MOLECULAR PHYLOGENETIC ANALYSIS OF LIFE-HISTORY EVOLUTION IN ASTERINID STARFISH

We analyzed phylogenetic relationships among 12 nominal species of starfish in the genera Patiriella and Asterina (Order Valvatida, Family Asterinidae), based on complete sequences for a mitochondrial protein coding gene (cytochrome oxidase subunit I) and five mitochondrial transfer RNA genes (alanine, leucine, asparagine, glutamine, and proline) (1923 bp total). The resulting phylogeny was used to test a series of hypotheses about the evolution of life-history traits. (1) A complex, feeding, planktonic larva is probably ancestral for these starfish, but this is not the most parsimonious reconstruction of ancestral larval states. (2) The feeding larval form was lost at least four times among these species, and three of these losses occurred among members of a single clade. (3) Small adult size evolved before both cases of hermaphroditism and viviparous brooding, but viviparity was not always preceded by an intermediate form of external brooding. (4) An ordered transformation series from feeding planktonic development to viviparous brooding has been predicted for starfish, but we could not find an example of this transformation series. (5) Viviparity evolved recently (< 2 Mya). (6) Both species selection and transformation of lineages may have contributed to the accumulation of species with nonfeeding development among these starfish. (7) Neither Asterina nor Patiriella are monophyletic genera. Larval forms and life-history traits of these starfish have evolved freely under no obvious constraints, contrary to the widely assumed evolutionary conservatism of early development.     

EVOLUTION OF MORPHOLOGICAL NOVELTY: A PHYLOGENETIC ANALYSIS OF GROWTH PATTERNS IN STREPTOCARPUS (GESNERIACEAE)

Abstract.— Streptocarpus shows great variation in vegetative architecture. In some species a normal shoot apical meristem never forms and the entire vegetative plant body may consist of a single giant cotyledon, which may measure up to 0.75 m (the unifoliate type) or with further leaves arising from this structure (the rosulate type). A molecular phylogeny of 87 taxa (77 Streptocarpus species, seven related species, and three outgroup species) using the internal transcribed spacers and 5.8S region of nuclear ribosomal DNA suggests that Streptocarpus can be divided into two major clades. One of these broadly corresponds to the caulescent group (with conventional shoot architecture) classified as subgenus Streptocarpella, whereas the other is mainly composed of acaulescent species with unusual architecture (subgenus Streptocarpus). Some caulescent species (such as S. papangae) are anomalously placed with the acaulescent clade. Available cytological data are, however, completely congruent with the two major clades: the caulescent clade is x = 15 and the acaulescent clade (including the caulescent S. papangae) is x = 16 (or polyploid multiples of 16). The genera Linnaeopsis, Saintpaulia, and Schizoboea are nested within Streptocarpus. The sequenced region has evolved, on average, 2.44 times faster in the caulescent clade than in the acaulescent clade and this is associated with the more rapid life cycle of the caulescents. Morphological variation in plant architecture within the acaulescent clade is homoplastic and does not appear to have arisen by unique abrupt changes. Instead, rosulate and unifoliate growth forms have evolved several times, reversals have occurred, and intermediate architectures are found. An underlying developmental plasticity seems to be a characteristic of the acaulescent clade and is reflected in a great lability of form.     

HOW LIZARDS TURN INTO SNAKES: A PHYLOGENETIC ANALYSIS OF BODY-FORM EVOLUTION IN ANGUID LIZARDS

Abstract One of the most striking morphological transformations in vertebrate evolution is the transition from a lizardlike body form to an elongate, limbless (snakelike) body form. Despite its dramatic nature, this transition has occurred repeatedly among closely related species (especially in squamate reptiles), making it an excellent system for studying macroevolutionary transformations in body plan. In this paper, we examine the evolution of body form in the lizard family Anguidae, a clade in which multiple independent losses of limbs have occurred. We combine a molecular phylogeny for 27 species, our morphometric data, and phylogenetic comparative methods to provide the first statistical phylogenetic tests of several long‐standing hypotheses for the evolution of snakelike body form. Our results confirm the hypothesized relationships between body elongation and limb reduction and between limb reduction and digit reduction. However, we find no support for the hypothesized sequence going from body elongation to limb reduction to digit loss, and we show that a burrowing lifestyle is not a necessary correlate of limb loss. We also show that similar degrees of overall body elongation are achieved in two different ways in anguids, that these different modes of elongation are associated with different habitat preferences, and that this dichotomy in body plan and ecology is widespread in limb‐reduced squamates. Finally, a recent developmental study has proposed that the transition from lizardlike to snakelike body form involves changes in the expression domains of midbody Hox genes, changes that would link elongation and limb loss and might cause sudden transformations in body form. Our results reject this developmental model and suggest that this transition involves gradual changes occurring over relatively long time scales.     

THE EVOLUTION OF OVIPARITY WITH EGG GUARDING AND VIVIPARITY IN LIZARDS AND SNAKES: A PHYLOGENETIC ANALYSIS

This paper investigates the evolution of viviparity and of egg guarding in lizards and snakes in which three modes of reproduction can be described: oviparity without egg guarding, oviparity with egg guarding, and viviparity. All possible transitions of reproductive modes were detected in each taxon using Maddison's method. We then tested two specific hypotheses. First, egg guarding can be regarded as an alternative to viviparity. A relatively frequent association of egg guarding and viviparous species in the same taxon may be due to similar environmental conditions or species characteristics leading to two different solutions. Second, egg guarding may facilitate the evolution of viviparity. This hypothesis is supported by the high frequency of viviparous species in taxa containing egg guarding species and by a tendency for prolonged uterine retention of eggs in brooding squamates. Our analyses demonstrate that the first hypothesis is the best supported. Egg guarding and viviparity most often evolved independently. If a major benefit of egg guarding is the repulsion of potential predators, size is one of the most obvious morphological characters that should be correlated with the evolution of reproductive modes. The two reproductive traits were correlated to a reduction in body size for viviparous species and an increase in body size for egg guarding species. This could partly explain why the evolution of these reproductive modes seems almost antagonist.     

PHYLOGENETIC ANALYSIS OF THE EVOLUTION OF ALTERNATIVE SOCIAL BEHAVIOR IN THE MANAKINS (AVES: PIPRIDAE)

Phylogenetic analyses of lekking, lek spatial organization, and cooperative and coordinated lek display in the manakins (Aves: Pipridae) demonstrate that variation in social behavior in the group has a strong, phylogenetic component. Two of the three classes of social behavior examined also show significant phylogenetic constraints. Current adaptive plasticity models are insufficient to explain the phylogenetic variation in these behaviors in the manakins. These findings support the conclusion that vertebrate reproductive social behavior has an evolutionary history, and that it is not determined solely by adaptive individual plasticity to current conditions. The evolution of social behavior, particularly through sexual selection, can have historical consequences that can limit subsequent behavioral adaptation.     

A PHYLOGENETIC ANALYSIS OF BODY SIZE EVOLUTION AND BIOGEOGRAPHY IN CHUCKWALLAS (SAUROMALUS) AND OTHER IGUANINES

The evolution of body size was reconstructed in chuckwallas (genus Sauromalus), large herbivorous lizards of southwest North America, using a phylogeny derived from sequence variation in the mitochondrial cytochrome b gene. The body mass of two endemic island species (S. hispidus and S. varius) is typically fivefold larger than mainland species. We tested the hypothesis that large body size has evolved on these islands in response to local ecological conditions against the alternative hypothesis that large size is simply retained from large iguanine ancestors. The most parsimonious tree topology depicts the insular gigantic Sauromalus as monophyletic, having diverged from a common ancestor on the Baja California peninsula after the radiation of smaller bodied clades. In a robustness analysis of this topology, we found general support for this tree over alternative topologies representing minimum evolution hypotheses that imply large body size is retained from large iguanine ancestors. The most parsimonious reconstruction of body size evolution implies a change from large to small size after the Sauromalus ancestor diverged from Iguana, and one reversal back to large size within Sauromalus. The large size increase in the gigantic clade contrasts with evolutionary stasis of small body size (for an iguanine) in mainland populations. The gigantic species show 3–4% total sequence divergence from S. obesus populations on the nearby Baja California peninsula, and mainland populations of S. obesus obesus show similar levels of divergence from each other. An analysis of character transitions and comparative behavior implicates predation, and its relaxation on isolated islands, as a strong selective force in Sauromalus. Patterns of genetic differentiation in Sauromalus and biogeographic implications are discussed.     

A PHYLOGENETIC ANALYSIS OF CHARACTER DISPLACEMENT IN CARIBBEAN ANOLIS LIZARDS

Twenty-seven islands in the Lesser Antilles contain either one or two species of Anolis lizards. On nine of the ten two-species islands, the species differ substantially in size; 16 of the 17 one-species islands harbor an intermediate-sized species. Two processes could produce such a pattern: size adjustment (or character displacement), in which similar-sized species evolve in different directions in sympatry; and size assortment, in which only different-sized species can successfully colonize the same island together. Previous analyses implicitly have assumed that size is evolutionarily plastic and determined solely by recent ecological conditions, and consequently have tested the hypothesis that character displacement has occurred on each of the ten two-species islands. Other studies have focused only on size assortment. By analyzing such patterns in a phylogenetic context, I explicitly consider historical effects and can distinguish between size adjustment and size assortment. Using a minimum evolution algorithm, I assess evidence for size adjustment by partitioning changes in size along branches of the phylogenetic tree. Size evolution appears rare (a minimum of 4-7 instances of substantial size evolution). In the northern (but not the southern) Lesser Antilles, size change was significantly greater when a descendant taxon occurred on a two-species island and its hypothetical ancestor occurred on a one-species island, thus supporting the size adjustment hypothesis, though size adjustment might have occurred only once. The relative rarity of size evolution suggests that size assortment might be responsible for nonrandom patterns. In both the northern and southern Lesser Antilles, a null model of no size assortment is convincingly rejected. Closely related taxa, however, are usually similar in size, and hybridization between species has been reported. Consequently, similar-sized species might not coexist because they interbreed and coalesce into one gene pool. A null model that only allows species from different “clades” to co-occur is rejected for the northern Lesser Antilles, but is ambiguous with regard to the southern Lesser Antilles. Thus, competitive exclusion is probably responsible for the pattern of size assortment in the northern Lesser Antilles; both competitive exclusion and interbreeding of closely related species of similar size might be responsible for the patterns evident in the southern Lesser Antilles.     

RAPID EVOLUTION OF GASTRULATION MECHANISMS IN A SEA URCHIN WITH LECITHOTROPHIC LARVAE

This study documents evolutionary modifications in mechanisms of gastrulation in Heliocidaris erythrogramma, an echinoid with lecithotrophic larvae. Radially symmetrical cell rearrangements and changes in cell shape drive elongation of the archenteron in the ancestral mode of echinoid gastrulation. Cell marking experiments indicate that in H. erythrogramma, however, prolonged movement of cells over the ventral lip of the blastopore accompanies extension of the archenteron. Evolutionary modifications to archenteron extension in H. erythrogramma thus include utilization of a different type of cellular movement as well as the imposition of dorsoventral asymmetry in cellular movements. The conservation of gastrulation mechanisms among phylogenetically divergent echinoids with planktotrophic development suggests that the plesiomorphic condition has persisted at least 250 million years and perhaps much longer. Yet H. erythrogramma diverged from an ancestor with planktotrophic development only about 10 mya, indicating that morphogenetic mechanisms of early development can undergo substantial evolutionary changes, even after long periods of stasis.     

PHYLOGENETIC ANALYSIS OF PHENOTYPIC COVARIANCE STRUCTURE. II. RECONSTRUCTING MATRIX EVOLUTION

A modified minimum evolution approach is used to estimate covariance matrices for hypothetical ancestors. Branch lengths are calculated as the mean disparity in corresponding ancestor-descendent covariances. Branches are longest leading to terminal populations and subspecies, while interspecific branches are relatively short, indicating a general conservation of covariance structure among species despite a high degree of intraspecific variability. Absolute deviations in covariance structure are not correlated with phenotypic divergence. Interpreted in light of other studies, the analyses suggest that deviations in covariance structure are most strongly associated with the formation of diagnosably distinct taxa and stochastic sampling of genotypes at the population level. There is no evidence for restructuring of phenotypic covariance structure in association with reproductive isolation. The results suggest that phenotypic covariances are dynamic over short time scales and do not support attempts to extrapolate genetic covariance structure to explain or predict macroevolutionary change. This study further demonstrates that branch lengths, which are not usually analyzed in detail, contain valuable evolutionary information complementary to that residing in the branching pattern.     

PHYLOGENETIC ANALYSIS OF TRAIT EVOLUTION AND SPECIES DIVERSITY VARIATION AMONG ANGIOSPERM FAMILIES

Angiosperm families differ greatly from one another in species richness (S). Previous studies have attributed significant components of this variation to the influence of pollination mode (biotic/abiotic) and growth form (herbaceous/woody) on speciation rate, but these results suffer difficulties of interpretation because all the studies ignored the phylogenetic relationships among families. We use a molecular phylogeny of the angiosperm families to reanalyse correlations between S and family-level traits and use reconstructions of trait evolution to interpret the results. We confirm that pollination mode and growth form are correlated with S and show that the majority of changes in pollination mode involved a change from biotic to abiotic pollination with an associated fall in speciation rate. The majority of growth form changes involved the evolution of herbaceousness from woodiness with a correlated rise in speciation rate. We test the hypothesis of Ricklefs and Renner (1994) that “evolutionary flexibility” rather than other trait changes triggered increased speciation rates in some families, but find little support for the hypothesis.     

EVOLUTION OF VIVIPARITY: A PHYLOGENETIC TEST OF THE COLD‐CLIMATE HYPOTHESIS IN PHRYNOSOMATID LIZARDS

The evolution of viviparity is a key life‐history transition in vertebrates, but the selective forces favoring its evolution are not fully understood. With >100 origins of viviparity, squamate reptiles (lizards and snakes) are ideal for addressing this issue. Some evidence from field and laboratory studies supports the “cold‐climate” hypothesis, wherein viviparity provides an advantage in cold environments by allowing mothers to maintain higher temperatures for developing embryos. Surprisingly, the cold‐climate hypothesis has not been tested using both climatic data and phylogenetic comparative methods. Here, we investigate the evolution of viviparity in the lizard family Phrynosomatidae using GIS‐based environmental data, an extensive phylogeny (117 species), and recently developed comparative methods. We find significant relationships between viviparity and lower temperatures during the warmest (egg‐laying) season, strongly supporting the cold‐climate hypothesis. Remarkably, we also find that viviparity tends to evolve more frequently at tropical latitudes, despite its association with cooler climates. Our results help explain this and two related patterns that seemingly contradict the cold‐climate hypothesis: the presence of viviparous species restricted to low‐elevation tropical regions and the paucity of viviparous species at high latitudes. Finally, we examine whether viviparous taxa may be at higher risk of extinction from anthropogenic climate change.     

PHYLOGENETIC ANALYSES OF THE CORRELATED EVOLUTION OF CONTINUOUS CHARACTERS: A SIMULATION STUDY

We use computer simulation to compare the statistical properties of several methods that have been proposed for estimating the evolutionary correlation between two continuous traits, and define alternative evolutionary correlations that may be of interest. We focus on Felsenstein's (1985) method and some variations of it and on several “minimum evolution” methods (of which the procedure of Huey and Bennett [1987] is a special case), as compared with a nonphylogenetic correlation. The last, a simple correlation of trait values across the tips of a phylogeny, virtually always yields inflated Type I error rates, relatively low power, and relatively poor estimates of evolutionary correlations. We therefore cannot recommend its use. In contrast, Felsenstein's (1985) method yields acceptable significance tests, high power, and good estimates of what we term the input correlation and the standardized realized evolutionary correlation, given complete phylogenetic information and knowledge of the rate and mode of character change (e.g., gradual and proportional to time [“Brownian motion”] or punctuational, with change only at speciation events). Inaccurate branch length information may affect any method adversely, but only rarely does it cause Felsenstein's (1985) method to perform worse than do the others tested. Other proposed methods generally yield inflated Type I error rates and have lower power. However, certain minimum evolution methods (although not the specific procedure used by Huey and Bennett [1987]) often provide more accurate estimates of what we term the unstandardized realized evolutionary correlation, and their use is recommended when estimation of this correlation is desired. We also demonstrate how correct Type I error rates can be obtained for any method by reference to an empirical null distribution derived from computer simulations, and provide practical suggestions on choosing an analytical method, based both on the evolutionary correlation of interest and on the availability of branch lengths and knowledge of the model of evolutionary change appropriate for the characters being analyzed. Computer programs that implement the various methods and that will simulate (correlated) character evolution along a known phylogeny are available from the authors on request. These programs can be used to test the effectiveness of any new methods that might be proposed, and to check the generality of our conclusions with regard to other phylogenies.     

PHYLOGENETIC ANALYSES REVEAL UNEXPECTED PATTERNS IN THE EVOLUTION OF REPRODUCTIVE MODES IN FROGS

Understanding phenotypic diversity requires not only identification of selective factors that favor origins of derived states, but also factors that favor retention of primitive states. Anurans (frogs and toads) exhibit a remarkable diversity of reproductive modes that is unique among terrestrial vertebrates. Here, we analyze the evolution of these modes, using comparative methods on a phylogeny and matched life‐history database of 720 species, including most families and modes. As expected, modes with terrestrial eggs and aquatic larvae often precede direct development (terrestrial egg, no tadpole stage), but surprisingly, direct development evolves directly from aquatic breeding nearly as often. Modes with primitive exotrophic larvae (feeding outside the egg) frequently give rise to direct developers, whereas those with nonfeeding larvae (endotrophic) do not. Similarly, modes with eggs and larvae placed in locations protected from aquatic predators evolve frequently but rarely give rise to direct developers. Thus, frogs frequently bypass many seemingly intermediate stages in the evolution of direct development. We also find significant associations between terrestrial reproduction and reduced clutch size, larger egg size, reduced adult size, parental care, and occurrence in wetter and warmer regions. These associations may help explain the widespread retention of aquatic eggs and larvae, and the overall diversity of anuran reproductive modes.     

SELFISH LARVAE: DEVELOPMENT AND THE EVOLUTION OF PARASITIC BEHAVIOR IN THE HYMENOPTERA

Queens of hymenopteran social parasites manipulate the workers of other social species into raising their offspring. However, nonconspecific brood care may also allow the parasite larvae to control their own development to a greater extent than possible in nonparasitic species. An evolutionary consequence of this may be the loss of the parasite's worker caste if the larvae can increase their fitness by developing into sexuals rather than workers. We argue that this loss is particularly likely in species in which there is little inclusive fitness benefit in working. Retention of a worker caste correlates with characteristics that increase the fitness of working relative to becoming a sexual, such as worker-production of males, high intracolony relatedness, and seasonal environments where the hosts of potential parasite queens are not always available. Further evidence strongly suggests that when the worker caste is evolutionarily lost in perennial species like ants, it disappears rapidly and through a reduction in caste threshold and queen size, so that parasite larvae become queens with less food than required to produce host workers. This evolutionary process, however, appears to lower overall population fitness, resulting in workerless parasite species having small populations and being geographically restricted. Conversely, in annual species like bees and wasps, workerless social parasitism evolves with no size reduction in queens, which is consistent with an expected lower level of queen/offspring conflict.     

COMBINATORIAL WEIGHTS IN PHYLOGENETIC ANALYSIS: A STATISTICAL PARSIMONY PROCEDURE

Abstract— A data dependent weighting procedure is developed to allow the comparison of phylogenetic trees based on nucleic acid sequence data. The sampling error of this cladogram "cost" is then examined, permitting statistical evaluation of the cost differential.     

QUANTITATIVE CHARACTERS IN PHYLOGENETIC ANALYSIS

Abstract— When analysing phylogcnetic relationships at low laxonomic levels it is often the ease that many of the features that can be used to separate taxa show continuous variation. The theoretical and practical problems for the use of such quantitative characters in phylogenetic analysis arc examined. Three methods of coding continuous data into discrete characters are assessed in detail: simple gap-coding, generalised gap-coding and segment-coding, a form of range-coding. The methods are applied to a data set gathered for Eucatyptus L'Hérit. informal subgenus Symphyomyrtus section Maidenana (Myrtaceae). Each method is capable of distorting relative differences between taxa, but segment-coding produces the least amount of distortion, provided the range of variation of the character is divided into a sufficient number of character states.
Continuous quantitative characters provide data for phylogenetic analysis that arc more noisy than those provided by discrete qualitative characters and should, therefore, only be used when the number of qualitative characters is insufficient for resolution of relationships. The results of such analyses should be recognised as provisional pending the discovery of more readily informative characters.