Extant mammal body masses suggest punctuated equilibrium

Is gradual microevolutionary change within species simultaneously the source of macroevolutionary differentiation between species? Since its first publication, Darwin''s original idea that phenotypic differences between species develop gradually over time, as the accumulation of small selection-induced changes in successive generations has been challenged by palaeontologists claiming that, instead, new species quickly acquire their phenotypes to remain virtually unchanged until going extinct again. This controversy, widely known as the ‘punctuated equilibrium’ debate, remained unresolved, largely owing to the difficulty of distinguishing biological species from fossil remains. We analysed body masses of 2143 existing mammal species on a phylogeny comprising 4510 (i.e. nearly all) extant species to estimate rates of gradual (anagenetic) and speciational (cladogenetic) evolution. Our Bayesian estimates from mammals as well as separate sub-clades such as primates and carnivores suggest that gradual evolution is responsible for only a small part of body size variation between mammal species.     

Ribosomal RNA Sequencing of Members of the Crypthecodinium cohnii (Dinophyceae) Species Complex; Comparison with Soluble Enzyme Studies

ABSTRACT. Sixty-five members of the Ctypthecodinium cohnii species complex were analyzed for sequence differences within the D2 region of the 23S ribosomal RNA molecule. On the basis of 46 sequence differences the strains fell into 19 distinct ribosets (strains of identical sequence), some with many members. Members of four of the seven major sibling species (widespread breeding groups) were each found within single ribosets. Members of three other major sibling species were each, however, divided into two ribosets by a single sequence difference correlated with geographic separation and with previously reported electrophoretic polymorphisms of soluble enzymes within the sibling species. In addition to members of major sibling species, some ribosets include many minor sibling species (each represented by only one strain). Of 38 minor sibling species, 22 shared sequence with a major sibling species. Of these 22, 14 were identical in soluble enzymes to their related major sibling species or differed by only one of three enzymes. Other minor sibling species appear to have diverged extensively from any others in both rRNA sequence and electrophoretic profile. As a group, major sibling species differ markedly in the number of minor sibling species associated with them, suggesting differences in frequency of sexually isolating events in their past histories. These findings are discussed in the context of the previously proposed model of sympatric speciation.     

Sequential colonization and diversification of Galapágos endemic land snail genus Bulimulus (Gastropoda, Stylommatophora)

Species richness on island or islandlike systems is a function of colonization, within-island speciation, and extinction. Here we evaluate the relative importance of the first two of these processes as a function of the biogeographical and ecological attributes of islands using the Galápagos endemic land snails of the genus Bulimulus, the most species-rich radiation of these islands. Species in this clade have colonized almost all major islands and are found in five of the six described vegetation zones. We use molecular phylogenetics (based on COI and ITS 1 sequence data) to infer the diversification patterns of extant species of Bulimulus, and multiple regression to investigate the causes of variation among islands in species richness. Maximum-likelihood, Bayesian, and maximum-parsimony analyses yield well-resolved trees with similar topologies. The phylogeny obtained supports the progression rule hypothesis, with species found on older emerged islands connecting at deeper nodes. For all but two island species assemblages we find support for only one or two colonization events, indicating that within-island speciation has an important role in the formation of species on these islands. Even though speciation through colonization is not common, island insularity (distance to nearest major island) is a significant predictor of species richness resulting from interisland colonization alone. However, island insularity has no effect on the overall bulimulid species richness per island. Habitat diversity (measured as plant species diversity), island elevation, and island area, all of which are indirect measures of niche space, are strong predictors of overall bulimulid land snail species richness. Island age is also an important independent predictor of overall species richness, with older islands harboring more species than younger islands. Taken together, our results demonstrate that the diversification of Galápagos bulimulid land snails has been driven by a combination of geographic factors (island age, size, and location), which affect colonization patterns, and ecological factors, such as plant species diversity, that foster within-island speciation.     

Patterns of speciation in two sibling species of Graomys (Rodentia,Cricetidae) based on mtDNA sequences

To increase our understanding of the speciation process occurred in the sibling species Graomys griseoflavus and Graomys centralis, a phylogeographic study was conducted based on sequences of a hypervariable segment of the mtDNA D‐loop region. The resulting haplotype phylogenetic network showed two well‐defined clusters, one for each species. The clusters were connected by two haplotypes from localities that are almost 300 km apart, one situated in the Monte eco‐region and the other, in the Chaco. This result is in agreement with a previous hypothesis about the geographical context in which the cladogenetic process occurred. A divergence time of 0.15–0.28 million years was estimated, which is consistent with a process of recent speciation. An amova test confirmed that at present gene flow between species does not exist. The mismatch distribution analyses suggest that the geographical and demographic expansion undergone by the species is related to the climatic events that occurred in the region during the Quaternary.     

Multi-species outcomes in a common model of sympatric speciation

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.     

Patterns of evolution in Graomys griseoflavus (Rodentia,Muridae). IV. A case of rapid speciation

The South American group of rodents known as Graomys griseoflavus comprises two sibling species differing only in diploid chromosomal complement: G. griseoflavus (2n = 36, 37 and 38) and G. centralis (2n=42). Reproductive barriers comprising postzygotic as well as precopulatory mechanisms prevent gene exchange between these species. We have studied genetic polymorphism in two populations of G. centralis and four of G. griseoflavus by means of gel electrophoresis of enzymes and other proteins giving information on a total of 30 loci. Values of interspecific genetic identity were similar to those obtained for conspecific populations, suggesting that fixation of Robertsonian fusions would have occurred without significant bottlenecks. It would also indicate that the cladogenetic process must be relatively recent. F_IS values showed no evidence of inbreeding. Fixation indices (F_ST) for the ancestral species showed a tendency to form demes with very low gene flow among them, while in the derived species such tendency was not shown. However, because of the characteristics of the region they occupy, populations are of moderate size, and genic flow is low. Lack of correlation between gene flow levels and geographical distance between population pairs would indicate a recent and fast colonization of its distribution areas by the derived species. It is possible that fixation of Robertsonian fusions occurred in a marginal deme of the ancestral species, e.g. in a parapatric geographical context.