Evolutionists and ecologists are motivated to understand the forces that generate and maintain biological diversity. In turn, attention has focused on the relative roles of natural and sexual ...selection in vertebrate groups that are considered to be exemplars of evolutionary radiation. Here, we argue that recent phylogenetic and population genetic evidence shows that seemingly different vertebrate radiations follow similar evolutionary trajectories. Groups diverge along axes of habitat, trophic morphology and communication, often in that order. Notably, clades proceed to different stages of the radiation process, with the endpoint being correlated with species richness. Divergence along axes one and two (habitat and trophic morphology, respectively) is likely to follow ecological selection models; diversification along axis three (communication) probably proceeds according to sexual selection models. We speculate that a paucity of genetic variation for means of signaling that are independent of fitness traits might be the most important constraint limiting the diversification of vertebrate groups. The stages of evolutionary radiation discussed here do not apply to all organisms, but might represent a common trend.
Patterns of colonization and diversification on islands provide valuable insights into evolutionary processes. Due to their unique geographic position and well known history, the Galapagos Islands ...are an important model system for evolutionary studies. Here we investigate the evolutionary history of a winged grasshopper genus to infer its origin and pattern of colonization in the Galapagos archipelago. The grasshopper genus Sphingonotus has radiated extensively in the Palaearctic and many species are endemic to islands. In the New World, the genus is largely replaced by the genus Trimerotropis. Oddly, in the Caribbean and on the Galapagos archipelago, two species of Sphingonotus are found, which has led to the suggestion that these might be the result of anthropogenic translocations from Europe. Here, we test this hypothesis using mitochondrial and nuclear DNA sequences from a broad sample of Sphingonotini and Trimerotropini species from the Old World and New World. The genetic data show two distinct genetic clusters representing the New World Trimerotropini and the Old World Sphingonotini. However, the Sphingonotus species from Galapagos and the Caribbean split basally within the Old World Sphingonotini lineage. The Galapagos and Caribbean species appear to be related to Old World taxa, but are not the result of recent anthropogenic translocations as revealed by divergence time estimates. Distinct genetic lineages occur on the four investigated Galapagos Islands, with deep splits among them compared to their relatives from the Palaearctic. A scenario of a past wider distribution of Sphingonotus in the New World with subsequent extinction on the mainland and replacement by Trimerotropis might explain the disjunct distribution.
The traits involved in sexual selection, such as male secondary sexual characteristics and female mate choice, often co-evolve which can promote population differentiation. However, the genetic ...architecture of these phenotypes can influence their evolvability and thereby affect the divergence of species. The extraordinary diversity of East African cichlid fishes is often attributed to strong sexual selection and thus this system provides an excellent model to test predictions regarding the genetic architecture of sexually selected traits that contribute to reproductive isolation. In particular, theory predicts that rapid speciation is facilitated when male sexual traits and female mating preferences are controlled by a limited number of linked genes. However, few studies have examined the genetic basis of male secondary sexual traits and female mating preferences in cichlids and none have investigated the genetic architecture of both jointly. In this study, we artificially hybridized a pair of behaviorally isolated cichlid fishes from Lake Malawi and quantified both melanistic color pattern and female mate choice. We investigated the genetic architecture of both phenotypes using quantitative genetic analyses. Our results suggest that 1) many non-additively acting genetic factors influence melanistic color patterns, 2) female mate choice may be controlled by a minimum of 1-2 non-additive genetic factors, and 3) F2 female mate choice is not influenced by male courting effort. Furthermore, a joint analysis of color pattern and female mate choice indicates that the genes underlying these two traits are unlikely to be physically linked. These results suggest that reproductive isolation may evolve rapidly owing to the few genetic factors underlying female mate choice. Hence, female mate choice likely played an important role in the unparalleled speciation of East African cichlid fish.
A new class of glutathione transferases has been discovered by analysis of the expressed sequence tag data base and sequence alignment. Glutathione S-transferases (GSTs) of the new class, named ...Omega, exist in several mammalian species andCaenorhabditis elegans. In humans, GSTO 1-1 is expressed in most tissues and exhibits glutathione-dependent thiol transferase and dehydroascorbate reductase activities characteristic of the glutaredoxins. The structure of GSTO 1-1 has been determined at 2.0-Å resolution and has a characteristic GST fold (Protein Data Bank entry code 1eem). The Omega class GSTs exhibit an unusual N-terminal extension that abuts the C terminus to form a novel structural unit. Unlike other mammalian GSTs, GSTO 1-1 appears to have an active site cysteine that can form a disulfide bond with glutathione.
Rapid evolutionary radiations provide insight into the fundamental processes involved in species formation. Here we examine the diversification of one such group, the cichlid fishes of Lake Malawi, ...which have radiated from a single ancestor into more than 400 species over the past 700 000 years. The phylogenetic history of this group suggests: (i) that their divergence has proceeded in three major bursts of cladogenesis; and (ii) that different selective forces have dominated each cladogenic event. The first episode resulted in the divergence of two major lineages, the sand‐ and rock‐dwellers, each adapted to a major benthic macrohabitat. Among the rock‐dwellers, competition for trophic resources then drove a second burst of cladogenesis, which resulted in the differentiation of trophic morphology. The third episode of cladogenesis is associated with differentiation of male nuptial colouration, most likely in response to divergent sexual selection. We discuss models of speciation in relation to this observed pattern. We advocate a model, divergence with gene flow, which reconciles the disparate selective forces responsible for the diversification of this group and suggest that the nonadaptive nature of the tertiary episode has significantly contributed to the extraordinary species richness of this group.
Lake Malawi contains a flock of >500 species of cichlid fish that have evolved from a common ancestor within the last million years. The rapid diversification of this group has been attributed to ...morphological adaptation and to sexual selection, but the relative timing and importance of these mechanisms is not known. A phylogeny of the group would help identify the role each mechanism has played in the evolution of the flock. Previous attempts to reconstruct the relationships among these taxa using molecular methods have been frustrated by the persistence of ancestral polymorphisms within species. Here we describe results from a DNA fingerprinting technique that overcomes this problem by examining thousands of polymorphisms distributed across the genome. The resulting dendrogram averages the evolutionary history of thousands of genes and should accurately reflect the evolutionary history of these species. Our tree resolves relationships among closely related Lake Malawi cichlids and provides insights into the pattern of speciation in this group. We demonstrate that adaptive divergence in trophic morphology played an important role during the early history of the lake. Subsequent species diversity has arisen with little change in trophic morphology, which suggests that other forces are responsible for the continued speciation of these fishes.
ABSTRACT
The genetic structure of a species is shaped by the interaction of contemporary and historical factors. Analyses of individuals from the same population sampled at different points in time ...can help to disentangle the effects of current and historical forces and facilitate the understanding of the forces driving the differentiation of populations. The use of such time series allows for the exploration of changes at the population and intraspecific levels over time. Material from museum collections plays a key role in understanding and evaluating observed population structures, especially if large numbers of individuals have been sampled from the same locations at multiple time points. In these cases, changes in population structure can be assessed empirically. The development of new molecular markers relying on short DNA fragments (such as microsatellites or single nucleotide polymorphisms) allows for the analysis of long‐preserved and partially degraded samples. Recently developed techniques to construct genome libraries with a reduced complexity and next generation sequencing and their associated analysis pipelines have the potential to facilitate marker development and genotyping in non‐model species. In this review, we discuss the problems with sampling and available marker systems for historical specimens and demonstrate that temporal comparative studies are crucial for the estimation of important population genetic parameters and to measure empirically the effects of recent habitat alteration. While many of these analyses can be performed with samples taken at a single point in time, the measurements are more robust if multiple points in time are studied. Furthermore, examining the effects of habitat alteration, population declines, and population bottlenecks is only possible if samples before and after the respective events are included.
"The steep rise in neighborhood associations in post-Katrina New Orleans is commonly presented in starkly positive or negative terms--either romanticized narratives of community influence or ...dismissals of false consciousness and powerlessness to elite interests. In A Neighborhood Politics of Last Resort Stephen Danley offers a messier and ultimately more complete picture of these groups as simultaneously crucial but tenuous social actors. Through a comparative case study based on extensive fieldwork in post-Katrina New Orleans, Danley follows activists in their efforts to rebuild their communities, while also examining the dark underbelly of NIMBYism ("not in my back yard"), characterized by racism and classism. He elucidates how neighborhood activists were tremendously inspired in their defense of their communities, at times outwitting developers or other perceived threats to neighborhood life, but they could be equally creative in discriminating against potential neighbors and fighting to keep others outof their communities. Considering the plight of grassroots activism in the context of national and global urban challenges, A Neighborhood Politics of Last Resort immerses the reader in the daily minutiae of post-Katrina life to reveal how multiple groups responded to the same crisis with inconsistent and often ad-hoc approaches, visions, and results."--
A novel testing apparatus is presented which affords the researcher maximum control over the testing environment, but allows for the scoring of actual spawning events. Female Metriaclima zebra and ...Metriaclima benetos chose the appropriate conspecific mate in every mating trial performed. This apparatus provides support for a critical assumption of many cichlid speciation models: that female cichlids use visual cues to recognize conspecifics. These results demonstrate that females are able to identify conspecific mates using only visual cues, and provide further support for the importance of sexual selection in the speciation of these fishes.
Within the past 2 million years, the cichlids of Lake Malawi have diversified into well over 500 species resulting in one of the worlds largest lacustrine fish radiations. As a result, many of the ...habitats within the lake support a high diversity of species. In these highly species rich communities, male cichlids must acquire and defend a territory to successfully reproduce. Within the rock-dwelling cichlids of Lake Malawi (mbuna), this has resulted in the formation of polyspecific leks on the heterogeneous rocky benthos. Aggression is fairly common in these leks and has been tied not only to individual reproduction but to the larger phenomenon of community assembly and the maintenance of biological diversity. In this study, I examined the patterns of aggressive acts of four species within the mbuna genus
Maylandia
at two locations in the southern Lake Malawi. The number of aggressive acts of two sympatric species was examined at each location. At each site, one species defends territories over bedrock and the other over cobble. The number of aggressive acts across the four species was compared. The influence of habitat type on male aggression was examined and the targets of male aggression were identified to evaluate several hypotheses concerning the evolution of male aggression. The results show that aggression quantitatively varied among species, was largely directed towards heterospecifics, and was strongly influenced by habitat type. The aggressive behavior of one sympatric species pair,
Maylandia benetos
and
Maylandia zebra
, was observed under controlled laboratory conditions. Laboratory results support field observations: the bedrock associated species performed more aggressive acts and aggressive acts were directed equally at con- and heterospecifics. The results of this study suggest that habitat complexity plays a larger role in shaping aggressive behavior than other suggested factors such as competition for resources.
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