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PubMed1998

Speciation in the lesser galagos.

Species and their origins remain one of the outstanding enigmas of evolutionary biology. Many different views of the problem exist, but few have concrete predictions that open the problem to investigation. This study formalises predictions arising from the Recognition Concept of species (RC) and the Organization Theory of Speciation (OTS), and applies them to the pattern demonstrated by the lesser galago radiation. The RC and OTS are in agreement that one of the primary adaptive responses during animal speciation events involves the system of sexual recognition and reproduction, and indeed the lesser galago taxa show significant divergences in their communication systems relating to specific-mate recognition. On the other hand, only the RC predicts a concomitant adaptive response to the ecological conditions prevailing during speciation, and there are strong indications of shifts in habitat preference among these taxa. The predictions of the RC are supported above those of the OTS.

Adaptation, Physiological↗

PubMed2000

Natural selection and parallel speciation in sympatric sticklebacks.

Natural selection plays a fundamental role in most theories of speciation, but empirical evidence from the wild has been lacking. Here the post-Pleistocene radiation of threespine sticklebacks was used to infer natural selection in the origin of species. Populations of sticklebacks that evolved under different ecological conditions show strong reproductive isolation, whereas populations that evolved independently under similar ecological conditions lack isolation. Speciation has proceeded in this adaptive radiation in a repeatable fashion, ultimately as a consequence of adaptation to alternative environments.

Adaptation, Physiological↗

PubMed2000

Bimodal hybrid zones and speciation.

Contact zones exemplify a series of stages in speciation. In unimodal hybrid zones intermediates predominate; in bimodal zones hybrids are rare and parental forms predominate; and finally, species might overlap, but never hybridize. Recent studies show bimodality to be associated strongly with assortative mating or fertilization, and only weakly with overall levels of genetic divergence or intrinsic genomic incompatibility. Ecological divergence across most bimodal hybrid zones suggests that ecology contributes more to speciation than genomic incompatibility. This continuum of stable contact zones provides empirical evidence for a route to speciation, which does not require allopatry.

Journal Article↗

PubMed2001

Reproductive isolation caused by colour pattern mimicry.

Speciation is facilitated if ecological adaptation directly causes assortative mating, but few natural examples are known. Here we show that a shift in colour pattern mimicry was crucial in the origin of two butterfly species. The sister species Heliconius melpomene and Heliconius cydno recently diverged to mimic different model taxa, and our experiments show that their mimetic coloration is also important in choosing mates. Assortative mating between the sister species means that hybridization is rare in nature, and the few hybrids that are produced are non-mimetic, poorly adapted intermediates. Thus, the mimetic shift has caused both pre-mating and post-mating isolation. In addition, individuals from a population of H. melpomene allopatric to H. cydno court and mate with H. cydno more readily than those from a sympatric population. This suggests that assortative mating has been enhanced in sympatry.

Animals↗

PubMed2001

The biotic crisis and the future of evolution.

The biotic crisis overtaking our planet is likely to precipitate a major extinction of species. That much is well known. Not so well known but probably more significant in the long term is that the crisis will surely disrupt and deplete certain basic processes of evolution, with consequences likely to persist for millions of years. Distinctive features of future evolution could include a homogenization of biotas, a proliferation of opportunistic species, a pest-and-weed ecology, an outburst of speciation among taxa that prosper in human-dominated ecosystems, a decline of biodisparity, an end to the speciation of large vertebrates, the depletion of "evolutionary powerhouses" in the tropics, and unpredictable emergent novelties. Despite this likelihood, we have only a rudimentary understanding of how we are altering the evolutionary future. As a result of our ignorance, conservation policies fail to reflect long-term evolutionary aspects of biodiversity loss.

Biological Evolution↗

PubMed2001

Genetic composition and diploid hybrid speciation of a high mountain pine, Pinus densata, native to the Tibetan plateau.

Pinus densata has been suggested to have originated from hybridization events involving P. tabulaeformis and P. yunnanensis. In this study, allozyme differentiation at 12 loci was studied in 14 populations of P. tabulaeformis, P. densata, and P. yunnanensis from China. The observed genetic composition of P. densata supported the hybrid hypothesis and showed varying degrees of contribution from P. yunnanensis and P. tabulaeformis among its populations. These data, together with previous chloroplast DNA results, indicated different evolutionary histories among P. densata populations. To examine the possibility of ongoing hybridization among the three species, we analyzed patterns of linkage disequilibria between allozyme loci in ovule, pollen, and zygote pools. None of these tests suggested that there is significant ongoing gene exchange, implying that populations of P. densata have a stabilized hybrid nature. The normal fertility and high fecundity of P. densata indicate that this hybrid is maintained through sexual reproduction. P. densata represents an example of diploid hybrid speciation in an extreme ecological habitat that is both spatially and ecologically separated from that of its parents.

Alleles↗

PubMed2001

Host-induced assortative mating in host races of the larch budmoth.

The likelihood of sympatric speciation is enhanced when assortative mating is a by-product of adaptation to different habitats. Pleiotropy of this kind is recognized as important in parasites that use their hosts as a long-range cue for finding mates, but is generally assumed to have limited applicability for most other organisms. In the larch budmoth, Zeiraphera diniana (Lepidoptera: Tortricidae), sympatric host races feed on larch or pine. Zeiraphera diniana females attract males (call) by releasing host-independent long-range pheromones. Pheromone composition differs strongly between host races, but we show in an experimental field study that cross-attraction can occur at a rate of 0.03-0.38. Cross-attraction to larch females increases when they call from neighborhoods (8-m radius) rich in pine or from pine trees. Cross-attraction to pine females similarly increases when calling from neighborhoods rich in larch, but there is no significant effect of calling substrate. Males, as well as females, of this species preferentially alight on their own host, and in neighborhoods where their own host is common. This effect of tree species and host neighborhood on assortative mating is therefore due, at least in part, to the numbers of males of each host race present within approximately 200 m2 surrounding the female. This proximity effect is enhanced by the clumped distributions of the hosts themselves. Host chemistry might also affect pheromone production and/or response directly, but we have evidence neither for nor against this. This work provides empirical evidence that host adaptation has a pleiotropic effect on assortative mating in a species with host-independent long-range mating signals. Sympatric speciation via pleiotropy between ecological traits and assortative mating may thus be more common than generally supposed: Clumped resource distributions and habitat choice by adults are widespread.

Alleles↗

PubMed1985

Phylogenetics and the future of helminth systematics.

Phylogenetic systematics is a relatively new formal technique that increases the precision with which one can make direct estimates of the history of phylogenetic descent. These estimates are made in the form of phylogenetic trees, or cladograms. Cladograms may be converted directly into classifications or they may be used to test various hypotheses about the evolutionary process. More than 20 phylogenetic analyses of helminth groups have been published already, and these have been used to investigate evolutionary questions in developmental biology, biogeography, speciation, coevolution, and evolutionary ecology.

Animals↗

PubMed1998

Modelling Coevolution in Multispecies Communities.

We introduce the Webworld model, which links together the ecological modelling of food web structure with the evolutionary modelling of speciation and extinction events. The model describes dynamics of ecological communities on an evolutionary time-scale. Species are defined as sets of characteristic features, and these features are used to determine interaction scores between species. A simple rule is used to transfer resources from the external environment through the food web to each of the species, and to determine mean population sizes. A time step in the model represents a speciation event. A new species is added with features similar to those of one of the existing species and a new food web structure is than calculated. The new species may (i) add stably to the web, (ii) become extinct immediately because it is poorly adapted, or (iii) cause one or more other species to become extinct due to competition for resources. We measure various properties of the model webs and compare these with data on real food webs. These properties include the proportions of basal, intermediate and top species, the number of links per species and the number of trophic levels. We also study the evolutionary dynamics of the model ecosystem by following the fluctuations in the total number of species in the web. Extinction avalanches occur when novel organisms arise which are significantly better adapted than existing ones. We discuss these results in relation to the observed extinction events in the fossil record, and to the theory of self-organized criticality.Copyright 1998 Academic Press

Journal Article↗

PubMed1997

Selective introgression of paracentric inversions between two sibling species of the Anopheles gambiae complex.

The Anopheles gambiae complex includes the major vectors of malaria in sub-Saharan Africa where > 80% of all world-wide cases occur. These mosquitoes are characterized by chromosomal inversions associated to the speciation process and to intraspecific ecological and behavioral flexibility. It has been postulated that introgressive hybridization has selectively transferred inversions on the second chromosome between A. gambiae and A. arabiensis, the two most important vectors of malaria. Here we directly test this hypothesis with laboratory experiments in which hybrid populations were established and the fate of chromosomal inversions were followed. Consistent with the hypothesis, "foreign" X chromosomes were eliminated within two generations, while some "foreign" second chromosomes persisted for the duration of the experiments and, judging from the excess of heterozygotes established stable heterotic polymorphisms. Only those second chromosome inversions found naturally in the species could be introgressed.

Animals↗

PubMed2026

Improved Genomic Resources for the swordtail cricket, Laupala kohalensis Otte 1994.

Advances in genetic tools such as next and third generation sequencing, paired with a focus on representative clades, provide insight into how processes including adaptation, admixture, and genome structure shape the evolution and maintenance of species. However, our understanding of the genomics of speciation is dominated by systems where ecological adaptations are thought to cause initial barriers to gene exchange. In contrast to other model systems, the 38 species of the genus Laupala constitute a very rapid radiation, where evolution of reproductive barriers and speciation is thought to be driven by sexual selection. Here, with novel PacBio HiFi reads and RNA- and Iso-Seq data, we provide a highly contiguous, chromosome-level genome and markedly improved annotation of the endemic Hawaiian cricket, Laupala kohalensis Otte, 1994. Our new resources advance previous efforts, placing 99% of 47 scaffolds on 7 autosomes and 1 sex chromosome in the 1.67 Gb assembly, with a 98.8% BUSCO score (insecta_db10), N50 of ~268 Mb, and L50 of 3. Using a custom repeat library, we estimate the genome to have 46.09% repeat content, and the new annotation includes an increased estimate of 17,670 genes, which coincides with that known from other Orthopterans. Notably, we find a large nuclear DNA segment of mitochondrial origin on chromosome 7. This new resource provides a powerful tool to identify and compare genomic causes of phenotypic diversification in a system characterized by strong signatures of sexual differentiation, representing an underappreciated but potentially widespread cause of speciation.

Hawaii↗

PubMed1999

A test of alternative models of diversification in tropical rainforests: ecological gradients vs. rainforest refugia.

Comparison of mitochondrial and morphological divergence in eight populations of a widespread leaf-litter skink is used to determine the relative importance of geographic isolation and natural selection in generating phenotypic diversity in the Wet Tropics Rainforest region of Australia. The populations occur in two geographically isolated regions, and within each region, in two different habitats (closed rainforest and tall open forest) that span a well characterized ecological gradient. Morphological differences among ancient geographic isolates (separated for several million years, judging by their mitochondrial DNA sequence divergence) were slight, but morphological and life history differences among habitats were large and occurred despite moderate to high levels of mitochondrial gene flow. A field experiment identified avian predation as one potential agent of natural selection. These results indicate that natural selection operating across ecological gradients can be more important than geographic isolation in similar habitats in generating phenotypic diversity. In addition, our results indicate that selection is sufficiently strong to overcome the homogenizing effects of gene flow, a necessary first step toward speciation in continuously distributed populations. Because ecological gradients may be a source of evolutionary novelty, and perhaps new species, their conservation warrants greater attention. This is particularly true in tropical regions, where most reserves do not include ecological gradients and transitional habitats.

Animals↗

PubMed1998

Parasite extinction and colonisation and the evolution of parasite communities: a simulation study.

We determined what evolutionary processes influence the likelihood of detecting an effect of host ecological characteristics on the richness of parasite communities in comparative analyses among related host species. We used a mathematical model to generate phylogenies of hosts in which parasite communities varied over evolutionary time as parasite species were either gained or lost during host speciation events. Gain or loss of parasites were stochastic and could either be strongly, moderately, weakly, or not, affected by host ecological characteristics. The model was evaluated over this range of effects of host ecology, and for various mean probabilities of parasite gain and loss and various rates of change in host ecological characteristics at speciation events. Our results suggest that phylogenetic effects (the passing of parasite species from mother to daughter host species) are likely to obscure ecological effects (the effect of host body size, diet, habitat, lifespan, etc.) except when the effects of host ecology are strong, and the probabilities of gain or loss of parasites are high, or host ecological characteristics change markedly at speciation events. This outcome was not influenced by the shape of the phylogenetic tree used in the simulations. Sensitivity analysis of our model also shows this result to be robust to a wide range of assumptions and parameter values. Thus, because the composition of parasite communities tends to reflect their ancestry, the effect of host ecology will often be very difficult to detect.

Animals↗

PubMed1999

Conservatism of ecological niches in evolutionary time

Theory predicts low niche differentiation between species over evolutionary time scales, but little empirical evidence is available. Reciprocal geographic predictions based on ecological niche models of sister taxon pairs of birds, mammals, and butterflies in southern Mexico indicate niche conservatism over several million years of independent evolution (between putative sister taxon pairs) but little conservatism at the level of families. Niche conservatism over such time scales indicates that speciation takes place in geographic, not ecological, dimensions and that ecological differences evolve later.

Journal Article↗

PubMed2001

Modeling the ecological impact of heavy metals on aquatic ecosystems: a framework for the development of an ecological model.

In this paper, an ecological model is proposed to predict the effects of heavy metals on aquatic ecosystems. The bioavailable concentration of metals and a concept of toxicity strength (TS) are combined. The integrated ecological model relates the transport, distribution and speciation of heavy metals and their toxicity, and the effect of environmental variability on metal toxicity. It also emphasizes the link between physical and chemical processes of heavy metals in rivers and ecological effects. Based on the data obtained from research in the CERP project (Co-operative Ecological Research Project), the ecological impact of heavy metals on the aquatic ecosystem of the Le An River (polluted by heavy metals from a copper mine) was predicted. The results show that the estimated values of toxicity strength for surface water are in agreement with the percentage inhibition for the test organism (P. phosphoreum) and that the predicted ecological effect of polluted sediment is consistent with natural variability in aquatic ecosystems.

Animals↗

PubMed2000

Natural selection and sympatric divergence in the apple maggot Rhagoletis pomonella.

In On the Origin of Species, Darwin proposed that natural selection had a fundamental role in speciation. But this view receded during the Modern Synthesis when allopatric (geographic) models of speciation were integrated with genetic studies of hybrid sterility and inviability. The sympatric hypothesis posits that ecological specialization after a host shift can result in speciation in the absence of complete geographic isolation. The apple maggot, Rhagoletis pomonella, is a model for sympatric speciation in progress. Hawthorn (Crataegus spp.) is the native host for R. pomonella in N. Americas. But in the mid-1800s, a new population formed on introduced, domesticated apple (Malus pumila). Recent studies have conferred 'host race' status on apple flies as a potentially incipient species, partially isolated from haw flies owing to host-related adaptation. However, the source of selection that differentiates apple and haw flies is unresolved. Here we document a gene-environment interaction (fitness trade-off) that is related to host phenology and that genetically differentiates the races.

Animals↗

PubMed2002

Arthropods on islands: colonization, speciation, and conservation.

Islands have traditionally been considered to be any relatively small body of land completely surrounded by water. However, their primary biological characteristic, an extended period of isolation from a source of colonists, is common also to many situations on continents. Accordingly, theories and predictions developed for true islands have been applied to a huge array of systems, from rock pools, to single tree species in forests, to oceanic islands. Here, we examine the literature on islands in the broadest sense (i.e., whether surrounded by water or any other uninhabitable matrix) as it pertains to terrestrial arthropods. We categorize islands according to the features they share. The primary distinction between different island systems is "darwinian" islands (formed de novo) and "fragment" islands. In the former, the islands have never been in contact with the source of colonists and have abundant "empty" ecological niche space. On these islands, species numbers will initially increase through immigration, the rate depending on the degree of isolation. If isolation persists, over time species formation will result in "neo-endemics." When isolation is extreme, the ecological space will gradually be filled through speciation (rather than immigration) and adaptive radiation of neo-endemics. Fragment islands are fundamentally different. In these islands, the ecological space will initially be filled as a consequence of connection to the source of colonists prior to insularization. Species numbers will decrease following fragmentation through the process of relaxation. If these islands become more isolated, species will eventually arise through relictualization with the formation of "paleo-endemics." Given sufficient time, this process can result in generic level endemism on ancient fragment islands, a phenomenon well illustrated in Madagascar and New Zealand. Recognizing the distinction between the different kinds of islands is fundamental for understanding emerging patterns on each, in particular speciation, biodiversity (e.g., neo-endemics versus paleo-endemics), and conservation (e.g., naiveté in interactions with alien species).

Animals↗

PubMed1998

Francolin phylogenetics: molecular, morphobehavioral, and combined evidence.

The phylogenetics of francolins (Francolinus species) were reassessed by obtaining 660 bp of sequence of the mitochondrial DNA (mtDNA) cytochrome b gene from 20 species, the Common Quail Coturnix coturnix africana, and the Madagascar Partridge Margaroperdix madagarensis. Published sequences of the Japanese Quail C. c. japonica, Alectoris partridges, and the Junglefowl Gallus gallus were also included. Separate analysis of the 200 phylogenetically informative cytochrome b characters and the 25 informative morphobehavioral characters, as well as a combined analysis of molecular and morphobehavioral data, do not support francolin monophyly but provide strong evidence for two previously suggested clades--the quail-francolins (or partridges) and the partridge-francolins (pheasants/francolins). The quail-francolin clade comprises three groups of African francolins and three Asian species that were previously considered more closely related to the partridge-francolins. The partridge-francolin clade, which includes four groups of African francolins, forms a sister group to the Coturnix quails, the Madagascar Partridge, and the Alectoris partridges. The molecular data suggest that the two francolin clades diverged approximately 3-6 MYA. Climatic fluctuations of the past 2.5 MYA may have led to the diversification of the ecologically different francolin species groups and speciation within them.

Animals↗