Parallel ecological diversification in Antarctic notothenioid fishes as evidence for adaptive radiation

Antarctic notothenioid fishes represent a rare example of a marine species flock. They evolved special adaptations to the extreme environment of the Southern Ocean including antifreeze glycoproteins. Although lacking a swim bladder, notothenioids have diversified from their benthic ancestor into a wide array of water column niches, such as epibenthic, semipelagic, cryopelagic and pelagic habitats. Applying stable carbon (C) and nitrogen (N) isotope analyses to gain information on feeding ecology and foraging habitats, we tested whether ecological diversification along the benthic–pelagic axis followed a single directional trend in notothenioids, or whether it evolved independently in several lineages. Population samples of 25 different notothenioid species were collected around the Antarctic Peninsula, the South Orkneys and the South Sandwich Islands. The C and N stable isotope signatures span a broad range (mean δ¹³C and δ¹⁵N values between ?25.4‰ and ?21.9‰ and between 8.5‰ and 13.8‰, respectively), and pairwise niche overlap between four notothenioid families was highly significant. Analysis of isotopic disparity‐through‐time on the basis of Bayesian inference and maximum‐likelihood phylogenies, performed on a concatenated mitochondrial (cyt b) and nuclear gene (myh6, Ptr and tbr1) data set (3148 bp), showed that ecological diversification into overlapping feeding niches has occurred multiple times in parallel in different notothenioid families. This convergent diversification in habitat and trophic ecology is a sign of interspecific competition and characteristic for adaptive radiations.     

Evolutionary diversification of clades of squamate reptiles

We analysed the diversification of squamate reptiles (7488 species) based on a new molecular phylogeny, and compared the results to similar estimates for passerine birds (5712 species). The number of species in each of 36 squamate lineages showed no evidence of phylogenetic conservatism. Compared with a random speciation-extinction process with parameters estimated from the size distribution of clades, the alethinophidian snakes (2600 species) were larger than expected and 13 clades, each having fewer than 20 species, were smaller than expected, indicating rate heterogeneity. From a lineage-through-time plot, we estimated that a provisional rate of lineage extinction (0.66 per Myr) was 94% of the rate of lineage splitting (0.70 per Myr). Diversification in squamate lineages was independent of their stem age, but strongly related to the area of the region within which they occur. Tropical vs. temperate latitude exerted a marginally significant influence on species richness. In comparison with passerine birds, squamates share several clade features, including: (1) independence of species richness and age; (2) lack of phylogenetic signal with respect to clade size; (3) general absence of exceptionally large clades; (4) over-representation of small clades; (5) influence of region size on clade size; and (6) similar rates of speciation and extinction. The evidence for both groups suggests that clade size has achieved long-term equilibrium, suggesting negative feedback of species richness on the rate of diversification.     

To speciate,or not to speciate? Resource heterogeneity,the subjectivity of similarity,and the macroevolutionary consequences of niche‐width shifts in plant‐feeding insects

Coevolutionary studies on plants and plant‐feeding insects have significantly improved our understanding of the role of niche shifts in the generation of new species. Evolving plant lineages essentially constitute moving islands and archipelagoes in resource space, and host shifts by insects are usually preceded by colonizations of novel resources. Critical to hypotheses concerning ecological speciation is what happens immediately before and after colonization attempts: if an available plant is too similar to the current host(s), it simply will be incorporated into the existing diet, but if it is too different, it will not be colonized in the first place. It thus seems that the probability of speciation is maximized when alternative hosts are at an ‘intermediate’ distance in resource space. In this review, I wish to highlight the possibility that resource similarity and, thus, the definition of ‘intermediate’, are subjective concepts that depend on the herbivore lineage's tolerance to dietary variation. This subjectivity of similarity means that changes in tolerance can either decrease or increase speciation probabilities depending on the distribution of plants in resource space: insect lineages with narrow tolerances are likely to speciate by ‘island‐hopping’ on young, species‐rich plant groups, whereas more generalized lineages could speciate by shifting among resource archipelagoes formed by higher plant taxa. Repeated and convergent origins of traits known to broaden or to restrict host‐plant use in multiple different insect groups provide opportunities for studying how tolerance and resource heterogeneity may interact to determine speciation rates.     

Nonadaptive radiation: Pervasive diet specialization by drift in scale insects?

At least half of metazoan species are herbivorous insects. Why are they so diverse? Most herbivorous insects feed on few plant species, and adaptive host specialization is often invoked to explain their diversification. Nevertheless, it is possible that the narrow host ranges of many herbivorous insects are nonadaptive. Here, we test predictions of this hypothesis with comparative phylogenetic analyses of scale insects, a group for which there appear to be few host‐use trade‐offs that would select against polyphagy, and for which passive wind‐dispersal should make host specificity costly. We infer a strong positive relationship between host range and diversification rate, and a marked asymmetry in cladogenetic changes in diet breadth. These results are consonant with a system of pervasive nonadaptive host specialization in which small, drift‐ and extinction‐prone populations are frequently isolated from persistent and polyphagous source populations. They also contrast with the negative relationship between diet breadth and taxonomic diversification that has been estimated in butterflies, a disparity that likely stems from differences in the average costs and benefits of host specificity and generalism in scale insects versus butterflies. Our results indicate the potential for nonadaptive processes to be important to diet‐breadth evolution and taxonomic diversification across herbivorous insects.