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"Allmon, Warren D"
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How we study cryptic species and their biological implications: A case study from marine shelled gastropods
Methodological and biological considerations are intertwined when studying cryptic species. A potentially large component of modern biodiversity, the frequency of cryptic species among taxonomic groups is not well documented. The term “cryptic species” is imprecisely used in scientific literature, causing ambiguity when interpreting their evolutionary and ecological significance. This study reviews how cryptic species have been defined, discussing implications for taxonomy and biology, and explores these implications with a case study based on recently published literature on extant shelled marine gastropods. Reviewed gastropods were recorded by species. Records of cryptic gastropods were presented by authors with variable levels of confidence but were difficult to disentangle from inherent biases in the study effort. These complexities notwithstanding, most gastropod species discussed were not cryptic. To the degree that this review's sample represents extinct taxa, the results suggest that a high proportion of shelled marine gastropod species are identifiable for study in the fossil record. Much additional work is needed to provide a more adequate understanding of the relative frequency of cryptic species in shelled marine gastropods, which should start with more explicit definitions and targeted case studies. Methodological and biological considerations are inseparable when studying cryptic species. Using shelled marine gastropods, we review how cryptic species have been defined, reported, and the potential implications of cryptic species occurrences for taxonomy and evolution. We find most gastropod species are considered not cryptic, suggesting many species can be confidently identified and studied for living and fossil taxa.
Journal Article
Body-size evolution in gastropods across the Plio-Pleistocene extinction in the western Atlantic
The Plio-Pleistocene turnover event in the western Atlantic following the closure of the Central American Seaway involved high rates of extinction for both gastropod and bivalve molluscs. This extinction was associated with declining nutrient conditions and has been presumed to be associated with a decrease in molluscan body size. Previous work which has been concordant with this expectation, however, has either focused on bivalves or not considered the effects of the recovery post extinction. In three phylogenetically diverse clades, we found that body-size evolution in gastropods across the turnover event is likely tied to ecology. One clade increased in size, one decreased, and another exhibited no substantial change. Individual species lineages exhibit a mixture of microevolutionary changes from the Pliocene to today. This study indicates that gastropod body-size evolution may be more complex than in bivalves, with ecology and other functional traits playing a significant role. Macroevolutionary processes, especially whether a clade re-radiated post extinction, were found to be important. Indeed, a low portion of extant diversity consists of survivors from clades that increased in size or have similar size distributions among their species relative to the Pliocene.
Journal Article
Patterns and processes in the history of body size in turritelline gastropods, Jurassic to Recent
Body size is an important trait with implications for energy use and ecology as well as generation time and evolutionary rates. Turritelline gastropods are widely distributed through geologic time and space, making them an excellent group for evaluating macroevolutionary patterns. To evaluate the pattern of body-size change in turritelline gastropods, we compiled a dataset of shell lengths of 316 species of turritelline gastropods spanning the Jurassic to Recent. Type specimens were almost always significantly larger than specimen distributions from the same species. We found that turritelline gastropod size was inversely correlated with latitude, a trend likely driven by the Neogene–Recent diversification of small-bodied Southern Hemisphere taxa. A time series model was applied to distinguish among three possible macroevolutionary patterns: unbiased random walk (no directional trend), biased random walk (directional trend), and stasis (no net change). We determined that turritelline gastropods have experienced stasis in body size throughout their evolutionary history, adding to the growing literature documenting directionless body-size trends in marine invertebrate clades. Stasis of geographically widespread clades may be the result of ecological variability across the environmental range occupied by the group or differential diversification into opposing environments. Turritelline life-history patterns, especially their reproductive strategy that combines a short life span and decline in growth rate around 1 year of age to reallocate energy to reproduction, might circumvent selection for longevity and larger size, while further decrease in minimum size is likely limited by feeding efficiency and anti-predatory defense. The expectation that species or clades should continue to evolve to occupy larger size classes conflicts with the evolutionary advantages of small size, which in turritelline gastropods include high generational turnover and larger population sizes that yield opportunities for genetic variance.
Journal Article
Cretaceous marine nutrients, greenhouse carbonates, and the abundance of turritelline gastropods
Modern marine carbonate sediments accumulate where carbonate-producing organisms are abundant and siliciclastic input is low. Such accumulations occur today in two main environments and may be characterized as warm, low-nutrient (WLN; also known as tropical or chlorozoan) carbonates or cool, high-nutrient (CHN; also known as cool-water, temperate, nontropical, foramol, or heterozoan) carbonates. Few carbonates form today in warm, high-nutrient (WHN) conditions because such environments are very rare. Modern WLN environments occur mainly in low latitudes, where nutrients from upwelling or terrestrial runoff are low. Modern CHN environments occur mainly in middle to high latitudes, where nutrients, mostly from upwelling, are abundant. However, WHN depositional environments may have been widespread during the Cretaceous and Paleogene. During these times, upwelled nutrient-rich waters would, in general, have been warmer than they are today. Nutrients in these waters supported diverse carbonate-producing biotas that are not equivalent to either modern chlorozoan or heterozoan biotas. These conclusions derive from analysis of a growing data set of more than 40 turritelline gastropod-dominated fossil assemblages (TDAs), a term herein redefined, ranging in age from Lower Cretaceous to Pleistocene. Today TDAs occur mainly in CHN conditions. They were widespread in both carbonate and siliciclastic facies in the Cretaceous and Paleogene but (with only a single known exception in the Pliocene of Chile) occur in the Neogene only in siliciclastic sediments. This change in environmental distribution can be explained by positing changes in (1) modal environmental preferences of turritellines, from WHN to CHN, and (2) carbonate depositional environments, from WLN+WHN to WLN+CHN. Both changes were roughly coincident with the global change from \"greenhouse\" to \"icehouse\" conditions in the Oligocene.
Journal Article
Invertebrate Paleontology and Evolutionary Thinking in the US and Britain, 1860–1940
The role of paleontology in evolutionary biology between the publication of
The Origin of Species
in 1859 and the Evolutionary Synthesis of the 1940s (the post-Darwin, pre-Synthesis [PDPS] period) is frequently described as mostly misguided failure. However, a significant number of American and British PDPS invertebrate paleontologists of this period did devote considerable attention to evolution, and their evolutionary theories and conclusions were a good deal more diverse and nuanced than previous histories have suggested. This paper brings into focus a number of important but underrecognized aspects of the history of paleontology within the history of biology, including that PDPS paleontologists were not all as theoretically backward as they have been portrayed; that the post-Synthesis narrative of the history of evolution should be continually reevaluated, in part to decouple historical understanding from the agendas of authors who have used history to advance particular views of evolution; and that there is a much richer story to be told about the history of evolutionary biology in both the pre- and post-Synthesis eras.
Journal Article
Evolutionary remnants as widely accessible evidence for evolution: the structure of the argument for application to evolution education
Evolution education, in both schools and informal education, often focuses on natural selection and the fit of organisms through natural selection to their environment and way of life. Examples of evidence that evolution has occurred are therefore often limited to a modest number of classic but exotic cases, with little attention to how one might apply principles to more familiar organisms. Many of these classic examples are examples of adaptation; adaptation to local environments is, however, an outcome that could in principle also be explained by supernatural creation or design. A frequent result is the perception among the public is that examples of evolution are rare, and that the existence of well-adapted organisms may just as easily be explained metaphysically. We argue that among categories of evidence of evolution accessible to non-specialists in any environment, the most compelling evidence of common ancestry consists of remnants of evolutionary history evident in homologous features, particularly when those homologies are related to lack of fit of organisms to their way of life (“vestiges”) or to better fit that involves complicated combinations of parts usually assigned other functions (“contrivances”). Darwin emphasized the critical nature of this argument from imperfections, and it has been part of traditional catalogs of “evidence for evolution” for more than a century. Yet while remnants of history are widely used as a category of evidence for evolution, their utility in education of comparative anatomy to document body parts passed on through descent is underemphasized in evolution education at all levels. We explore the use of evolutionary remnants to document common ancestry and evidence for evolution, for application to evolution education.
Journal Article
Review and revision of the Olivoidea (Neogastropoda) from the Paleocene and Eocene of the U.S. Gulf Coastal Plain
Numerous species of “oliviform” gastropods have been recognized in the Paleogene of the U.S. Gulf Coastal Plain, many of which have previously been allied to the “Bullia group” in the family Nassariidae, and placed in a variety of poorly defined genera. We review these species, revise their generic and familial placement, and present a phylogenetic analysis. Of 19 species considered valid, all are assigned to Olivoidea, six to Olividae—one to Oliva, five to Agaronia—and the rest to Ancillariidae. The highly variable species Ancillaria altile Conrad is referred in the genus Ancillopsis and appears to have evolved anagenetically over an interval of perhaps 20 million years. Ancillaria tenera Conrad and Ancillaria scamba Conrad are placed in the new genus Palmoliva. Monoptygma Lea is demonstrated to belong to Ancillariidae, and to contain only a single species. Specimens assigned to Lisbonia expansa Palmer are split into adults assigned to Ancillopsis altilis and juveniles (together with several other species) in the long-lived species Anbullina elliptica (Whitfield). Coastal Plain ancillariids may have evolved from one or more species of the Cretaceous–Paleocene genus Eoancilla. We agree with previous authors who have suggested that the late Eocene species Oliva mississippiensis Conrad is the earliest known representative of this genus and the subfamily Oliviinae, perhaps derived from a species of Agaronia. The oldest Agaronia is lower Eocene (Ypresian). UUID: http://zoobank.org/b7d9f79b-c68b-4385-aba3-bb07c6d6dc87
Journal Article