New work on speciation

Just lately there's been a flurry of papers on speciation that I haven't had time to digest properly. Several of them seem to support "sympatric" or localised speciation based on selection for local resources with reproductive isolation a side effect of divergent selection. So here they are below the fold with abstracts and my comments...

Evolution of reproductive isolation in plants

Heredity advance online publication 23 July 2008; doi: 10.1038/hdy.2008.69

A Widmer, C Lexer and S Cozzolino

Reproductive isolation is essential for the process of speciation and much has been learned in recent years about the ecology and underlying genetics of reproductive barriers. But plant species are typically isolated not by a single factor, but by a large number of different pre- and postzygotic barriers, and their potentially complex interactions. This phenomenon has often been ignored to date. Recent studies of the relative importance of different isolating barriers between plant species pairs concluded that prezygotic isolation is much stronger than postzygotic isolation. But studies of the patterns of reproductive isolation in plants did not find that prezygotic isolation evolves faster than postzygotic isolation, in contrast to most animals. This may be due to the multiple premating barriers that isolate most species pairs, some of which may be controlled by few genes of major effect and evolve rapidly, whereas others have a complex genetic architecture and evolve more slowly. Intrinsic postzygotic isolation in plants is correlated with genetic divergence, but some instrinsic postzygotic barriers evolve rapidly and are polymorphic within species. Extrinsic postzygotic barriers are rarely included in estimates of different components of reproductive isolation. Much remains to be learned about ecological and molecular interactions among isolating barriers. The role of reinforcement and reproductive character displacement in the evolution of premating barriers is an open topic that deserves further study. At the molecular level, chromosomal and genic isolation factors may be associated and act in concert to mediate reproductive isolation, but their interactions are only starting to be explored.

Keywords: genic isolation, chromosomal rearrangement, natural selection, prezygotic, postzygotic, speciation

Comment: plants often hybridise, and as gamete broadcasters they are often susceptible also to introgression of genes from one species to another. So it makes sense that what would, post hoc, isolate plants is going to be all over the map like this.
Widmer, A., Lexer, C., Cozzolino, S. (2008). Evolution of reproductive isolation in plants. Heredity DOI: 10.1038/hdy.2008.69

Reproductive isolation in Saccharomyces

Heredity advance online publication 23 July 2008; doi: 10.1038/hdy.2008.73

D Greig

Although speciation is one of the most interesting processes in evolution, the underlying causes of reproductive isolation are only partially understood in a few species. This review summarizes the results of many experiments on the reproductive isolation between yeast species of the Saccharomyces sensu stricto group. Hybrids between these species form quite readily in the laboratory, but, if given a choice of species to mate with, some are able to avoid hybridization. F1 hybrids are viable but sterile: the gametes they produce are inviable. For one pair of species, hybrid sterility is probably caused by chromosomal rearrangements, but for all the other species, the major cause of hybrid sterility is antirecombination—the inability of diverged chromosomes to form crossovers during F1 hybrid meiosis. Surprisingly, incompatibility between the genes expressed from different species' genomes is not a major cause of F1 hybrid sterility, although it may contribute to reproductive isolation at other stages of the yeast life cycle.

Keywords: Saccharomyces cerevisiae, paradoxus, reproductive isolation, evolution, speciation
Otto, S.P., Servedio, M.R., Nuismer, S.L. (2008). Frequency-dependent selection and the evolution of assortative mating. Genetics DOI: 10.1534/genetics.107.084418

Isolation-by-Distance and Outbreeding Depression Are Sufficient to Drive Parapatric Speciation in the Absence of Environmental Influences

PLoS Comput Biol 4(7): e1000126. doi:10.1371/journal.pcbi.1000126

Guy A. Hoelzer, Rich Drewes, Jeffrey Meier, René Doursat

A commonly held view in evolutionary biology is that speciation (the emergence of genetically distinct and reproductively incompatible subpopulations) is driven by external environmental constraints, such as localized barriers to dispersal or habitat-based variation in selection pressures. We have developed a spatially explicit model of a biological population to study the emergence of spatial and temporal patterns of genetic diversity in the absence of predetermined subpopulation boundaries. We propose a 2-D cellular automata model showing that an initially homogeneous population might spontaneously subdivide into reproductively incompatible species through sheer isolation-by-distance when the viability of offspring decreases as the genomes of parental gametes become increasingly different. This simple implementation of the Dobzhansky-Muller model provides the basis for assessing the process and completion of speciation, which is deemed to occur when there is complete postzygotic isolation between two subpopulations. The model shows an inherent tendency toward spatial self-organization, as has been the case with other spatially explicit models of evolution. A well-mixed version of the model exhibits a relatively stable and unimodal distribution of genetic differences as has been shown with previous models. A much more interesting pattern of temporal waves, however, emerges when the dispersal of individuals is limited to short distances. Each wave represents a subset of comparisons between members of emergent subpopulations diverging from one another, and a subset of these divergences proceeds to the point of speciation. The long-term persistence of diverging subpopulations is the essence of speciation in biological populations, so the rhythmic diversity waves that we have observed suggest an inherent disposition for a population experiencing isolation-by-distance to generate new species.
Hoelzer, G.A., Drewes, R., Meier, J., Doursat, R., Bourne, P.E. (2008). Isolation-by-Distance and Outbreeding Depression Are Sufficient to Drive Parapatric Speciation in the Absence of Environmental Influences. PLoS Computational Biology, 4(7), e1000126. DOI: 10.1371/journal.pcbi.1000126

Comment: speciation by parapatry is when populations at the edge of a species' range evolve in ways that tend to increase their reproductive isolation from the main metapopulation.

Frequency-dependent selection and the evolution of assortative mating

Genetics. Published Articles Ahead of Print: July 27, 2008, doi:10.1534/genetics.107.084418

Sarah P. Otto, Maria R. Servedio and Scott L. Nuismer

A long-standing goal in evolutionary biology is to identify the conditions that promote the evolution of reproductive isolation and speciation. The factors promoting sympatric speciation have been of particular interest, both because it is notoriously difficult to prove empirically and because theoretical models have generated conflicting results, depending on the assumptions made. Here, we analyze the conditions under which selection favors the evolution of assortative mating, thereby reducing gene flow between sympatric groups, using a general model of selection, which allows fitness to be frequency dependent. Our analytical results are based on a two-locus diploid model, with one locus altering the trait under selection and the other locus controlling the strength of assortment (a "one-allele" model). Examining both equilibrium and non-equilibrium scenarios, we demonstrate that whenever heterozygotes are less fit, on average, than homozygotes at the trait locus, indirect selection for assortative mating is generated. While costs of assortative mating hinder the evolution of reproductive isolation, they do not prevent it unless they are sufficiently great. Assortative mating that arises because individuals mate within groups (formed in time or space) is most conducive to the evolution of complete assortative mating from random mating. Assortative mating based on female preferences is more restrictive, because the resulting sexual selection can lead to loss of the trait polymorphism and cause the relative fitness of heterozygotes to rise above homozygotes, eliminating the force favoring assortment. When assortative mating is already prevalent, however, sexual selection can itself cause low heterozygous fitness, promoting the evolution of complete reproductive isolation (akin to "reinforcement") regardless of the form of natural selection.
Otto, S.P., Servedio, M.R., Nuismer, S.L. (2008). Frequency-dependent selection and the evolution of assortative mating. Genetics DOI: 10.1534/genetics.107.084418

Key Words: Assortment, disruptive selection, frequency-dependent selection, sexual selection, sympatric speciation

Microbial Evolution: Stalking the Wild Bacterial Species

Current Biology Volume 18, Issue 13, 8 July 2008, Pages R565-R567 doi:10.1016/j.cub.2008.05.029

W. Ford Doolittle

A recent report suggests that, when habitats are disturbed, bacterial populations that would be considered to be separate species can merge, reversing the process of speciation. But, for bacteria, ‘species’ remains undefined and undefinable.
DOOLITTLE, W. (2008). Microbial Evolution: Stalking the Wild Bacterial Species. Current Biology, 18(13), R565-R567. DOI: 10.1016/j.cub.2008.05.029

Comment: Doolittle repeats a standard claim about microbial species, that they are not structured enough or lack a "grade of organization" (my words, not his) and so we cannot call them species. I argued against this view, which is unfortunately widely repeated without much support, in my HPLS paper on Microbial Species, which is not, unfortunately, online: The Concept and Causes of Microbial Species. Studies in History and Philosophy of the Life Sciences 28 (3): 389-408.

While on the topic of speciation, here's a nice wrap up at Biological Ramblings of a paper that finds, using the biological species concept no less, that the American Winter Wren is actually two species. And GrrlScientist has a nice review of another paper that finds that the Australian rosella is not a ring species. I am suspicious of ring species claims - most of them are not based on actual gene flow but on morphology. What the paper Grrl discusses indicates is that species are more like advancing and receding pools of water held together by surface tension, than the orderly advance of gene pools that ring species seem to indicate. Here's the abstract for that paper:

Where and when does a ring start and end? Testing the ring-species hypothesis in a species complex of Australian parrots.

Proc Biol Sci. 2008 Jul 29. [Epub ahead of print]

Joseph L, Dolman G, Donnellan S, Saint KM, Berg ML, Bennett AT.

Speciation, despite ongoing gene flow can be studied directly in nature in ring species that comprise two reproductively isolated populations connected by a chain or ring of intergrading populations. We applied three tiers of spatio-temporal analysis (phylogeny/historical biogeography, phylogeography and landscape/population genetics) to the data from mitochondrial and nuclear genomes of eastern Australian parrots of the Crimson Rosella Platycercus elegans complex to understand the history and present genetic structure of the ring they have long been considered to form. A ring speciation hypothesis does not explain the patterns we have observed in our data (e.g. multiple genetic discontinuities, discordance in genotypic and phenotypic assignments where terminal differentiates meet). However, we cannot reject that a continuous circular distribution has been involved in the group's history or indeed that one was formed through secondary contact at the 'ring's' east and west; however, we reject a simple ring-species hypothesis as traditionally applied, with secondary contact only at its east. We discuss alternative models involving historical allopatry of populations. We suggest that population expansion shown by population genetics parameters in one of these isolates was accompanied by geographical range expansion, secondary contact and hybridization on the eastern and western sides of the ring. Pleistocene landscape and sea-level and habitat changes then established the birds' current distributions and range disjunctions. Populations now show idiosyncratic patterns of selection and drift. We suggest that selection and drift now drive evolution in different populations within what has been considered the ring.

In addition there are papers on a peripatric origin for two sympatric species of field crickets, ecological speciation in sunflowers, sympatric speciation in fishes in Sulawesi, techniques for estimating speciation probabilities phylogenetically, ecological gradients in finches, habitat fragmentation in a Hispaniolan lizard, and a way to analyze and reconstruct reticulate (that is, joining, not splitting) phylogenies.

And finally, note that Stephen Colbert has had a spider named after him. There are too many levels of irony here for me to fully disentangle them all.