Speciation Research Articles

Ecological speciation in Darwin's finches: Ghosts of finches future.

The theory of ecological speciation posits that adaptive divergence among incipient species raises incidental barriers to reproduction, thus catalyzing the emergence of new species. In this study, we conducted an experimental test of this theory in Galápagos finches, a clade in which beaks and mating songs are mechanistically linked. We forecasted the acoustic structure of songs for a set of possible evolutionary futures (successive droughts spurring increasingly large beaks) and, in a field assay, presented resulting song simulations to territorial males. We found that responses to songs dropped off after six simulated drought events, to degrees roughly comparable to drops in response to songs that diverged through cultural drift and acoustic adaptation. Our results support, in Darwin's finches, the feasibility and mechanistic bases of an ecological speciation hypothesis.

Reconciling Santa Rosalia: Both Reproductive Isolation and Coexistence Constrain Diversification.

AbstractUnderstanding patterns of diversification necessarily requires accounting for both the generation and the persistence of species. Formal models of speciation genetics, however, focus on the generation of new species without explicitly considering the maintenance of biodiversity (e.g., coexistence, the focus of ecological studies of diversity). Consequently, it remains unclear whether and how new species will coexist following a speciation event, a gap limiting our ability to understand the rate-limiting controls of diversification over macroevolutionary timescales. To connect coexistence and speciation theory and assess the relative importance of ecological versus genetic constraints in diversification events, we develop a deterministic, three-locus, population-genetic model that includes a skewed distribution of available resources (to generate variation in fitness differences), frequency-dependent competition, and assortative mating. Both ecology and genetics play vital and interacting roles in shaping initial speciation events and long-term eco-evolutionary outcomes. Ecological constraints are especially important when fitness differences are large and competition remains strong among dissimilar phenotypes. Ephemeral species can occur in our model and are typically lost because of competitive exclusion, a result demonstrating that species persistence may serve as the rate-limiting control of long-term diversification rates. More broadly, our model adds evidence that the unification of ecological and evolutionary (including genetic) perspectives on biodiversity is needed to predict large-scale patterns.

Sharp decline in male fertility in F2 hybrids of the female-heterogametic silk moth Bombyx.

Sexual selection drives rapid evolution of morphological, physiological, and behavioral traits, especially in males, and it may also drive the rapid evolution of hybrid male sterility. Indeed, the faster male theory of speciation was once viewed as a major cause of Haldane's rule in male-heterogametic XY taxa, but is increasingly being replaced by the genetic conflict hypothesis partly because it cannot explain the faster evolution of hybrid female sterility in female-heterogametic ZW taxa. The theory nonetheless predicts that there should be more genes for hybrid male sterility than for hybrid female sterility even in such taxa, but this remains untested. Thus, finding evidence for the faster male theory of reproductive isolation beyond the F1 generation in ZW systems still represents a challenge to studying the impact of sexual selection. In this study, we examined F2 hybrids between the domesticated silkworm Bombyx mori and the wild silk moth Bombyx mandarina, which have ZW sex determination. We found that although only females showed reduced fertility in the F1 generation, the F2 hybrid males had a significant reduction in fertility compared with the parental and F1 males. Importantly, 27% of the F2 males and 15% of the F2 females were completely sterile, suggesting the presence of recessive incompatibilities causing male sterility in female-heterogametic taxa.

Social Plasticity Enhances Signal-Preference Codivergence.

AbstractThe social environment is often the most dynamic and fitness-relevant environment animals experience. Here we tested whether plasticity arising from variation in social environments can promote signal-preference divergence-a key prediction of recent speciation theory but one that has proven difficult to test in natural systems. Interactions in mixed social aggregations could reduce, create, or enhance signal-preference differences. In the latter case, social plasticity could establish or increase assortative mating. We tested this by rearing two recently diverged species of Enchenopa treehoppers-sap-feeding insects that communicate with plant-borne vibrational signals-in treatments consisting of mixed-species versus own-species aggregations. Social experience with heterospecifics (in the mixed-species treatment) resulted in enhanced signal-preference species differences. For one of the two species, we tested but found no differences in the plastic response between sympatric and allopatric sites, suggesting the absence of reinforcement in the signals and preferences and their plastic response. Our results support the hypothesis that social plasticity can create or enhance signal-preference differences and that this might occur in the absence of long-term selection against hybridization on plastic responses themselves. Such social plasticity may facilitate rapid bursts of diversification.

A Survey of Terrestrial Vertebrates of Tetepare Island, Solomon Islands, Including Six New Island Records1

The Solomon Islands host a diverse terrestrial vertebrate fauna which has played a formative role in the development of speciation theory. Yet, despite over a century of biological exploration in the region, there are many islands for which we have incomplete knowledge of the vertebrate fauna. In 2019, we spent 20 days on Tetepare Island in the Western Province, Solomon Islands. Tetepare has a long history of conservation action by local communities and it is now the largest uninhabited tropical island in the world. We recorded 57 species of birds, 13 mammals, 5 amphibians, and 21 reptiles. Of these, we documented six species for the first time on Tetepare by western scientists: one frog, three non-avian reptiles, and two mammals. These findings point to a continued need for basic biological inventory work to inform research, local conservation efforts, and to increase published knowledge of the biodiversity in the Solomon Islands.

Sympatric Speciation in Mole Rats and Wild Barley and Their Genome Repeatome Evolution: A Commentary.

The theories of sympatric speciation (SS) and coding and noncoding (cd and ncd =repeatome) genome function are still contentious. Studies on SS in our two new models, "Evolution Canyon" and "Evolution Plateau", in Israel, divergent microclimatically and geologically-edaphically, respectively, indicated that in ecologically divergent microsites SS is a common speciation model across life from bacteria to mammals. Genomically, the intergenic ncd repeatome was and is still regarded by many biologists as "selfish," "junk," and non-functional. In contrast, it is considered by the encyclopedia of DNA elements discovery as biochemically functional and regulatory, and the transposable elements were considered earlier by Barbara McClintock as "controlling elements" of genes. Remarkably, it is found that repeated elements can statistically identify significantly, the five species of subterranean mole rats of Spalax ehrenbergi superspecies adapted to increasingly arid climatic trend southward in Israel. Moreover, it is first discovered in the SS studies in two distant taxa, subterranean mole rats and wild barley, and later also in spiny mice in Israel and subterranean zokors in China, that the noncoding repeatome is genomically mirroring the image of the protein-coding genome in divergent ecologies. It is shown that this mirroring image is statistically significant both within and between the ecologically divergent taxa supporting the hypothesis that much of the repeatome might be regulatory and selected as the protein-coding genome by the same ecological stresses.

Three problems in the genetics of speciation by selection

Speciation is the process by which barriers to gene flow evolve between populations. Although we now know that speciation is largely driven by natural selection, knowledge of the agents of selection and the genetic and genomic mechanisms that facilitate divergence is required for a satisfactory theory of speciation. In this essay, we highlight three advances/problems in our understanding of speciation that have arisen from studies of the genes and genomic regions that underlie the evolution of reproductive isolation. First, we describe how the identification of “speciation” genes makes it possible to identify the agents of selection causing the evolution of reproductive isolation, while also noting that the link between the genetics of phenotypic divergence and intrinsic postzygotic reproductive barriers remains tenuous. Second, we discuss the important role of recombination suppressors in facilitating speciation with gene flow, but point out that the means and timing by which reproductive barriers become associated with recombination cold spots remains uncertain. Third, we establish the importance of ancient genetic variation in speciation, although we argue that the focus of speciation studies on evolutionarily young groups may bias conclusions in favor of ancient variation relative to new mutations.

Island and Mountain Ecosystems as Testbeds for Biological Control in the Anthropocene

For centuries, islands and mountains have incited the interest of naturalists, evolutionary biologists and ecologists. Islands have been the cradle for biogeography and speciation theories, while mountain ranges have informed how population adaptation to thermal floors shapes the distribution of species globally. Islands of varying size and mountains’ altitudinal ranges constitute unique “natural laboratories” where one can investigate the effects of species loss or global warming on ecosystem service delivery. Although invertebrate pollination or seed dispersal processes are steadily being examined, biological control research is lagging. While observations of a wider niche breadth among insect pollinators in small (i.e., species-poor) islands or at high (i.e., colder) altitudes likely also hold for biological control agents, such remains to be examined. In this Perspective piece, we draw on published datasets to show that island size alone does not explain biological control outcomes. Instead, one needs to account for species’ functional traits, habitat heterogeneity, host community make-up, phenology, site history or even anthropogenic forces. Meanwhile, data from mountain ranges show how parasitism rates of Noctuid moths and Tephritid fruit flies exhibit species- and context-dependent shifts with altitude. Nevertheless, future empirical work in mountain settings could clarify the thermal niche space of individual natural enemy taxa and overall thermal resilience of biological control. We further discuss how global databases can be screened, while ecological theories can be tested, and simulation models defined based upon observational or manipulative assays in either system. Doing so can yield unprecedented insights into the fate of biological control in the Anthropocene and inform ways to reinforce this vital ecosystem service under global environmental change scenarios.

Sympatric genetic divergence between early- and late-season weedy rice populations.

Summary Sympatric genetic divergence is the most appealing and controversial pattern in the theory of ecological speciation. Examples that support sympatric genetic divergence in plant species are extremely rare. Solid evidence of sympatric genetic divergence will provide deep insights for revealing the underlying mechanisms of ecological speciation.We analysed the total genomic DNA sequences of 120 weedy rice (WR; Oryza sativa f. spontanea) plants, representing three WR population pairs separately from three early‐ and late‐season rice fields, in comparison with those of the co‐occurring rice cultivars and other rice materials.We detected substantial genetic divergence within the pairs of the sympatric early‐ and late‐season WR populations, although genetic divergence was unevenly distributed across the genomes. Restricted gene flow was determined between the sympatric WR populations, resulting in their distinct genetic structures. We also detected relatively low genetic diversity that was likely to be associated with stronger selection in early‐season WR populations.Our findings provide strong evidence for sympatric genetic divergence between the WR populations in the same fields but in different seasons. We conclude that temporal isolation plays an important role in creating genetic divergence between sympatric populations/species in plants.

Ecological speciation by sympatric host shifts in a clade of herbivorous sea slugs, with introgression and localized mitochondrial capture between species

Host shifting in insect-plant systems was historically important to the development of ecological speciation theory, yet surprisingly few studies have examined whether host shifting drives diversification of marine herbivores. When small-bodied consumers feed and also mate on a preferred host, disruptive selection can split a population into host races despite gene flow. Support for host shifts is notably lacking for invertebrates associated with macroalgae, where the scale of dispersal by planktonic larvae often far exceeds the grain of host patchiness, and adults are typically less specialized than terrestrial herbivores. Here, we present a candidate example of ecological speciation in a clade of sea slugs that primarily consume green algae in the genus Caulerpa, including highly invasive species. Ancestral character state reconstructions supported ‘sea grapes’ (C. racemosa, C. lentillifera) as the ancestral host for a tropical radiation of 12 Elysia spp., with one shift onto alternative Caulerpa spp. in the Indo-Pacific. A Caribbean radiation of three species included symaptric host shifts to Rhipocephalus brevicaulis in the ancestor of E. pratensis Ortea & Espinosa, 1996, and to C. prolifera in E. hamanni Krug, Vendetti & Valdes 2016, plus a niche expansion to a range of Caulerpa spp. in E. subornata Verrill, 1901. All three species are broadly sympatric across the Caribbean but are host-partitioned at a fine grain, and distinct by morphology and at nuclear loci. However, non-recombining mtDNA revealed a history of gene flow between E. pratensis and E. subornata: COI haplotypes from E. subornata were 10.4% divergent from E. pratensis haplotypes from four sites, but closely related to all E. pratensis haplotypes sampled from six Bahamian islands, indicating historical introgression and localized “mitochondrial capture.” Disruptive selective likely fueled divergence and adaptation to distinct host environments, indicating ecological speciation may be an under-appreciated driver of diversification for marine herbivores as well as epibionts and other resource specialists.

Is Côte D'Ivoire a new high hybridization zone for the two major malaria vectors, Anopheles coluzzii and An. gambiae (Diptera, Culicidae)?

Anopheles gambiae and An. coluzzii are very closely related and recently differentiated species representing the main malaria vectors in the Afrotropical region and responsible of up to >3 infective bites/person/night in Côte D'Ivoire, where prevention and control has stagnated in recent years. The aim of the present study was to genetically and ecologically characterize An. gambiae and An. coluzzii populations from two villages of Côte D'Ivoire, lying in the coastal forest belt and 250 km inland in the Guinean savannah mosaic belt, respectively. Results reveal high frequencies of both species in both study sites and high frequencies of hybrids (4-33%) along the whole year of sampling. Consistently with observations for the well-known high hybridization zone at the far-west of the species range, hybrid frequencies were higher in the coastal village and highest when the two species occurred at more balanced frequencies, supporting the "frequency-dependent hybridization" ecological speciation theory. Pilot genotyping revealed signatures of genomic admixture in both chromosome-X and -3. Coupled with previous reports of hybrids in the region, the results point to the coastal region of Côte D'Ivoire as a possible regions of high hybridization. Preliminary characterization of parameters relevant for malaria transmission and control (e.g. possibly higher sporozoite rates and indoor biting preferences in hybrids than in the parental species) highlight the possible relevance of the breakdown of reproductive barriers between An. gambiae and An. coluzzii not only in the field of ecological evolution, but also in malaria epidemiology and control.

The evolution of age-specific choosiness and reproductive isolation in a model with overlapping generations.

The strength of mate choice (choosiness) often varies with age, but theory to understand this variation is scarce. Additionally, theory has investigated the evolution of choosiness in speciation scenarios but has ignored that most organisms have overlapping generations. We investigate whether speciation can result in variation of choosiness with age, and whether such variation can in turn affect speciation. We develop a population-genetic model of the evolution of choosiness in organisms with overlapping generations in the context of secondary contact between two divergent populations. We assume that females choose males that match their phenotype, such that choosiness evolves by sexual selection. We demonstrate that speciation can result in the evolution of age-specific choosiness when the mating trait is under divergent ecological selection and age is not used as a mating cue. The cause of this result is that allele frequencies differ between choosy females and males. However, we find that the evolution of age-specific choosiness does not affect the overall level of reproductive isolation compared to a case without age-structure, supporting previous speciation theory. Overall, our results connect life history and speciation theory, and the mechanisms that we highlight have implications for the understanding of the role of sex-specific selection in the evolution ofchoosiness.

Advances in speciation theories and their verifications based on the biological species concept

Advances in speciation theories and their verifications based on the biological species concept

Speciation of yttrium and the rare earth elements in seawater: Review of a 20-year analytical journey

This review describes a program that we have conducted for more than 20 years, first at the University of South Florida and then at the University of Maryland Center for Environmental Science, to systematically measure stability constants for solution complexes of yttrium and all rare earth elements (YREE), except Pm, at the ionic strength of seawater (I ~ 0.7). This effort informs our goal of better characterizing and understanding YREE speciation in the marine environment, in the presence of both inorganic and organic ligands (and sorbents). Since all YREE appear to show similar behavior as a function of ionic strength, patterns of stability constants for individual ligands have been extrapolated to I = 0 whenever sufficient data for at least one member of the series are available. Complexes with hydroxide, fluoride and chloride, carbonate, and sulfate, are treated in detail, being the most prominent inorganic YREE ligands. A natural siderophore (DFOB) and synthetic polyaminopolycarboxylic acids (NTA, EDTA, DTPA) are given as examples of strong organic YREE complexation which, while remaining poorly quantified, is thought to be important. Several YREE ligands that play a negligible role in seawater, or for which data are as yet scarce, are also briefly discussed. Many different and sometimes novel analytical approaches were used in our experiments and these are summarized in the introductory sections, along with a primer on the theory of chemical speciation. This paper is intended as a retrospective of our work, not as a critical review of all available data. Literature values are thus cited primarily for comparison with our results, to support specific arguments, or to highlight prior discrepancies that prompted our own studies.

A phylogeny of white-eyes based on ultraconserved elements

White-eyes are an iconic radiation of passerine birds that have been the subject of studies in evolutionary biology, biogeography, and speciation theory. Zosterops white-eyes in particular are thought to have radiated rapidly across continental and insular regions of the Afro- and Indo-Pacific tropics, yet their phylogenetic history remains equivocal. Here, we sampled 77% of the genera and 47% of known white-eye species and sequenced thousands of ultraconserved elements to infer the phylogeny of the avian family Zosteropidae. We used concatenated maximum likelihood and species tree methods and found strong support for seven clades of white-eyes and three clades within the species-rich Zosterops radiation.

Speciation Analysis of Seleno-Compounds in Hair and Finger Nails

Speciation Analysis of Seleno-Compounds in Hair and Finger Nails

Evolution of specialization in a plant-microbial mutualism is explained by the oscillation theory of speciation.

Specialization in mutualisms is thought to be a major driver of diversification, but few studies have explored how novel specialization evolves, or its relation to the evolution of other niche axes. A fundamental question is whether generalist interactions evolve to become more specialized (i.e., oscillation hypothesis) or if partner switches evolve without any change in niche breadth (i.e., musical chairs hypothesis). We examined alternative models for the evolution of specialization by estimating the mutualistic, climatic, and edaphic niche breadths of sister plant species, combining phylogenetic, environmental, and experimental data on Acmispon strigosus and Acmispon wrangelianus genotypes across their overlapping ranges in California. We found that specialization along all three niche axes was asymmetric across species, such that the species with broader climatic and edaphic niches, Acmispon strigosus, was also able to gain benefit from and invest in associating with a broader set of microbial mutualists. Our data are consistent with the oscillation model of specialization, and a parallel narrowing of the edaphic, climatic, and mutualistic dimensions of the host species niche. Our findings provide novel evidence that the evolution of specialization in mutualism is accompanied by specialization in other niche dimensions.

Speciation inDaphnia

The microcrustacean Daphnia is arguably one of the most studied zooplankton species, having a well understood ecology, life history, and a relatively well studied evolutionary history. Despite this wealth of knowledge, species boundaries within closely related species in this genus often remain elusive and the major evolutionary forces driving the diversity of daphniids remain controversial. This genus contains more than 80 species with multiple cryptic species complexes, with many closely related species able to hybridize. Here, we review speciation research in Daphnia within the framework of current speciation theory. We evaluate the role of geography, ecology, and biology in restricting gene flow and promoting diversification. Of the 253 speciation studies on Daphnia, the majority of studies examine geographic barriers (55%). While evidence shows that geographic barriers play a role in species divergence, ecological barriers are also probably prominent in Daphnia speciation. We assess the contribution of ecological and nonecological reproductive isolating barriers between closely related species of Daphnia and found that none of the reproductive isolating barriers are restricting gene flow completely. Research on reproductive isolating barriers has disproportionally focused on two species complexes, the Daphnia pulex and Daphnia longispina species complexes. Finally, we identify areas of research that remain relatively unexplored and discuss future research directions that build our understanding of speciation in daphniids.

Molecular phylogeny of the “salty” Liolaemus species (Reptilia: Squamata): anomalus group, endemic of the Argentina Monte Desert

In Argentina, the Monte ecoregion extends for more than 2,000 km from NW Argentina to borealPatagonia, and includes many salt-pans and barren flatlands colonized by the monophyleticLiolaemus anomalus group that includes seven species. Some of their external morphologicalcharacteristics and behaviors are very unusual for the genus, and this has led to a complextaxonomic history. The group is very poorly known except for a recent paper with descriptionsof three species and morphological-based phylogenetic hypotheses. Of the four recognizedspecies, two are threatened and one is vulnerable, and while the conservation status of thethree recently described species is unknown, they are suspected to also be under some degreeof threat. We reviewed all georeferenced localities known to produce a distribution map, andsequenced two mitochondrial and four nuclear genes, for representatives of all species. Weinferred a time-calibrated species tree showing that the evolutionary history of this clade occurredentirely during the Pleistocene, with most of the divergences very recent. The climaticand geomorphological changes driving this divergence started during the Great PatagonianGlaciation, initially separating the two northwestern-most distributed species (L. pipanaco +L. pseudoanomalus) from the rest. Given the very recent evolutionary history of the group andtheir unique and conserved morphology, incongruent topologies among datasets are expected; adetailed genome-wide dataset will be needed to fully assess and resolve their speciation history.

Raw, activated and modified biosorbents for the speciation of C. I. acid red 2 from aqueous solutions: An adsorption study

Adsorptive removal of C. I. Acid Red 2 dye in the aqueous solution has been carried out on the various surfaces. This study describes the comparative adsorption of dye using mustard cakes (MC), sugarcane bagasse ash (SBA) and leaves of water hyacinth (WH) in different forms. The effects of initial adsorbate concentration, contact time, adsorbent dosage, temperature and pH of the medium on the adsorption of C. I. Acid Red 2 (C.I.A.R-2) or Methyl Red (MR) were investigated. The biosorbents were characterized by using modern technique (FTIR, SEM, and XRD). The maximum adsorption capacity of MR occurred at pH 2 and 4 for different adsorbent materials. Adsorption of the dyes was found to increase as the adsorbent dosage increases but decreases as initial concentration of the adsorbate (dye) increases. This is due to the fact that the active site of the adsorbent can adsorb a certain concentration of the dye. The C. I. Acid Red 2 adsorption kinetics was analyzed by using pseudo-first order and pseudo-second order kinetic models. The results indicated that the adsorption of C. I. Acid Red 2 (C-Colour, I -Index) onto various surfaces is best described using a pseudo-second order kinetic model. The Langmuir, Freundlich and Temkin adsorption isotherms were also studied and the data are well fitted with Freundlich isotherm. On the basis of the results it was demonstrated that mustard oil cake, sugarcane bagasse ash and leaves of water hyacinth are promising adsorbent materials which can be used to clean up textile dyes from wastewater. Key words: C.I.A.R-2, sugarcane bagasse ash (SBA), mustard cakes (MC) and water hyacinth (WH), adsorption.