While models of sympatric speciation are motivated in part by multi-species adaptive radiations such as the Cameroon crater lake cichlids, existing models have focused on bifurcation into a single ...pair of daughter species. This paper shows that a familiar model of sympatric speciation, driven by intraspecific competition and assortative mating based on ecological characters values, can yield multiple daughter species if individual niche widths are sufficiently restricted. Surprisingly, the multi-species outcome is not produced by successive bifurcation events, but by simultaneous divergence resulting in a hard polytomy. This result is sensitive to a number of assumptions, whose violation may prevent speciation. In some cases when speciation fails, the population instead ends in a state that closely resembles incipient species pairs, with an ecological polymorphism and partial reproductive isolation. However, this polymorphism is stable and does not lead to complete reproductive isolation, suggesting that empirical cases of incipient species pairs may not always end in speciation.
We introduce the concept of many-to-one mapping of form to function and suggest that this emergent property of complex systems promotes the evolution of physiological diversity. Our work has focused ...on a 4-bar linkage found in labrid fish jaws that transmits muscular force and motion from the lower jaw to skeletal elements in the upper jaws. Many different 4-bar shapes produce the same amount of output rotation in the upper jaw per degree of lower jaw rotation, a mechanical property termed Maxillary KT. We illustrate three consequences of many-to-one mapping of 4-bar shape to Maxillary KT. First, many-to-one mapping can partially decouple morphological and mechanical diversity within clades. We found with simulations of 4-bars evolving on phylogenies of 500 taxa that morphological and mechanical diversity were only loosely correlated (R2 = 0.25). Second, redundant mapping permits the simultaneous optimization of more than one mechanical property of the 4-bar. Labrid fishes have capitalized on this flexibility, as illustrated by several species that have Maxillary KT = 0.8 but have different values of a second property, Nasal KT. Finally, many-to-one mapping may increase the influence of historical factors in determining the evolution of morphology. Using a genetic model of 4-bar evolution we exerted convergent selection on three different starting 4-bar shapes and found that mechanical convergence only created morphological convergence in simulations where the starting forms were similar. Many-to-one mapping is widespread in physiological systems and operates at levels ranging from the redundant mapping of genotypes to phenotypes, up to the morphological basis of whole-organism performance. This phenomenon may be involved in the uneven distribution of functional diversity seen among animal lineages.
While it now appears likely that sympatric speciation is possible, its generality remains contentious. If it really is rare, then most natural populations must not fit the assumptions of sympatric ...speciation theory. A better understanding of these assumptions may help identify when sympatric speciation is or is not likely. This paper investigates two such assumptions: that genetic variation for stringent assortative mating is not limiting and that females are not penalized for mating assortatively. Simulations demonstrate that the speed of sympatric speciation is very sensitive to the population's capacity for stringent assortative mating and is potentially extremely slow. The rapid divergence often thought to be a hallmark of sympatric speciation may only occur in a restricted area of parameter space.
Optimal foraging theory predicts that individuals should become more opportunistic when intraspecific competition is high and preferred resources are scarce. This density-dependent diet shift should ...result in increased diet breadth for individuals as they add previously unused prey to their repertoire. As a result, the niche breadth of the population as a whole should increase. In a recent study, R. Svanbäck and D. I. Bolnick confirmed that intraspecific competition led to increased population diet breadth in threespine stickleback (Gasterosteus aculeatus). However, individual diet breadth did not expand as resource levels declined. Here, we present a new method based on complex network theory that moves beyond a simple measure of diet breadth, and we use the method to reexamine the stickleback experiment. This method reveals that the population as a whole added new types of prey as stickleback density was increased. However, whereas foraging theory predicts that niche expansion is achieved by individuals accepting new prey in addition to previously preferred prey, we found that a subset of individuals ceased to use their previously preferred prey, even though other members of their population continued to specialize on the original prey types. As a result, populations were subdivided into groups of ecologically similar individuals, with diet variation among groups reflecting phenotype-dependent changes in foraging behavior as prey density declined. These results are consistent with foraging theory if we assume that quantitative trait variation among consumers affects prey preferences, and if cognitive constraints prevent individuals from continuing to use their formerly preferred prey while adding new prey.
Molecular clock methods allow biologists to estimate divergence times, which in turn play an important role in comparative studies of many evolutionary processes. It is well known that molecular age ...estimates can be biased by heterogeneity in rates of molecular evolution, but less attention has been paid to the issue of potentially erroneous fossil calibrations. In this study we estimate the timing of diversification in Centrarchidae, an endemic major lineage of the diverse North American freshwater fish fauna, through a new approach to fossil calibration and molecular evolutionary model selection. Given a completely resolved multi‐gene molecular phylogeny and a set of multiple fossil‐inferred age estimates, we tested for potentially erroneous fossil calibrations using a recently developed fossil cross‐validation. We also used fossil information to guide the selection of the optimal molecular evolutionary model with a new fossil jackknife method in a fossil‐based model cross‐validation. The centrarchid phylogeny resulted from a mixed‐model Bayesian strategy that included 14 separate data partitions sampled from three mtDNA and four nuclear genes. Ten of the 31 interspecific nodes in the centrarchid phylogeny were assigned a minimal age estimate from the centrarchid fossil record. Our analyses identified four fossil dates that were inconsistent with the other fossils, and we removed them from the molecular dating analysis. Using fossil‐based model cross‐validation to determine the optimal smoothing value in penalized likelihood analysis, and six mutually consistent fossil calibrations, the age of the most recent common ancestor of Centrarchidae was 33.59 million years ago (mya). Penalized likelihood analyses of individual data partitions all converged on a very similar age estimate for this node, indicating that rate heterogeneity among data partitions is not confounding our analyses. These results place the origin of the centrarchid radiation at a time of major faunal turnover as the fossil record indicates that the most diverse lineages of the North American freshwater fish fauna originated at the Eocene‐Oligocene boundary, approximately 34 mya. This time coincided with major global climate change from warm to cool temperatures and a signature of elevated lineage extinction and origination in the fossil record across the tree of life. Our analyses demonstrate the utility of fossil cross‐validation to critically assess individual fossil calibration points, providing the ability to discriminate between consistent and inconsistent fossil age estimates that are used for calibrating molecular phylogenies.
Objective To evaluate the effect of infertility and intracytoplasmic sperm injection (ICSI) on DNA methylation of offspring. Design Microarray analysis of DNA methylation in archived neonatal ...bloodspots of in vitro fertilization (IVF)/ICSI-conceived children compared with controls born to fertile and infertile parents. Setting Academic research laboratory. Patient(s) : Neonatal blood spots of 137 newborns conceived spontaneously, through intrauterine insemination (IUI), or through ICSI using fresh or cryopreserved (frozen) embryo transfer. Intervention(s) None. Main Outcome Measure(s) The Illumina Infinium HumanMethylation450k BeadChip assay determined genome-wide DNA methylation. Methylation differences between conception groups were detected using a Bioconductor package, ChAMP, in conjunction with Adjacent Site Clustering (A-clustering). Result(s) The methylation profiles of assisted reproductive technology and IUI newborns were dramatically different from those of naturally (in vivo) conceived newborns. Interestingly, the profiles of ICSI-frozen (FET) and IUI infants were strikingly similar, suggesting that cryopreservation may temper some of the epigenetic aberrations induced by IVF or ICSI. The DNA methylation changes associated with IVF/ICSI culture conditions and/or parental infertility were detected at metastable epialleles, suggesting a lasting impact on a child's epigenome. Conclusion(s) Both infertility and ICSI alter DNA methylation at specific genomic loci, an effect that is mitigated to some extent by FET. The impact of assisted reproductive technology and/or fertility status on metastable epialleles in humans was uncovered. This study provides an expanded set of loci for future investigations on IVF populations.
Predation has long influenced how ecologists think about ecological processes from the small to the large. At the microscale, predation-related stress can yield permanent changes in prey physiology ...or morphology; at the macroscale, altered predation regimes in grazer-dominated systems can profoundly alter ecosystem function.
Like many phenotypic traits, biomechanical systems are defined by both an underlying morphology and an emergent functional property. The relationship between these levels may have a profound impact ...on how selection for functional performance is translated into morphological evolution. In particular, complex mechanical systems are likely to be highly redundant, because many alternative morphologies yield equivalent functions. We suggest that this redundancy weakens the relationship between morphological and functional diversity, and we illustrate this effect using an evolutionary model of the four‐bar lever system of labrid fishes. Our results demonstrate that, when traits are complex, the morphological diversity of a clade may only weakly predict its mechanical diversity. Furthermore, parallel or convergent selection on function does not necessarily produce convergence in morphology. Empirical observations suggest that this weak form‐function relationship has contributed to the morphological diversity of labrid fishes, as functionally equivalent species may nevertheless possess morphologically distinct jaws. We suggest that partial decoupling of morphology and mechanics due to redundancy is a major factor in morphological diversification.
Background: Many theoretical models of speciation and niche evolution assume that the ecological similarity between conspecific individuals depends on their phenotypic similarity. Thus, competition ...between individuals is expected to depend on their phenotypic similarity. Theoretical models often assume that this intraspecific competition function is Gaussian. Questions: Are morphological similarity and diet similarity positively correlated? If so, is this relationship non-linear? Data: Stomach contents, stable isotope ratios (d super(13)C and d super(15)N), and trophic morphology (standard length, gape width, body width, gill raker number, arid gill raker length) for 265 threespine sticklebacks (Gasterosteus aculeatus) from a single population from a lake in British Columbia. Analysis: We calculated the diet similarity and morphological similarity between all pairs of individuals in our sample. We examined the correlation between diet and morphological similarity, and tested whether the relationship exhibits any non-linearity. Conclusions: Similarity in trophic morphology is correlated with dietary similarity between individuals. However, both body size and trophic morphology explained remarkably small percentages of the variance in diet overlap. Also, we found no evidence of curvature in the intraspecific competition function.
Question: How do post-glacial colonization history and current watershed geomorphology affect population genetic structure in lacustrine fish populations? Study system: Eleven populations of ...lacustrine threespine sticklebacks (Gasterosteus aculeatus) from a single watershed, and two adjacent marine populations of G. aculeatus as outgroups. Methods: Individuals were genotyped using six microsatellite loci and several population genetic parameters were calculated (including genetic differentiation, genetic diversity, migration, and divergence time). We regressed population genetic parameters against environmental variables such as stream gradient and length, distance from the mouth of the watershed, and lake area. Results: Genetic differentiation was highest between lakes separated by high-gradient streams, but was unaffected by stream length. Estimated divergence times between adjoining populations declined from >10,000 years ago for lake pairs close to the ocean to less than 5000 years for lake pairs near the top of the watershed. Genetic diversity declined as a function of geographic distance from the mouth of the watershed but was positively correlated with lake area. Conclusions: Our findings confirm expectations that landscape features such as stream gradient and stream branching structure strongly influence patterns of genetic divergence. However, in contrast to previous studies, we found no significant relationship between geographic distance and genetic divergence among lake pairs. We present novel evidence that post-glacial colonization occurred gradually over a significant period of time, rather than a single rapid invasion into all lakes.
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