometric parameters on million‐year, cross‐species time scales, there is limited evidence for microevolutionary changes in allometric slopes. Accordingly, we find that static allometries often predict evolutionary allometries on the subspecies level, but less so across species. Although there is a large body of work on allometry in a broad sense that includes all kinds of morphological trait–size relationships, we found relatively little information about the evolution of allometry in the narrow sense of a power relationship. Despite the many claims of microevolutionary changes of static allometries in the literature, hardly any of these apply to narrow‐sense allometry, and we argue that the hypothesis of strongly constrained static allometric slopes remains viable.     

HYBRID SPECIATION AND INDEPENDENT EVOLUTION IN LINEAGES OF ALPINE BUTTERFLIES

The power of hybridization between species to generate variation and fuel adaptation is poorly understood despite long‐standing interest. There is, however, increasing evidence that hybridization often generates biodiversity, including via hybrid speciation. We tested the hypothesis of hybrid speciation in butterflies occupying extreme, high‐altitude habitats in four mountain ranges in western North America with an explicit, probabilistic model, and genome‐wide DNA sequence data. Using this approach, in concert with ecological experiments and observations and morphological data, we document three lineages of hybrid origin. These lineages have different genome admixture proportions and distinctive trait combinations that suggest unique and independent evolutionary histories.     

THE CONTRIBUTION OF EDAPHIC HETEROGENEIYT TO THE EVOLUTION AND DIVERSITY OF BURSERACEAR TREES IN THE WESTERN AMAZON

Abstract —Environmental heterogeneity in the tropics is thought to lead to specialization in plants and thereby contribute to the diversity of the tropical flora. We examine this idea with data on the habitat specificity of 35 western Amazonian species from the genera Protium, Crepidospermum, and Tetragastris in the monophyletic tribe Protieae (Burseraceae) mapped on a molecular‐based phylogeny. We surveyed three edaphic habitats that occur throughout terra firme Amazonia: white‐sand, clay, and terrace soils in eight forests across more than 2000 km in the western Amazon. Twenty‐six of the 35 species were found to be associated with only one of three soil types, and no species was associated with all three habitats; this pattern of edaphic specialization was consistent across the entire region. Habitat association mapped onto the phylogenetic tree shows association with terrace soils to be the probable ancestral state in the group, with subsequent speciation events onto clay and white‐sand soils. The repeated gain of clay association within the clade likely coincides with the emergence of large areas of clay soils in the Miocene deposited during the Andean uplift. Character optimizations revealed that soil association was not phylogenetically clustered for white‐sand and clay specialists, suggesting repeated independent evolution of soil specificity is common within the Protieae. This phylogenetic analysis also showed that multiple cases of putative sister taxa with parapatric distributions differ in their edaphic associations, suggesting that edaphic heterogeneity was an important driver of speciation in the Protieae in the Amazon basin.     

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.