We performed a tree-based analysis of trilobite postembryonic development in a sample of 60 species for which quantitative data on segmentation and growth increments between putative successive instars are available, and that spans much of the temporal, phylogenetic, and habitat range of the group. Three developmental traits were investigated: the developmental mode of trunk segmentation, the average per-molt growth rate, and the conformity to a constant per-molt growth rate (Dyar's rule), for which an original metric was devised. Growth rates are within the normal range with respect to other arthropods and show overall conformity to Dyar's rule. Randomization tests indicate statistically significant phylogenetic signal for growth in early juveniles but not in later stages. Among five evolutionary models fit via maximum likelihood, one in which growth rates vary independently among species, analogous to Brownian motion on a star phylogeny, is the best supported in all ontogenetic stages, although a model with a single, stationary peak to which growth rates are attracted also garners nontrivial support. These results are not consistent with unbounded, Brownian-motion-like evolutionary dynamics, but instead suggest the influence of an adaptive zone. Our results suggest that developmental traits in trilobites were relatively labile during evolutionary history.
Abstract Aim Little is known about how marine biodiversity responds to oceanographic and climatic changes over the decadal to centennial time‐scales which are most relevant for predicted climate changes due to greenhouse gas forcing. This paper aims to reveal decadal–centennial scale deep‐sea biodiversity dynamics for the last 20,000 years and then explore potential environmental drivers. Location The North A tlantic O cean. Methods We investigated deep‐sea benthic microfossil records to reveal biodiversity dynamics and subsequently applied comprehensive ecological modelling to test possible environmental factors (i.e. surface productivity, seasonality of productivity or deepwater circulation related to bottom‐water temperature) that may have influenced deep‐sea biodiversity over these time‐scales. Results Deep‐sea biodiversity changed synchronously with stadial–interstadial climate changes over the last 20,000 years across a large area of the N orth A tlantic in both ostracod crustaceans and foraminiferan protozoa (in spite of their different dispersal abilities). Species diversity rapidly increased during abrupt stadial events during the last deglacial and the H olocene interglacial periods. These include the well‐known H einrich 1, the Y ounger D ryas and the 8.2 ka events when the strength of A tlantic M eridional O verturning C irculation ( AMOC ) decreased. There is also evidence for quasi‐cyclic changes in biodiversity at a c . 1500‐year periodicity, consistent with the well‐known ‘1500‐year climatic cycle’. Statistical analyses revealed that AMOC variability (probably specifically the variability in AMOC ‐driven bottom‐water temperature) is correlated with deep‐sea biodiversity. Main conclusions Our finding of a significant AMOC –diversity relationship may indicate pervasive control of the diversity of deep‐sea benthic species by rapidly changing climate, specifically bottom‐water temperature, over decadal to centennial time‐scales. Our results, based on highly resolved fossil records, may portend pervasive, synchronous and sudden ecosystem responses to human‐induced changes to climate and ocean circulation in this century.
For almost 30 years, paleontologists have analyzed evolutionary sequences in terms of simple null models, most commonly random walks. Despite this long history, there has been little discussion of how model parameters may be estimated from real paleontological data. In this paper, I outline a likelihood-based framework for fitting and comparing models of phyletic evolution. Because of its usefulness and historical importance, I focus on a general form of the random walk model. The long-term dynamics of this model depend on just two parameters: the mean (μ step ) and variance (σ 2 step ) of the distribution of evolutionary transitions (or “steps”). The value of μ step determines the directionality of a sequence, and σ 2 step governs its volatility. Simulations show that these two parameters can be inferred reliably from paleontological data regardless of how completely the evolving lineage is sampled. In addition to random walk models, suitable modification of the likelihood function permits consideration of a wide range of alternative evolutionary models. Candidate evolutionary models may be compared on equal footing using information statistics such as the Akaike Information Criterion (AIC). Two extensions to this method are developed: modeling stasis as an evolutionary mode, and assessing the homogeneity of dynamics across multiple evolutionary sequences. Within this framework, I reanalyze two well-known published data sets: tooth measurements from the Eocene mammal Cantius , and shell shape in the planktonic foraminifera Contusotruncana . These analyses support previous interpretations about evolutionary mode in size and shape variables in Cantius , and confirm the significantly directional nature of shell shape evolution in Contusotruncana . In addition, this model-fitting approach leads to a further insight about the geographic structure of evolutionary change in this foraminiferan lineage.
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 time-series 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.
The nature of evolutionary changes recorded by the fossil record has long been controversial, with particular disagreement concerning the relative frequency of gradual change versus stasis within lineages. Here, I present a large-scale, statistical survey of evolutionary mode in fossil lineages. Over 250 sequences of evolving traits were fit by using maximum likelihood to three evolutionary models: directional change, random walk, and stasis. Evolution in these traits was rarely directional; in only 5% of fossil sequences was directional evolution the most strongly supported of the three modes of change. The remaining 95% of sequences were divided nearly equally between random walks and stasis. Variables related to body size were significantly less likely than shape traits to experience stasis. This finding is in accord with previous suggestions that size may be more evolutionarily labile than shape and is consistent with some but not all of the mechanisms proposed to explain evolutionary stasis. In general, similar evolutionary patterns are observed across other variables, such as clade membership and temporal resolution, but there is some evidence that directional change in planktonic organisms is more frequent than in benthic organisms. The rarity with which directional evolution was observed in this study corroborates a key claim of punctuated equilibria and suggests that truly directional evolution is infrequent or, perhaps more importantly, of short enough duration so as to rarely register in paleontological sampling.
Trait variation and covariation are understood to influence the response of populations to natural selection on generational time scales, but their role, if any, in shaping long-term macroevolutionary divergence is still unclear. The present study uses the rich fossil record of the ostracode genus Poseidonamicus to reconstruct in great detail the evolutionary history of a set of landmark-based morphometric characters. This reconstruction included two kinds of evolutionary inferences: ancestor-descendant transitions among populations repeatedly sampled at the same location and divergence between lineages measured as independent contrasts on a phylogeny. This reconstructed history was then used to test if evolutionary changes were concentrated in directions (traits or combinations of traits) with high phenotypic variance. Two different statistics of association between evolution and variation tested the null hypothesis that evolutionary changes occur in random directions with respect to trait variability. The first of these measured the similarity between the directions of evolutionary change and the axis of maximum variance, and the second measured the degree to which evolutionary changes were concentrated in directions of high phenotypic variation. Randomization tests indicated that both kinds of evolutionary inferences (ancestor-descendant and phylogenetic contrasts) occurred preferentially in directions of high phenotypic variance (and close to the axis of maximal variation), suggesting that within-population variation can structure long-term divergence. This effect decayed after a few million years, but at least for one metric, never disappeared completely. These results are consistent with Schluter's genetic constraints model in which evolutionary trajectories on adaptive landscapes are deflected by variation within and covariation among traits.
Evolutionary change during the interval in which a fossil sample accumulates will inflate the variance of that sample relative to the population-level standing variation. If this effect is widespread and severe, paleontological samples will not provide reliable estimates of population variation. Although the few published studies conducted to test this possibility have found similar levels of variation in samples differing greatly in temporal acuity, the paucity of case studies prevents assessing the generality of this pattern. In this paper, two independent, literature-based approaches are used to greatly expand the data available to address this issue. The first approach compares morphometric variability in Quaternary mammal samples with samples from related modern populations. The second approach artificially lumps separate samples from evolving lineages and calculates the variance effects of this analytical time-averaging. Both approaches yield consistent results indicating that variance observed in time-averaged samples is typically only slightly inflated (approximately 5%) relative to population-level values. This finding suggests that rates of evolution are typically slow when scaled to within-population variation, providing support for relative stasis as the dominant mode of within-lineage evolution. An important practical consequence of these findings is that time-averaged fossil samples generally show trait variances and covariances that are similar to population-level parameters, which has been an important but implicit assumption in many paleontological studies of phenotypic variation.
Patterns preserved in the fossil record are of the highest importance in addressing questions about long-term evolutionary processes, yet both the description of pattern and its translation into process can be difficult. With respect to gradual phyletic change, we know that randomly generated sequences may exhibit characteristics of a “trend” apparent patterns, therefore, must be interpreted with caution. Furthermore, even when the claim of a gradual trend can be statistically justified, interpretation of the underlying mechanisms may be challenging. Given that we can observe populations changing rapidly over tens or hundreds of years, it is now more difficult to explain instances of geologically gradual (as opposed to punctuated) change. Here we describe morphologic change in two bivalve lineages from the late Miocene Lake Pannon. We evaluate change according to the model-based methods of Hunt. Both lineages exhibit size increases and shape changes over an interval of nearly 4 million years. Size and two shape variables in the conjungens lineage are best fit by a model of directional evolution; remaining shape variables mostly conform to unbiased random walks. Body-size evolution in the diprosopum lineage is also significantly directional but all shape variables are best fit by the unbiased random walk model; the small number of sampling intervals available for this lineage ( n = 6) makes determination of the actual pattern more difficult. Model-fitting results indicate that the parallel trajectories of increasing log shell height over time in the two lineages can be accounted for by an underlying trend shared by both lineages, suggesting that the size increases may be a shared response to the same cause. The pace of phenotypic change, measured as Lynch's Δ, is slower than the neutral expectation for all size and shape traits. Our examples illustrate well the paradox of gradualism; the sequences exhibit significant directional morphological evolution, but rates of change as measured over the long-term are apparently too slow for directional selection or even drift to be the cause. Viewing long-term phenotypic evolution in terms of populations tracking peaks on adaptive landscapes is useful in this context. Such a view allows for intervals of directional selection (during times of peak movement–resulting in the overall trends we can detect) interspersed with intervals of stasis (during times of peak stability–resulting in overall changes that appear to proceed more slowly than the neutral expectation). The paradox of gradualism thus reduces to (1) peak movements and their drivers, which are not restricted in rate as are population-genetic drivers, and (2) the maintenance of stasis, on which no consensus exists. We can identify no environmental parameter in the central European Neogene that exhibits consistent change across the interval of gradual morphologic change. It may be that in Lake Pannon the long-term persistence of generally ameliorating conditions (plentiful resources and habitat space, few predators or competitors) resulted in geologically slow but consistent peak shifts, which in turn facilitated size increase and shape change in these lineages.
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