SIZE VARIATION,GROWTH STRATEGIES,AND THE EVOLUTION OF MODULARITY IN THE MAMMALIAN SKULL

Allometry is a major determinant of within‐population patterns of association among traits and, therefore, a major component of morphological integration studies. Even so, the influence of size variation over evolutionary change has been largely unappreciated. Here, we explore the interplay between allometric size variation, modularity, and life‐history strategies in the skull from representatives of 35 mammalian families. We start by removing size variation from within‐species data and analyzing its influence on integration magnitudes, modularity patterns, and responses to selection. We also carry out a simulation in which we artificially alter the influence of size variation in within‐taxa matrices. Finally, we explore the relationship between size variation and different growth strategies. We demonstrate that a large portion of the evolution of modularity in the mammalian skull is associated to the evolution of growth strategies. Lineages with highly altricial neonates have adult variation patterns dominated by size variation, leading to high correlations among traits regardless of any underlying modular process and impacting directly their potential to respond to selection. Greater influence of size variation is associated to larger intermodule correlations, less individualized modules, and less flexible responses to natural selection.     

FIFTY YEARS OF INTERSPECIFIC HYBRIDIZATION: GENETICS AND MORPHOMETRICS OF A CONTROLLED EXPERIMENT ON THE LAND SNAIL CERION IN THE FLORIDA KEYS

An unusually well documented case of secondary hybridization and introgression involving two morphologically dissimilar species of land snails is described. In 1915, 55 Bahamian Cerion casablancae were established on Bahia Honda Key, Florida. The introduced snails thrived, bred true to form, and the colony grew until about 1928 when it began to hybridize with C. incanum, the unrelated Florida species. In 1977, morphological hybrids occupied an area of about 3.5 ha. Spatial and temporal aspects of the interaction were characterized morphologically (18 shell characters were studied in source populations, founders, and hybrids sampled in 1933 and 1977) and genetically (variation at 17 allozyme loci in relevant populations sampled in 1977). In addition, special circumstances permitted us to discern the genetics of the founders: C. incanum is isogenic locally, C. casablancae is variable (P_o = 0.29–0.35), and the two species are well-differentiated (Nei's D? = 0.27). Hybrid intermediacy of form and continued enhanced variation appeared in characters from three covariance sets, while some unique hybrid morphologies resulted from characters in a fourth set. Comparison of 1933 with 1977 samples showed that the hybrids are gradually approaching C. incanum in phenotype. Regardless of phenotype, the snails on Bahia Honda Key were panmictic and outbreeding. There was no evidence for strong selection against hybrids, and by 1977 introduced alleles had spread over 5 ha. However, no pure C. casablancae remain; low (m = 0.026/generation) but persistent gene flow has brought about a 30% diminution of the introduced genome. These observations are useful in interpreting Cerion's remarkable variability as colonization following hurricane dispersal has undoubtedly played a role in the group's complex evolution. More generally, the results are relevant to the problem of interpreting hybrid zones of unknown origin, and the differences in the generally concordant patterns of morphological and genetic introgression reveal constraints on the way components of different coadapted genomes interact.     

ONE SIZE FITS ALL? RELATIONSHIPS BETWEEN THE SIZE AND DEGREE OF VARIATION IN GENITALIA AND OTHER BODY PARTS IN TWENTY SPECIES OF INSECTS AND SPIDERS

Hypotheses regarding the function of elaborate male genitalia were tested in a sample of insects and spiders by comparing their allometric values (slopes in log-log regressions on indicators of body size) with those of other body parts. Male genitalia consistently had lower slopes than other body parts. Perhaps as a consequence of this pattern, genitalic size also tended, though less consistently, to have lower coefficients of variation than did the size of other body parts. The morphological details of coupling between males and females in several species clearly indicated that selection favoring mechanical fit is not responsible for these trends. Sexual selection on male courtship structures that are brought into contact with females in precise ways may favor relatively low allometric values, in contrast to the high values seen in the other sexually selected characters (usually visual display devices) that have been studied previously, because a female's own size will influence her perception of the contact courtship devices of a male.     

CONTRIBUTION OF GENETIC AND ENVIRONMENTAL FACTORS TO SHELL SHAPE VARIATION IN THE LOTIC SNAIL SEMISULCOSPIRA REINIANA (PROSOBRANCHIA: PLEUROCERIDAE)

The genetic and environmental factors affecting shell shapein the freshwater snail Semisulcospira reiniana at Takahashiin Kyoto, central Japan, were studied by means of a rearingexperiment and field observations. Shell shape was characterizedby three parameters; W (whorl expansion rate), T (whorl translationrate), and S (roundness of generating curve). Estimated heritabilitieswere low in all three parameters and the largest component ofthe great shell variation in the Takahashi population was environmentalvariance, suggesting that the main source of shell variationwas phenotypic modulation, in response to the external environment.In the field, snails that were active in fast currents had largerW and smaller T, that is, a larger body whorl and a lower spire,than snails in slow currents. Substratum conditions relatedto T in resting periods although its cause was unclear. Mechanismsare suggested which in the absence of selection of genetic variationcould cause and maintain shell variation in S. reiniana in differentmicrohabitats. (Received 8 March 1996; accepted 3 November 1997)     

VARIATION IN SPECIES DIVERSITY AND SHELL SHAPE IN HAWAIIAN LAND SNAILS: IN SITU SPECIATION AND ECOLOGICAL RELATIONSHIPS

The native land-snail fauna of the Hawaiian islands was investigated from a combined perspective of ecological and historical, vicariant, and dispersalist biogeography. There were more than 750 described, valid species; almost all were endemic to the archipelago, many to single islands. Path analysis showed that island area, per se, had the strongest influence on numbers of species. Island altitude and number of plant communities, both strongly related to area and both dimensions of habitat diversity, also had major influences. The influence of island age was complex. A direct effect, older islands having more species, was more than counterbalanced by the strong indirect effects of age on area and altitude: older islands are smaller and lower, and smaller, lower islands had fewer species. Distance of an island from a source of colonization was of minor importance. Species richness thus appears to be related almost exclusively to evolutionary radiation in situ and not to an equilibrium between immigration and extinction. Islands need not be extremely isolated for evolutionary radiation to be more important than immigration/extinction dynamics in determining species richness, but isolation is a relative term dependent on the dispersal abilities of the organisms in question. Numbers of recorded species were also strongly correlated with collecting effort on each island, a result that stands as a warning to others involved in such studies. Numbers of species in different families were not evenly distributed across islands. Notably, Kauai had more amastrids and helicinids and fewer achatinellids than predicted; Oahu had more amastrids but fewer pupillids and succineids than predicted; Hawaii exhibited the opposite pattern from Oahu. These patterns may partly reflect the vagaries of collecting/describing effort, but some may be due to the combined effects of historical factors and competitive exclusion. The distribution of shell height/diameter was bimodal with a distinct absence of more or less equidimensional species, a general pattern seen in other faunas. Among the pulmonates, tall species predominated, suggesting a relative lack of opportunity for globular/flat species. Notably, amastrids occurred in both modes, evidence that, at least in part, ecological not taxonomic factors underlie the bimodality. The proportions of tall and globular/flat species did not vary among islands. Prosobranchs were mostly low-spired but generally less flat than the pulmonates in the low-spired mode. The islands were probably colonized originally by small taxa. Large, tall shells are found only on Kauai and Niihau, the oldest of the main islands, suggesting that opportunities for such species are probably available on other islands.     

SHORT-TERM EVOLUTION IN THE SIZE AND SHAPE OF PEA APHIDS

Phenotypic evolution in contemporary populations can generally be witnessed only when novel selective forces produce rapid evolution. Examples of conditions that have led to rapid evolution include drastic environmental change, invasion of a new predator, or a host-range expansion. In cyclical parthenogens, however, yearly cycles of phenotypic evolution may occur due to the loss of adaptation during recombination in the sexual phase (genetic slippage), permitting an opportunity to observe adaptive evolutionary change in contemporary populations that are not necessarily subject to new patterns of natural selection. In insect herbivores, comparative studies suggest that morphological features that aid individuals in remaining on the plant or exploiting it as a food source are likely targets for selection. Here, we estimated the genetic variability of morphological traits in a cyclical parthenogen, the pea aphid (Acyrthosiphon pisum), to determine the potential for their evolution and we tested the hypothesis that size and/or shape evolves by clonal selection during one season of parthenogenetic reproduction. Genetic variation in a set of morphological traits was estimated using laboratory-reared descendents of clones collected from a single alfalfa field in May 1988 and April 1989 (henceforth, the “early” collections). In both years, there was significant clonal heritability early in the season both for overall morphology and for several individual aspects of size and shape. Because the course of short-term evolutionary change in the multivariate phenotype is a function of patterns of genetic covariance among characters, genetic correlations between size and 12 shape variables were also estimated for these early collections. A comparison between the mean phenotype of each early collection and that of a corresponding “late” collection made from the same field seven to eight clonal generations later in the same years revealed qualitatively similar changes in the average multivariate morphological phenotypes between the time periods in both years, although the difference was only significant for the 1989 samples. The pattern of genetic correlations that we estimated early in the 1989 season between overall size and various shape variables suggests that the observed short-term evolutionary changes in shape could have been due to natural selection acting only to increase overall size. We tested this hypothesis by estimating selection on size using a separate data set in which both demographic and morphological variables were measured on individuals reared under field conditions. Highly significant regressions of individual relative fitness on size were found for two major fitness components. Thus, it is likely that the evolutionary change in morphology that we observed is attributable to natural selection, possibly acting primarily through body size. A shift back to smaller size between the late 1988 and early 1989 collections from the same field suggests that either a cost of recombination or opposing selective forces during overwintering may produce persistent yearly cycles of morphological evolution in this cyclically parthenogenetic species.     

BODY AND LIMB SIZE DISSOCIATION AT THE ORIGIN OF BIRDS: UNCOUPLING ALLOMETRIC CONSTRAINTS ACROSS A MACROEVOLUTIONARY TRANSITION

The origin of birds and powered flight is a classic major evolutionary transition. Research on their origin often focuses on the evolution of the wing with trends of forelimb elongation traced back through many nonavian maniraptoran dinosaurs. We present evidence that the relative forelimb elongation within avian antecedents is primarily due to allometry and is instead driven by a reduction in body size. Once body size is factored out, there is no trend of increasing forelimb length until the origin of birds. We report that early birds and nonavian theropods have significantly different scaling relationships within the forelimb and hindlimb skeleton. Ancestral forelimb and hindlimb allometric scaling to body size is rapidly decoupled at the origin of birds, when wings significantly elongate, by evolving a positive allometric relationship with body size from an ancestrally negative allometric pattern and legs significantly shorten by keeping a similar, near isometric relationship but with a reduced intercept. These results have implications for the evolution of powered flight and early diversification of birds. They suggest that their limb lengths first had to be dissociated from general body size scaling before expanding to the wide range of fore and hindlimb shapes and sizes present in today's birds.     

QUANTITATIVE GENETICS OF SHELL FORM OF AN INTERTIDAL SNAIL: CONSTRAINTS ON SHORT-TERM RESPONSE TO SELECTION

We investigated the genetic and environmental determinants of shell form in an intertidal snail (Prosobranchia: Littorina sp.) to identify constraints on the short-term response to selection. Our quantitative genetic parameters were estimated from a half-sib experimental design using 288 broods of snails. Each brood was divided into two treatments differing in snail population density, and therefore in grazing area per snail. Differences in population density induced marked differences in shell form. Snails in the low density treatment grew faster and had lighter shells with narrower whorls and narrower apertures than their siblings at high density. Despite this environmental plasticity in shell shape we found significant additive genetic variance for components of shell shape. We discuss two mechanisms that may maintain additive genetic variance for shell shape in intertidal snail populations: migration between environments with different selective pressures and migration between environments with different mean growth rates. We also estimated a genetic variance-covariance matrix for shell form traits and used the matrix to identify constraints on the short-term response to selection. We predict the rate of response to selection for predator-resistant morphology such as would occur upon invasion of predatory crabs. The large negative genetic correlation between relative spire height and shell weight would facilitate simultaneous selection for a lower spire and a heavier shell, both of which would increase resistance to predatory crabs.     

INTER AND INTRA-SPECIFIC VARIATION OF THE SHELL OF TWO SPECIES OF THE TROPICAL FRESHWATER SNAIL BELLAMYA (MOLLUSCA: PROSOBRANCHIA)

Shell variation in two species of the tropical freshwater snailBellamya from two different habitats, was studied. The variation of the shell features studied, which includedthe shell width, spire height, aperture height and aperturewidth, were found to be related to shell height. Furthermore,significant difference in the shell shape was found not onlybetween the two species but also between males and females ofeach species. (Received 18 August 1983;     

HISTORICAL ALLOPATRY AND THE BIOGEOGRAPHY OF SPECIATION IN THE PROSOBRANCH SNAIL GENUS NUCELLA

Two recently diverged northeastern Pacific sibling snail species, Nucella ostrina and N. emarginata, currently inhabit adjacent zoogeographic provinces. Their distributions overlap in central California to the north of a major faunal boundary at Point Conception, California (PC). To test the hypothesis that modern sympatry is due to a recent northward range expansion by N. emarginata, I analyzed the population structures of both species with nuclear (allozyme) and mitochondrial DNA (mtDNA) markers. Populations of N. emarginata in the region of overlap exhibit significantly lower heterozygosity and allelic diversity than either populations to the south of PC or populations of N. ostrina. A single mtDNA haplotype characterizes all but one population of N. emarginata sampled in this region, but no haplotype to the south of PC is found at more than one locality. MtDNA haplotypes and allozyme allele frequencies also indicate monophyly of central California populations of N. emarginata. Sharp differences in allelic diversity over small geographic distances may reflect the action of natural selection, but because both nuclear and mtDNA markers display concordant patterns, a range expansion across PC best explains patterns of genetic variation in N. emarginata. Allozymes and mtDNA also reveal that the geologically older N. ostrina is paraphyletic with respect to N. emarginata. This pattern is consistent with, but not indicative of, a peripheral isolation model of speciation. Low genetic diversity is also expected if a significant bottleneck occurred at speciation. However, low allelic diversity is not universal throughout the geographic range of N. emarginata; high allelic diversity at the southern end of the distribution of N. emarginata suggests that in the past N. emarginata has been geographically restricted much further south than PC. A northward range expansion across PC by N. emarginata may thus represent only the most recent postglacial movement by the species. The thermal and oceanographic discontinuities found at PC may not have been directly involved in geographic isolation if N. emarginata originated much further south of this modern boundary. Despite uncertainty regarding the exact spatial distribution of populations at speciation, genetic data indicate that even though N. ostrina and N. emarginata currently exhibit a broad range of geographic overlap, speciation was likely allopatric and was initiated by physical isolation of populations in different zoogeographic provinces.     

APPARENT SELECTIVE NEUTRALITY OF MITOCHONDRIAL DNA SIZE VARIATION: A TEST IN THE DEEP-SEA SCALLOP PLACOPECTEN MAGELLANICUS

Animal mitochondrial DNA (mtDNA) is believed to have evolved under intense selection for economy of the size of the molecule. Among scallop species mtDNA size may vary by a factor of two and among conspecific individuals by as much as 25%. We have examined the possibility that large mtDNA size differences may be associated with fitness in the deep sea scallop Placopecten magellanicus by comparing shell lengths of individuals with different copy numbers of a large mtDNA repeated sequence. Among juvenile cohorts of same age, shell length is known to be a good index of overall fitness in marine bivalves and it is shown here to be affected by differences in nuclear genotype, expressed as the degree of enzyme heterozygosity. We have observed no correlation between shell length and mtDNA length and interpreted this to mean that variation in the size of animal mtDNA is effectively neutral to the forces of natural selection acting on the individual. This type of mtDNA variation must, therefore, be explained in terms of biases in the molecular mechanisms causing expansion or contraction of the molecule, differential replication rates of mtDNA molecules of different size, and the stochastic assortment of mtDNA size classes among individuals.     

INTRASPECIFIC LIFE HISTORY VARIATION IN A POND SNAIL: THE ROLES OF POPULATION DIVERGENCE AND PHENOTYPIC PLASTICITY

Two approaches were used to determine the degree of divergence in life histories among populations of the pond snail, Lymnaea elodes. Juvenile snails were reciprocally transferred between ponds differing in permanence and productivity, and the resulting variation in life history traits was recorded. In a second experiment, parents and their offspring from both a vernal and a permanent pond population were reared in the same pond. Proximal factors had by far the greatest effects on life history traits in the transfer experiment, with snails reared in a more productive pond showing earlier reproduction at a larger size, higher fecundity, and longer life cycle length. Snails from the more uncertain pond in terms of drying date did reproduce at an earlier age and smaller size and grew less in each pond. However, these population differences, for the most part, disappeared when snails were reared for two generations in the same environment. Much of the intraspecific variation in life histories seen in this species must therefore be considered the result of phenotypic plasticity. I argue that the plasticity in life histories itself may be adaptive to this inhabitant of unpredictable, vernal ponds.     

SPECIES RECOGNITION AND PATTERNS OF POPULATION VARIATION IN THE REPRODUCTIVE STRUCTURES OF A DAMSELFLY GENUS

The selection pressures imposed by mate choice for species identity should impose strong stabilizing selection on traits that confer species identity to mates. Thus, we expect that such traits should show nonoverlapping distributions among closely related species, but show little to no variance among populations within a species. We tested these predictions by comparing levels of population differentiation in the sizes and shapes of male cerci (i.e., the clasper structures used for species identity during mating) of six Enallagma damselfly species. Cerci shapes were nonoverlapping among Enallagma species, and five of six Enallagma species showed no population variation across their entire species ranges. In contrast, cerci sizes overlapped among species and varied substantially among populations within species. These results, taken with previous studies, suggest that cerci shape is a primary feature used in species recognition used to discriminate conspecific from heterospecifics during mating.     

AN ANALYSIS OF SHELL SHAPE VARIATION IN THE PAINTED TOPSHELL CALLIOSTOMA ZIZYPHINUM (L.) (PROSOBRANCHIA: TROCHIDAE

Littoral and sublittoral Calliostoma zizyphinum (L.) from fourteensites around Ireland are compared by a new method of shell shapeanalysis. The apparatus used is described and results show clearvariation between populations. It is also used to show similaritybetween the variegated and white morphs found in StrangfordLough. Patterns of growth are shown to be largely in responseto the habitat conditions encountered at each site. The meanapical angle of each population is shown to be of great importancein distinguishing populations of topshells, as it varies withthe relative exposure of each site. (Received 30 July 1986;     

GEOGRAPHIC VARIATION WITHIN AN ISLAND: UNIVARIATE AND MULTIVARIATE CONTOURING OF SCALATION,SIZE, AND SHAPE OF THE LIZARD GALLOTIA GALLOTI

Microgeographic variation of the vegetarian lizard Gallotia galloti within the island of Tenerife is described using univariate analysis, correlation, multiple group principal component analysis, canonical analysis, transects, and contours. The size varies locally in a mosaic pattern while head shape shows a WNW–ESE cline in the south. The scalation (scale and femoral pore counts) has two facets to its geographic variation, both of which are incongruent with the primary variation in the size and shape. The scalation shows categorical variation (stepped cline) between northern and southern populations and also a strong clinal relationship with altitude. The possible causes of this variation are considered and adaptation to current ecological conditions appears to be implicated for at least the altitudinal variation, although the pertinent factors are not obvious. It is apparent that vertebrate populations distributed across small islands do not necessarily offer the advantage of a discrete homogeneous unit for evolutionary studies but can offer the opportunity for studying microgeographic variation.     

MITOCHONDRIAL PHYLOGEOGRAPHY OF THE LAND SNAIL ALBINARIA IN CRETE: LONG-TERM GEOLOGICAL AND SHORT-TERM VICARIANCE EFFECTS

The land snail genus Albinaria exhibits an extreme degree of morphological differentiation in Greece, especially in the island of Crete. Twenty-six representatives of 17 nominal species and a suspected hybrid were examined by sequence analysis of a PCR-amplified mitochondrial DNA fragment of the large rRNA subunit gene. Maximum parsimony and neighbor-joining phylogenetic analyses demonstrate a complex pattern of speciation and differentiation and suggest that Albinaria species from Crete belong to at least three distinct monophyletic groups, which, however, are not monophyletic with reference to the genus as a whole. There is considerable variation of genetic distance within and among “species” and groups. The revealed phylogenetic relations do not correlate well with current taxonomy, but exhibit biogeographical coherence. Certain small- and large-scale vicariance events can be traced, although dispersal and parapatric speciation may also be present. Our analysis suggests that there was an early and rapid differentiation of Albinaria groups across the whole of the range followed by local speciation events within confined geographical areas.