Convergent evolution of life habit and shell shape in scallops (Bivalvia: Pectinidae) with a description of a new genus

Phenotypic convergence is a fascinating evolutionary pattern. Many taxa evolve remarkable similarities, often due to similar selection pressures, which suggest that there may be a limited number of solutions to a particular ecological challenge. However, some phenotypes are only superficially similar. For example, convergent evolution may occur at one phenotypic level such as behavior, but other components (morphology) of the same phenotype exhibit divergence. In other words, the solutions might be the same, but how taxa approach the problem could be very different. This then suggests that there may be multiple phenotypic optima that can tackle similar ecological challenges. In this dissertation, I investigate the evolutionary patterns of life habits and shell shapes in scallops and how these phenotypic traits contribute to biological diversity. I found many scallop species have converged in life habit, but only a few lineages converged in shell shape. Rather, shell shape variation tends to be greater in species with less specific life habit requirements. However, scallop species with the gliding life habit have lesser shell shape variation. Interestingly, gliding can be performed by two distinct shell shapes, indicating that only some components of shell shape are important for the life habit, and others are free to vary. This research indicates that there is likely greater diversity in form, despite phenotypic similarities in function

Convergent and parallel evolution in life habit of the scallops (Bivalvia: Pectinidae)

We employed a phylogenetic framework to identify patterns of life habit evolution in the marine bivalve family Pectinidae. Specifically, we examined the number of independent origins of each life habit and distinguished between convergent and parallel trajectories of life habit evolution using ancestral state estimation. We also investigated whether ancestral character states influence the frequency or type of evolutionary trajectories

Morphological convergence of shell shape in distantly related scallop species (Mollusca: Pectinidae)

Morphological convergence is a central concept in evolutionary biology, but convergent patterns remain under-studied in nonvertebrate organisms. Some scallop species exhibit long-distance swimming, a behaviour whose biomechanical requirements probably generate similar selective regimes. We tested the hypothesis that shell shape similarity in long-distance swimming species is a result of convergent evolution. Using landmark-based geometric morphometrics, we quantified shell shape in seven species representing major behavioural habits. All species displayed distinct shell shapes, with the exception of the two long-distance swimmers, whose shells were indistinguishable. These species also displayed reduced morphological variance relative to other taxa. Finally, a phylogenetic simulation revealed that these species were more similar in their shell shape than was expected under Brownian motion, the model of character evolution that best described changes in shell shape. Together, these findings reveal that convergent evolution of shell shape occurs in scallops, and suggest that selection for shell shape and behaviour may be important in the diversification of the group

Shell shape convergence masks biological diversity in gliding scallops: description of Ylistrum n. gen. (Pectinidae) from the Indo-Pacific Ocean

The scallop genus Amusium Röding, 1798 is one of few genera of Pectinidae that includes taxa capable of long-distance swimming or gliding. Membership of the genus has been defined primarily by shell shape, and it currently includes only three species: the type species A. pleuronectes (Linnaeus, 1758), A. balloti (Bernardi, 1861) and A. japonicum (Gmelin, 1791). In this study, we use molecular data and aspects of shell morphology to resolve the systematics of the genus. Phylogenetic reconstruction of Pectinidae using nuclear and mitochondrial DNA sequence from four genes supports a polyphyletic Amusium. Differences in internal ribbing pattern provide morphological evidence for the recognition of the two clades identified in our phylogenetic analyses. In contrast, quantification of shell shape through geometric morphometric methods indicates that shape is a convergent phenotype and is not informative in terms of distinguishing between the two gliding lineages. Based on these results, we describe Ylistrum, n. gen, which includes two species previously assigned to Amusium. We provide characters that separate the now monotypic Amusium from the two species, Ylistrum balloti, n. comb. and Y. japonicum, n. comb

Parallel changes in genital morphology delineate cryptic diversification of planktonic nudibranchs

The relative roles of geographical and non-geographical barriers in the genesis of genetic isolation are highly debated in evolutionary biology, yet knowing how speciation occurs is essential to our understanding of biodiversity. In the open ocean, differentiating between the two is particularly difficult, because of the high levels of gene flow found in pelagic communities. Here, we use molecular phylogenetics to test the hypothesis that geography is the primary isolating mechanism in a clade of pelagic nudibranchs, Glaucinae. Our results contradict allopatric expectations: the cosmopolitan Glaucus atlanticus is panmictic, whereas the Indo-Pacific Glaucus marginatus contains two pairs of cryptic species with overlapping distributions. Within the G. marginatus species complex, a parallel reproductive change has occurred in each cryptic species pair: the loss of a bursa copulatrix. Available G. marginatus data are most consistent with non-geographical speciation events, but we cannot rule out the possibility of allopatric speciation, followed by iterative range extension and secondary overlap. Irrespective of ancestral range distributions, our results implicate a central role for reproductive character differentiation in glaucinin speciation—a novel result in a planktonic system

Trends in the sand: directional evolution in the shell shape of recessing scallops (Bivalvia: Pectinidae)

Directional evolution is one of the most compelling evolutionary patterns observed in macroevolution. Yet, despite its importance, detecting such trends in multivariate data remains a challenge. In this study, we evaluate multivariate evolution of shell shape in 93 bivalved scallop species, combining geometric morphometrics and phylogenetic comparative methods. Phylomorphospace visualization described the history of morphological diversification in the group; revealing that taxa with a recessing life habit were the most distinctive in shell shape, and appeared to display a directional trend. To evaluate this hypothesis empirically, we extended existing methods by characterizing the mean directional evolution in phylomorphospace for recessing scallops. We then compared this pattern to what was expected under several alternative evolutionary scenarios using phylogenetic simulations. The observed pattern did not fall within the distribution obtained under multivariate Brownian motion, enabling us to reject this evolutionary scenario. By contrast, the observed pattern was more similar to, and fell within, the distribution obtained from simulations using Brownian motion combined with a directional trend. Thus, the observed data are consistent with a pattern of directional evolution for this lineage of recessing scallops. We discuss this putative directional evolutionary trend in terms of its potential adaptive role in exploiting novel habitats

Molecular phylogeny of the Pectinoidea (Bivalvia) indicates Propeamussiidae to be a non-monophyletic family with one clade sister to the scallops (Pectinidae)

Scallops (Pectinidae) are one of the most diverse families of bivalves and have been a model system in evolutionary biology. However, in order to understand phenotypic evolution, the Pectinidae needs to be placed in a deeper phylogenetic framework within the superfamily Pectinoidea. We reconstructed a molecular phylogeny for 60 species from four of the five extant families within the Pectinoidea using a five gene dataset (12S, 16S, 18S, 28S rRNAs and histone H3). Our analyses give consistent support for the non-monophyly of the Propeamussiidae, with a subset of species as the sister group to the Pectinidae, the Propeamussiidae type species as sister to the Spondylidae, and the majority of propeamussiid taxa sister to the Spondylidae + Pr. dalli. This topology represents a previously undescribed relationship of pectinoidean families. Our results suggest a single origin for eyes within the superfamily and likely multiple instances of loss for these characters. However, it is now evident that reconstructing the evolutionary relationships of Pectinoidea will require a more comprehensive taxonomic sampling of the Propeamussiidae sensu lato

Convergent and parallel evolution in life habit of the scallops (Bivalvia: Pectinidae)

<p>Abstract</p> <p>Background</p> <p>We employed a phylogenetic framework to identify patterns of life habit evolution in the marine bivalve family Pectinidae. Specifically, we examined the number of independent origins of each life habit and distinguished between convergent and parallel trajectories of life habit evolution using ancestral state estimation. We also investigated whether ancestral character states influence the frequency or type of evolutionary trajectories.</p> <p>Results</p> <p>We determined that temporary attachment to substrata by byssal threads is the most likely ancestral condition for the Pectinidae, with subsequent transitions to the five remaining habit types. Nearly all transitions between life habit classes were repeated in our phylogeny and the majority of these transitions were the result of parallel evolution from byssate ancestors. Convergent evolution also occurred within the Pectinidae and produced two additional gliding clades and two recessing lineages. Furthermore, our analysis indicates that byssal attaching gave rise to significantly more of the transitions than any other life habit and that the cementing and nestling classes are only represented as evolutionary outcomes in our phylogeny, never as progenitor states.</p> <p>Conclusions</p> <p>Collectively, our results illustrate that both convergence and parallelism generated repeated life habit states in the scallops. Bias in the types of habit transitions observed may indicate constraints due to physical or ontogenetic limitations of particular phenotypes.</p

Convergent evolution of life habit and shell shape in scallops (Bivalvia: Pectinidae) with a description of a new genus

Phenotypic convergence is a fascinating evolutionary pattern. Many taxa evolve remarkable similarities, often due to similar selection pressures, which suggest that there may be a limited number of solutions to a particular ecological challenge. However, some phenotypes are only superficially similar. For example, convergent evolution may occur at one phenotypic level such as behavior, but other components (morphology) of the same phenotype exhibit divergence. In other words, the solutions might be the same, but how taxa approach the problem could be very different. This then suggests that there may be multiple phenotypic optima that can tackle similar ecological challenges. In this dissertation, I investigate the evolutionary patterns of life habits and shell shapes in scallops and how these phenotypic traits contribute to biological diversity. I found many scallop species have converged in life habit, but only a few lineages converged in shell shape. Rather, shell shape variation tends to be greater in species with less specific life habit requirements. However, scallop species with the gliding life habit have lesser shell shape variation. Interestingly, gliding can be performed by two distinct shell shapes, indicating that only some components of shell shape are important for the life habit, and others are free to vary. This research indicates that there is likely greater diversity in form, despite phenotypic similarities in function.</p

Convergent and parallel evolution in life habit of the scallops (Bivalvia: Pectinidae)

Background: We employed a phylogenetic framework to identify patterns of life habit evolution in the marine bivalve family Pectinidae. Specifically, we examined the number of independent origins of each life habit and distinguished between convergent and parallel trajectories of life habit evolution using ancestral state estimation. We also investigated whether ancestral character states influence the frequency or type of evolutionary trajectories. Results: We determined that temporary attachment to substrata by byssal threads is the most likely ancestral condition for the Pectinidae, with subsequent transitions to the five remaining habit types. Nearly all transitions between life habit classes were repeated in our phylogeny and the majority of these transitions were the result of parallel evolution from byssate ancestors. Convergent evolution also occurred within the Pectinidae and produced two additional gliding clades and two recessing lineages. Furthermore, our analysis indicates that byssal attaching gave rise to significantly more of the transitions than any other life habit and that the cementing and nestling classes are only represented as evolutionary outcomes in our phylogeny, never as progenitor states. Conclusions: Collectively, our results illustrate that both convergence and parallelism generated repeated life habit states in the scallops. Bias in the types of habit transitions observed may indicate constraints due to physical or ontogenetic limitations of particular phenotypes.This article is from BMC Evolutionary Biology 11 (2011): 164, doi:10.1186/1471-2148-11-164. Posted with permission.</p