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Parallel Evolution in Opisthobranch Gastropods and its Implications for Phylogenetic Methodology
Abstract
Parallel evolution, as exemplified by data on opisthobranch mollusks (a group in which snails have repeatedly evolved into
slugs), often exceeds divergent evolution. A similar level of parallelism has been described in an array of other organisms.
This fact presents a serious challenge to those systematists who claim that a cladogram with a minimal number of steps represents
a most parsimonious and most likely scenario of genealogical relationship. When rampant parallelism is present, pattern cladism
or other congruence networks which rely solely on ingroup comparison of characters are unsuccessful in resolving the problem.
We suggest that emphasis on unique innovation and divergent apomorphy presents a fruitful and scientifically consistent alternative
to this dilemma.
We contend that cladists have drastically altered the meaning of several terms—including “parallel evolution,” “parsimony,”
and “a priori”—to suit their own methodological and philosophical paradigm. These modifications reduce the testability of
their hypotheses and, therefore, the scientific credibility of their methods. The construction of a genealogy based on shared-derived
features, which is independent of a theoretical basis, represents a resurfacing of typological thinking which has little value
in explaining or even depicting the patterns of diversity of life.
... Also once again, we do not have the necessary sister-group data. We do, however, have evidence that high levels ofmorphological homoplasy exist among a variety ofgroups offree-living organisms, including prokaryotes (Bremer and Bremer, 1989); fungi (Heiland, 1987;Crisci et al., 1988); angiosperms (Crane, 1985;Freire, 1987;Bremer, 1987); opisthobranch molluscs (Gosliner and Ghiselin, 1984); amphipods (Myers, 1988); nemerteans (Sundberg, 1989); insects (Throckmorton, 1965;Saether, 1977); and vertebrates (Hecht and Edwards, 1976;Butler, 1982;Begle, 1991). Thus, while we cannot make the precise comparisons we would like, we can assess whether the parasitic platyhelminths fall into the "high homoplasy" group of taxa. ...
Patterns of parasite morphological diversification were investigated using a morphological data base for the parasitic platyhelminths comprising 1,459 characters analyzed using phylogenetic systematic methods. Only 10.8% of the 1,882 character transformations are losses, casting doubt on views that parasites are secondarily simplified and exhibit degenerate evolution. Chi-squared analysis indicates that character loss in the Digenea and Monogenea occurs in proportion to total change and is disproportionately lower within the Eucestoda. In the Digenea fewer female characters and more male characters have been lost than expected by the total number of characters in that group, and more male and more nonreproductive characters have been lost in proportion to their distribution across groups. In the Monogenea fewer nonreproductive and more larval characters have been lost than expected within the group, and female character loss is high relative to other groups. In the Eucestoda fewer female and more larval characters have been lost than expected within the group, whereas loss of male and nonreproductive character is low, and loss of larval characters is high, compared to the other groups. Patterns of character loss result partially from characters that show repeated (homoplasious) loss in different groups. High consistency index and low homoplasy slope ratio values indicate that the parasitic platyhelminths show unusually low levels of homoplasy, casting doubt on views that parasite morphology is unusually adaptively plastic. Homoplasy within the monogeneans occurs in proportion to overall character change, is slightly higher than expected in the digeneans, and is much lower than expected within the eucestodes. Homoplasy occurs less often than expected in larval characters, and more often than expected in nonreproductive characters in the Digenea. Monogeneans show more homoplasy than expected for larval characters both within and among groups. Eucestodes show fewer homoplasious male and nonreproductive, and more homoplasious larval, characters than expected within the group, and higher homoplasy in larval characters and lower homoplasy in female and nonreproductive characters among groups.
Animals synthesize simple lipids using a distinct fatty acid synthase (FAS) related to the type I polyketide synthase (PKS) enzymes that produce complex specialized metabolites. The evolutionary origin of the animal FAS and its relationship to the diversity of PKSs remain unclear despite the critical role of lipid synthesis in cellular metabolism. Recently, an animal FAS-like PKS (AFPK) was identified in sacoglossan molluscs. Here, we explore the phylogenetic distribution of AFPKs and other PKS and FAS enzymes across the tree of life. We found AFPKs widely distributed in arthropods and molluscs (>6300 newly described AFPK sequences). The AFPKs form a clade with the animal FAS, providing an evolutionary link bridging the type I PKSs and the animal FAS. We found molluscan AFPK diversification correlated with shell loss, suggesting AFPKs provide a chemical defense. Arthropods have few or no PKSs, but our results indicate AFPKs contributed to their ecological and evolutionary success by facilitating branched hydrocarbon and pheromone biosynthesis. Although animal metabolism is well studied, surprising new metabolic enzyme classes such as AFPKs await discovery.
We use scanning electron microscopy imaging to examine the shell microstructure of fossil and living species in five families of caenogastropods (Strombidae, Volutidae, Olividae, Pseudolividae, and Ancillariidae) to determine whether parallel or convergent evolution is responsible for the development of a unique caenogastropod trait, the extreme parietal callus (EPC). The EPC is defined as a substantial thickening of both the spire callus and the callus on the ventral shell surface such that it covers 50% or more of the surface. Caenogastropods as a whole construct the EPC convergently, using a variety of low-density, poorly organized microstructures that are otherwise uncommon in caenogastropod non-callus shell construction. Within clades, however, we see evidence for parallelism in decreased regulation in both the shell and callus microstructure. Low-density and poorly ordered microstructure—such as used for the EPC—uses less organic scaffolding and is less energetically expensive than normal shell microstructure. This suggests the EPC functions to rapidly and inexpensively increase shell thickness and overall body size. Tests of functional ecology suggest that the EPC might function both to defend against crushing predation through increased body size and dissipation of forces while aiding in shell orientation of highly mobile gastropods. These interpretations hinge on the current phylogenetic placement of caenogastropod families, emphasizing the essential contribution of phylogeny when interpreting homoplasy.
A cladistic analysis of 57 species in six genera of the Endogonaceae was performed using 27 morphological characters of asexual spores and the endophytic phase of mycorrhizal development. These species comprised a monophyletic group defined by two synapomorphies: mutualistic symbiosis with terrestrial plants and production of specialized dichotomously branched intraradical arbuscules interfacing host and fungal symbiont. Members of Endogone, another genus in the Endogonaceae, did not share these characters and thus were a polyphyletic group related to some arbuscular species in Glomus and Sclerocystis only by convergence. Two main branches were hypothesized to have evolved from a common arbuscular ancestor. One branch consisted of Gigaspora and Scutellospora, as defined by extraradical auxiliary cells and spores formed within a thin unit wall on a sporogenous cell. The other branch consisted of Glomus, Sclerocystis, Acaulospora, and Entrophospora, as defined by intraradical vesicles in mycorrhizal roots. Parallelisms were numerous within all descendant monophyletic fungal groups, lowering resolution of branching patterns. The cladogram and phylogenetic tree reconstructed from this analysis establish a theoretical framework for future studies.
Phylogenetic analysis of truncatellid gastropods using comparative anatomy and ribosomal RNA sequences shows that terrestrial truncatellids likely evolved three times independently in the Caribbean. The terrestrial subfamily Geomelaniinae, characterized in part by pallial fertilization and uniquely derived features of radula and protoconch, occurs in the Greater Antilles and Cayman Islands. Truncatellinae, with renopericardial fertilization, has several widespread amphibious species and two terrestrial species restricted to Trinidad and Barbados. The species in Barbados may be the most recent animal species to evolve full terrestriality; Barbados emerged above sea level only about one million years ago. By the mid-Cenozoic, truncatellids had traits enabling them to colonize land in appropriate tectonic settings. Parallel trends in character evolution occurred in the terrestrial lineages. In older terrestrial radiations, transitional character states would likely be lost, potentially allowing parallelism to confound phylogenetic analysis of morphological characters.
The "tendency" for homoplasy to appear in closely related taxa has been widely discussed but rarely quantified. This paper proposes statistical tests that examine the topological distribution of homoplasy within characters in phylogenies. They test whether character changes are localized (confined to some subtree), or clustered (occur in proximity to each other), relative to two null models of character evolution. Null Model I assumes that the observed number of character changes are dispersed randomly among the internodes of the tree, whereas Model II weights the probability that an internode contains a change by the length of that internode-estimated by the total number of character changes along that internode. Localization is measured by the largest furthest-neighbor distance between changes, clustering by the mean nearest neighbor distance. Distances are measured either by the number of intervening branches or the number of intervening character changes. Analyses of four cladistic data sets from the literature reveal very few characters that exhibit significant levels of clustering or localization-no more than would be expected by chance. In every data set a majority of characters exhibited at least weak tendencies, but in only one data set was there a significant excess of such characters. The present findings do not provide compelling evidence for the existence of "tendencies" in homoplasy, at least among characters used to reconstruct phylogenies. They should be sought elsewhere, in cladistic analyses of larger scope, probably among a class of characters defined a priori on a structural or functional basis.
Contemporary practice in the classification of nemerteans (phylum Nemertea) is critically discussed. It is argued that basing higher taxa on the existence of a unique combination of characters in a species (or genus) is unlikely to lead to monophyletic taxa, and that this approach should be abandoned in favour of a classification based on explicit hypotheses of phylogeny. These hypotheses should be based on all available characters and characters should not be excluded before the analysis. The classification should be based on a reconstruction of the phylogeny and reflect this phylogeny in an unambiguous way.
Oligophyly may be defined as a restrictive factor in evolution leading to minimization of the number of phyletic lines owing to an occasional reduction by means of mass extinction as well as to their recovery from scanty survivors. The monophyletic origin of the vast majority of taxa finds its explanation in this succession of events, namely in the diversity reduction (DR) - rediversification (RD) sequence. In turn, the recovery from a few or a single ancestral species (near-monophyly or monophyly) causes a number of consequences for the evolution of emerging new taxa. They produce a particular class of systematic groups called genealogical domains. Such groups display an exceptionally close affinity and a similar evolutionary potential exhibited i.e. an abundant parallelism. In other words, the paucity of ancestry (oligophyly) explains why both the monophyletic origin and evolutionary parallelism are such common features of the phylogeny in most fossil groups. Parallelism is caused by similarity of apomorphic tendencies (known as 'underlying synapomorphy' in phylogenetic systematics), which are among the most characteristic features of evolution within a genealogical domain. It is now evidenced that the vast majority of Late Silurian monograptid faunas are descendants of only two species - survivors from the severe lundgreni Event. Numerous cases of heterochronic parallelism and evolutionary repetitions observed within the repertoire of the Late Silurian monograptid faunas may be explained as a far reaching effect of oligophyly. Each ancestral species established its own genealogical domain displaying certain apomorphic tendencies. The same is true for the monophyletic origin and early radiation of Llandovery monograptids. Whilst graptolites provide numerous graphic examples substantiating the oligophyly concept, it is clear that the phenomena discussed are of a much more general nature.
The phylogenetic relationships among gastropod subgroups, with emphasis on the Euthyneura, were investigated through the analyses of nearly complete 18S rDNA sequences of 29 representative gastropods. Neighbor-joining, maximum-likelihood, and maximum-parsimony methods were used in the construction of phylogenetic trees. The 18S rDNA data support the monophyly of Vetigastropoda, the vetigastropod clade Trochoidea, and Caenogastropoda. However, the monophylies of two caenogastropod subgroups, Neotaenioglossa and Neogastropoda, are not supported. The basal position of Neritopsina is confirmed. Within the Euthyneura, the Stylommatophora and the Systellommatophora are monophyletic, but the Opisthobranchia, the Pulmonata, and the Basommatophora are not. The present study supports the inclusion of Succineidae within Stylommatophora. However the phylogenetic position of Systellommatophora within Gastropoda remains unresolved.
Morphological (including ultrastructural) and developmental characters utilized in recent literature are critically reviewed as the basis to reassess the phylogenetic relationships of gastropods. The purpose of this paper is to provide a framework of characters for future studies and a testable phylogenetic hypothesis. This is one of the first attempts to use such characters to assess the relationships of all major clades using parsimony methods. The analysis uses 117 characters and includes 40 taxa, predominantly 'prosobranchs'. Five outgroup taxa are included, representing four conchiferan groups and Polyplacophora. Of the 117 characters reviewed and included in the analyses, nine are shell characters (four of these are shell structure), two opercular, two muscular, four ctenidial, 12 renopericardial and 24 reproductive (including 17 based on sperm and spermatogenesis), 27 of the digestive system, 32 of the nervous system and sense organs; the remainder are developmental (3) and of the foot and hypobranchial gland. In the initial analysis the data set included a mixture of binary and multistate characters with all characters unordered. These data were also analysed after scaling so that each character had equal weight. A third data set was constructed in which all characters were coded as binary characters. These analyses resulted in some implausible character transformations, mainly involving the regaining of lost pallial structures. Additional analyses were run on all three sets of data after removing five characters showing the most unlikely transformations. These analyses resulted in generally similar topologies. The robustness of the clades was tested using clade decay. The adaptive radiation of gastropods and their life history traits are briefly described and discussed and the terminology for simultaneous hermaphroditism refined. A scenario for the evolution of torsion equated with the fossil record is proposed and the effects of torsion and coiling on gastropods are discussed along with asymmetry imposed by limpet-shaped body forms. It is suggested that the first gastropods were ultradextral. The idea that heterochrony has played a major part in gastropod evolution is developed and discussed, particularly the paedomorphic stamp imposed on the apogastropods. The veliger larvae of caenogastropods and heterobranchs are contrasted and found to differ in many respects. The evolution of planktotrophy within gastropods is discussed. Recent phylogenetic hypotheses for gastropods based on molecular data are generally in broad agreement with the present results. On the basis of our analyses we discuss the major monophyletic groups within gastropods. Gastropods appear to be a monophyletic clade, and divide into two primary groups, the Eogastropoda (incorporating the patellogastropods and their (probably sinistrally coiled) ancestors and the Orthogastropoda - the Correspondence to W.F. Ponder. E-mail: [email protected]
/* */ remainder of the gastropods. Orthogastropoda comprises several well defined clades. The vetigastropod clade encompasses most of the groups previously included in the paraphyletic Archaeogastropoda (fissurellids, trochoideans, scissurelloideans, halioroideans pleurotomarioideans) as well as lepetodriloidean and lepetelloidean limpets and seguenzids. The location of the hot vent taxa Peltospiridae and Neomphalidae varies with each analysis, probably because there is a lack of ultrastructural data for these taxa and parallelism in many characters. They either form a paraphyletic or monophyletic group at or near the base of the vetigastropods or a clade with the neritopsines and cocculinoideans. The neritopsines (Neritoidea etc.) consistently form a clade with the cocculinoidean limpets, but their position on the tree also differs depending on the data set used and (in the case of the scaled data) whether or not the full suite of characters is used. They are either the sister to the rest of the orthogastropods or to the apogastropods. Caenogastropods [Mesogastropoda (+ architaenioglossan groups) + Neogastropoda] are consistently monophyletic as are the heterobranchs ('Heterostropha' + Opisthobranchia + Pulmonata). The caenogastropods and heterobranchs also form a clade in all the analyses and the name Apogastropoda is redefined to encompass this group. New taxa are proposed, Sorbeoconcha for the caenogastropods exclusive of the architaenioglossan taxa, and Hypsogastropoda for the 'higher caenogastropods' - the Sorbeoconcha exclusive of the Cerithioidea and Campaniloidea.
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