Morphological stasis in an ongoing gastropod radiation from Lake Malawi

Abstract

Evolutionary processes leading to adaptive radiation regularly occur too fast to be accurately recorded in the fossil record but too slowly to be readily observed in living biota. The study of evolutionary radiations is thereby confronted with an epistemological gap between the timescales and approaches used by neontologists and paleontologists. Here we report on an ongoing radiation of extant Bellamya species (n = 4) from the African Rift Lake Malawi that provides an unusual opportunity to bridge this gap. The substantial molecular differentiation in this monophyletic Bellamya clade has arisen since Late Pleistocene megadroughts in the Malawi Basin caused by climate change. Morphological timeseries analysis of a high-resolution, radiocarbon-dated sequence of 22 faunas spanning the Holocene documents stasis up to the middle Holocene in all traits studied (shell height, number of whorls, and two variables obtained from geometric morphometrics). Between deposition of the last fossil fauna (∼5 ka) and the present day, a drastic increase in morphological disparity was observed (3.7-5.8 times) associated with an increase in species diversity. Comparison of the rates of morphological evolution obtained from the paleontological time-series with phylogenetic rates indicates that the divergence in two traits could be reconstructed with the slow rates documented in the fossils, that one trait required a rate reduction (stabilizing selection), and the other faster rates (divergent selection). The combined paleontological and comparative approach taken here allows recognition that morphological stasis can be the dominant evolutionary pattern within species lineages, even in very young and radiating clades.speciation | rates of evolution | punctuated equilibrium | Viviparidae E volutionary radiations are potentially responsible for much of the ecological and phenotypic diversity on earth (1, 2). Beyond rapid speciation through lineage splitting, radiations are often characterized by exceptional diversification into a variety of ecological niches, with divergence, at least in traits under selection, regularly occurring very rapidly (e.g., refs. 3 and 4). However, the controls on diversification may be complex and include historical contingencies, as well as ecological, genetic, and developmental factors (5, 6). One major challenge is that processes leading to evolutionary radiations occur regularly too fast to be accurately recorded in the fossil record where temporal resolutions finer than ∼10 4 y are unusual, but perhaps still too crude to see speciation and radiation unfold. On the other hand, speciation generally occurs too slowly to be readily observed in living biota, resulting in an "epistemological gap" between the timescales and approaches used by neontologists and those that paleontologists adopt for the study of organismal evolution (7,8). Moreover, it can be difficult to assess the adaptive significance of phenotypic variation among fossil taxa (e.g., ref. 9), partly because ecological, behavioral, physiological, ...

Tempo does not correlate with mode in the fossil record

“…Given a literal reading of the fossil record constraining all four extant species to have diverged from the fossil lineage during the last 5500 years, and assuming an equal ability to evolve during the radiation as observed in the fossil time series, seven out of eight trait states in the four extant taxa are easily reachable from the fossil trait states within the 5500 years that separates the fossil lineage and the extant taxa: All four extant taxa are less than 4.5 log units different in number of whorls compared to the mean in the fossil lineage, while three out of four extant taxa are less than 2.0 log units different in height compared to the mean in the fossil lineage (see Fig. in Van Bocxlaer and Hunt ). Speciation seems unlikely to be a necessary driver of high rates of evolutionary change.…”

Role of ancient lakes in genetic and phenotypic diversification of freshwater snails

“…Various evolutionary phenomena such as adaptive radiation, parallel evolution, and hybrid speciation have been documented in some ancient lakes (e.g., Seehausen, ; Sturmbauer & Meyer, ). In these circumstances, mollusks are an excellent model to compare the relationships among morphology, phylogeny, and geographical distribution patterns (Glaubrecht, ; Miura, Urabe, Nishimura, Nakai, & Chiba, ; von Rintelen, Wilson, Meyer, & Glaubrecht, ; Van Bocxlaer, ; Van Bocxlaer & Hunt, ). Because mollusks are less mobile they are more likely to become geographically isolated (Miura et al, ).…”

Novel polymorphic microsatellite markers for Bellamya and their application in population genetics of three species

“…These snails are also reported to be widespread invasive species in North American and African lakes (Sengupta et al, 2009;Solomon et al, 2010). Because Bellamya is a species-rich genus of primarily sessile freshwater snails that is widely distributed across Southeast Asia, India and Africa, it is a suitable model system for the study of population genetics and biological evolution (Sengupta et al, 2009;Schultheiβ et al, 2011;Van Bocxlaer and Hunt, 2013).…”