Sympatric speciation without borders?

The biogeography of speciation remains a controversial issue and the process of allopatric speciation reigns. Sympatric speciation differs from allopatric speciation in terms of geographic setting and the role of selection in bringing about reproductive isolating mechanisms, making it a particularly fascinating and controversial subject for evolutionary biologists. Mayr (1947) explained the difference eloquently: for allopatric speciation, populations spatially diverge and then become reproductively isolated; for sympatric speciation, populations first become reproductively isolated and then diverge. Because of this, sympatric speciation is difficult to show empirically and most evolutionary biologists agree that strict ecological, evolutionary, and geographic criteria must be met ( Coyne & Orr 2004 ). In this issue, Crow et al. (2010) challenge us to expand the definition of sympatric speciation by studying species of marine fishes that they propose have arisen by sympatric speciation in a setting that does not appear to conform to the usual geographical criteria.     

Are habitat islands islands? Woodliving beetles (Coleoptera) in deciduous forest fragments in boreal forest

Modern forestry has changed the fire dynamics in the boreal forest and as a result the size and number of deciduous forest patches have been reduced as well as the number of deciduous trees within the coniferous forest This has exaggerated the insularity of deciduous forest patches within the boreal coniferous forest zone In this paper I examine whether the diversity of beetles living in dead stems of deciduous trees follows the relationship with patch area to be predicted from island biogeographic theory, and to what extent the species assemblages differ between large and small patches and single deciduous trees within managed coniferous forests Three larger patches of deciduous forest (>120 ha) arisen as successions after forest fires in the late 1880's are compared with 6 small (<20 ha) patches of similar origin No difference in diversity could be detected between large and small patches A statistically significant difference between assembly composition was detected using matrix regression between a matrix of observed assembly similarities and a hypothetical similarity matrix based on the type of area in which the plot was situated (large- small or matrix) Finally I examined the distribution of the 56 most common beetle species over the habitat types investigated Thirty four species did not show any Significant habitat preference, 12 were found more often in clear-cut areas than expected 5 were over-represented in small patches, and 4 in large areas My explanation to this apparent lack of insularity effects is the relative recent commencement of intensive forestry in these areas and the fact that the matrix is of rather high quality that is inhabitable for numerous beetle species Thus the patches may be viewed as incipient islands still exchanging biota with their surroundings     

Are green islands red herrings? Significance of green islands in plant interactions with pathogens and pests

The term green island was first used to describe an area of living, green tissue surrounding a site of infection by an obligately biotrophic fungal pathogen, differentiated from neighbouring yellowing, senescent tissue. However, it has now been used to describe symptoms formed in response to necrotrophic fungal pathogens, virus infection and infestation by certain insects. In leaves infected by obligate biotrophs such as rust and powdery mildew pathogens, green islands are areas where senescence is retarded, photosynthetic activity is maintained and polyamines accumulate. We propose such areas, in which both host and pathogen cells are alive, be termed green bionissia. By contrast, we propose that green areas associated with leaf damage caused by toxins produced by necrotrophic fungal pathogens be termed green necronissia. A range of biotrophic/hemibiotrophic fungi and leaf-mining insects produce cytokinins and it has been suggested that this cytokinin secretion may be responsible for the green island formation. Indeed, localised cytokinin accumulation may be a common mechanism responsible for green island formation in interactions of plants with biotrophic fungi, viruses and insects. Models have been developed to study if green island formation is pathogen-mediated or host-mediated. They suggest that green bionissia on leaves infected by biotrophic fungal pathogens represent zones of host tissue, altered physiologically to allow the pathogen maximum access to nutrients early in the interaction, thus supporting early sporulation and increasing pathogen fitness. They lead to the suggestion that green islands are 'red herrings', representing no more than the consequence of the infection process and discrete changes in leaf senescence.     

Phosphorus in sediments — speciation and analysis

Characterization of sediment phosphorus is commonly based on sequential chemical extractions, in which phosphorus is supposed to be selectively removed from different compounds in the sediments. The first extraction schemes were designed to quantify discrete chemical or mineralogical compounds. As extraction schemes have been tested on different sediments, several systematic errors have been detected and the schemes have been modified and simplified accordingly. Other chemical extractions or treatments have attempted to determine phosphorus bound to particles with a certain strength or binding energy, the purpose being to determine the labile, loosely bound, exchangeable, mobile or algal-available fraction of sediment phosphorus. All extraction procedures yield operationally defined fractions and cannot be used for identification of discrete phosphorus compounds. The many methodological modifications make it necessary to be cautious when comparing results from the literature in this field.     

Sympatric speciation in parasites--what is sympatry?

Parasites account for a large part of known species diversity and are considered to have a high potential for sympatric speciation. However, the frequency of sympatric divergence in these organisms will depend on the definition of sympatry that one uses. Like many of our current species concepts, the typical definition of sympatry is not widely applicable to parasites. Revisiting the historically defined conditions for sympatric speciation and considering the situations in which we might regard parasites as being sympatric leads us to question the classic prediction that parasites have a greater tendency to speciate in sympatry than do free-living organisms.     

Speciation on islands: what are we learning?

Systematics and the Origin of Species from the Viewpoint of a Zoologist has remained an essential text on the bookshelves of evolutionary biologists since it was first published. Here, I expand upon several topics touched upon by Ernst Mayr to look at how our thinking has evolved, and is evolving, with particular reference to molecular phylogenetic studies on islands. At the time of publication, apart from the fossil record, inferences of temporal trends or patterns could only be speculative, deduced from the distributions of species and the patterns that these present. Much like the subject material itself, evolutionary biology evolves as a discipline, with an increasing availability of tools and resources. The development of molecular phylogenetics and molecular markers has given biologists a new window on the past and, as such, the ideas and explanations of Mayr have become more accessible to testing. © 2008 The Linnean Society of London, Biological Journal of the Linnean Society, 2008, 95 , 47–52.     

Are islands the end of the colonization road?

Ecologists have, up to now, widely regarded colonization of islands from continents as a one-way journey, mainly because of widely accepted assertions that less diverse island communities are easier to invade. However, continents present large targets and island species should be capable of making the reverse journey, considering they are the direct descendants of successful colonists and provided that they have not lost their dispersal abilities. Recent mapping of geography onto molecular phylogenies has revealed several cases of 'reverse colonization' (from islands to continents). We suggest this phenomenon warrants closer attention in ecology and biogeography. Assessing its significance will contribute to understanding the role of dispersal and establishment in biogeographic distributions and the assembly of natural biotas.     

How likely is speciation in neutral ecology?

Patterns of biodiversity predicted by the neutral theory rely on a simple phenomenological model of speciation. To further investigate the effect of speciation on neutral biodiversity, we analyze a spatially explicit neutral model based on population genetics. We define the metacommunity as a system of populations exchanging migrants, and we use this framework to introduce speciation with little or no gene flow (allopatric and parapatric speciation). We find that with realistic mutation rates, our metacommunity model driven by neutral processes cannot support more than a few species. Adding natural selection in the population genetics of speciation increases the number of species in the metacommunity, but the level of diversity found in the Barro Colorado Island is difficult to reach.     

Sympatric speciation: when is it possible?

This paper is written to compare the results of theoretical investigations of sympatric speciation with the relevant experimental data. We understand sympatric speciation as a formation of species out of a population whose spatial structure is not important genetically. A necessary prerequisite for speciation is an action of disruptive selection on sufficiently polymorphic traits. The present analysis confirms the view that such a selection is ecologically realistic. The genetical part of speciation begins with a development of reproductive isolation between those individuals that are opposed in some characters. It is shown that selection for reproductive isolation may be quite strong. Extinction of intermediate individuals, which completes speciation, proceeds under a wide range of conditions, including those when the newly formed species differ in quantitative characters, though most of the genes arc likely to remain the same in both species. The whole process seems possible if differences in several (up to 10) loci are sufficient to adapt the forming species to different niches and to establish reproductive isolation. It is shown that populations with bimodal distributions of some genetically determined quantitative characters can have a considerable life-time. Such distributions may be formed either as a transition stage of sympatric speciation or represent a stationary state under conditions close to those necessary to complete speciation. They are very important for experimental investigations. Sympatric speciation always follows the same principal course; it does not contradict the idea of a genome coadaptedness. The occurrence of sympatric speciation is different for different taxa depending rather on how frequently populations are subjected to the appropriate kind of selection than on their ability to obey it.     

Bioavailability and speciation—the case of inorganic compounds

Abstract

The stability constants of binary complexes of 2,4-dichlorophenoxyacetic acid (2,4-D), (4-chloro-2-methylphenoxy)acetic acid (2,4-MCPA) and (4-chloro-2-methylphenoxy) propionic acid (2,4-MCPP) with Hg(II), Pb(II) have been calculated at 298 K and at ionic strength μ = 0.1 (NaNO₃). Potentiometric measurements display two hydroxide complexes for Hg(II): HgH_?1A and HgH_?2A₂ whereas for Pb(II) we observe the formation of three species PbH_?1A, PbH_?1A₂ and PbH_?2A₂. With regard to the successive complexes, no other system represents this type of complexes under our experimental conditions. The order of capacity of complexation compared to metal for the three ligands is Hg(II)4Pb(II). Finally, the study in solution has been completed by a quantum examination of the structures of the complex of Hg(II) by the AM1 method.     

What,if anything,is sympatric speciation?

Sympatric speciation has always fascinated evolutionary biologists, and for good reason; it pits diversifying selection directly against the tendency of sexual reproduction to homogenize populations. However, different investigators have used different definitions of sympatric speciation and different criteria for diagnosing cases of sympatric speciation. Here, we explore some of the definitions that have been used in empirical and theoretical studies. Definitions based on biogeography do not always produce the same conclusions as definitions based on population genetics. The most precise definitions make sympatric speciation an infinitesimal end point of a continuum. Because it is virtually impossible to demonstrate the occurrence of such a theoretical extreme, we argue that testing whether a case fits a particular definition is less informative than evaluating the biological processes affecting divergence. We do not deny the importance of geographical context for understanding divergence. Rather, we believe this context can be better understood by modelling and measuring quantities, such as gene flow and selection, rather than assigning cases to discrete categories like sympatric and allopatric speciation.     

Gene transfer in bacteria: speciation without species?

Although Bacteria and Archaea reproduce by binary fission, exchange of genes among lineages has shaped the diversity of their populations and the diversification of their lineages. Gene exchange can occur by two distinct routes, each differentially impacting the recipient genome. First, homologous recombination mediates the exchange of DNA between closely related individuals (those whose sequences are sufficient similarly to allow efficient integration). As a result, homologous recombination mediates the dispersal of advantageous alleles that may rise to high frequency among genetically related individuals via periodic selection events. Second, lateral gene transfer can introduce novel DNA into a genome from completely unrelated lineages via illegitimate recombination. Gene exchange by this route serves to distribute genes throughout distantly related clades and therefore may confer complex abilities--not otherwise found among closely related lineages--onto the recipient organisms. These two mechanisms of gene exchange play complementary roles in the diversification of microbial populations into independent, ecologically distinct lineages. Although the delineation of microbial "species" then becomes difficult--if not impossible--to achieve, a cogent process of speciation can be predicted.     

What do we need to know about speciation?

Speciation has been a major focus of evolutionary biology research in recent years, with many important advances. However, some of the traditional organising principles of the subject area no longer provide a satisfactory framework, such as the classification of speciation mechanisms by geographical context into allopatric, parapatric and sympatry classes. Therefore, we have asked where speciation research should be directed in the coming years. Here, we present a distillation of questions about the mechanisms of speciation, the genetic basis of speciation and the relationship between speciation and diversity. Our list of topics is not exhaustive; rather we aim to promote discussion on research priorities and on the common themes that underlie disparate speciation processes.     

Microbial endemism: does phosphorus limitation enhance speciation?

There is increasing evidence for the existence of unique ecosystems that are dominated by locally adapted microbiota which harbour distinct lineages and biological capabilities, much like the macrobiota of Darwin's Galapagos Islands. As a primary example of such a system, we highlight key discoveries from the Cuatro Ciénegas basin in Mexico. We argue that high microbial endemism requires a combination of geographical isolation, long-term continuity and mechanisms for reducing the intensity of horizontal gene transfer (HGT). We also propose that strong phosphorus limitation has an important role in microbial diversification by reducing the intensity of HGT.     

Recombination-suppression: how many mechanisms for chromosomal speciation?

Over the past decade several theoretical and empirical studies have revived interest in the role of chromosomes in speciation. The resulting models do not suffer from the problems experienced by previously proposed mechanisms of chromosomal speciation, because they invoke suppression of recombination rather than a reduction in the fitness of heterokaryotypes as their core process. However, they are not free from difficulties. The evidence for recombination-suppression models is discussed here. The general conclusion is that a consensus opinion on which models best describe the real-world situation is currently unlikely because of an inability of the available empirical evidence to fully distinguish between them, which may be due in part to a lack of exclusivity. I argue that future work should take this lack of exclusivity into account. Resolving the biogeography of speciation is also suggested in order to tell the various models apart. Further study is needed which focuses on confirming the operation of individual elements of the various models, rather than attempting to validate any single mechanism as a whole.     

What does Drosophila genetics tell us about speciation?

Studies of hybrid inviability, sterility and 'speciation genes' in Drosophila have given insight into the genetic changes that result in reproductive isolation. Here, I survey some extraordinary and important advances in Drosophila speciation research. However, 'reproductive isolation' is not the same as 'speciation', and this Drosophila work has resulted in a lopsided view of speciation. In particular, Drosophila are not always well-suited to investigating ecological and other selection-driven primary causes of speciation in nature. Recent advances have made use of far less tractable, but more charismatic organisms, such as flowering plants, vertebrates and larger insects. Work with these organisms has complemented Drosophila studies of hybrid unfitness to provide a more complete understanding of speciation.     

Will sympatric speciation fail due to stochastic competitive exclusion?

Sympatric speciation requires coexistence of the newly formed species. If divergence proceeds by small mutational steps, the new species utilize almost the same resources initially, and full speciation may be impeded by competitive exclusion in stochastic environments. We investigate this primarily ecological problem of sympatric speciation by studying the population dynamics of a diverging asexual population in a fluctuating environment. Correlation between species responses to environmental fluctuation is assumed to decrease with distance in trait space. Rapidly declining correlation in combination with high environmental variability may delay full speciation or even render it impossible. Stochastic extinctions impeding speciation are most likely when correlation decays faster than competition, for example, when demographic stochasticity is strong or when divergence is not accompanied by niche separation, such as in speciation driven entirely by sexual selection. Our general theoretical results show an interesting connection between short-term ecological dynamics and long-term, large-scale evolution.