Congrats to lead author Haley Kenyon and coauthors Miguel Alcaide and David Toews on our publication comparing song and genetic variation across the Townsend’s / Black-throated Green Warbler hybrid zone. I think this paper makes a particularly interesting contribution by jointly considering cultural evolution, genetic evolution, and hybrid zone theory.

Kenyon, H.L., M. Alcaide, D.P.L. Toews, and D.E. Irwin. Cultural isolation is greater than genetic isolation across an avian hybrid zone. Journal of Evolutionary Biology, online Early View: doi:10.1111/jeb.12989  Link

The Abstract:
Elucidating the relationship between genetic and cultural evolution is important in understanding speciation, as learned premating barriers might be involved in maintaining species differences. Here, we test this relationship by examining a widely recognized premating barrier, bird song, in a hybrid zone between black-throated green (Setophaga virens) and Townsend’s warblers (S. townsendi). We use song analysis, genomic techniques and playback experiments to characterize the cultural and genetic backgrounds of individuals in this region, expecting that if song is an important reproductive barrier between these species, there should be a strong relationship between song and genotype. We show that songs in the hybrid zone correspond to the distinctly different songs found in allopatry but that song and genotype are not tightly coupled in sympatry. Allopatric individuals responded only to local songs, indicating that individuals may have learned to respond to songs they commonly hear. We observed discordance between song and genotype clines; a narrower cline suggests that cultural selection on song is stronger than natural selection on genotype. These findings indicate that song is unlikely to play a role in reproductive isolation between these species, and we suggest that spatial variation in song may nonetheless be maintained by frequency-dependent cultural selection. This decoupling of genes and culture may contribute to hybridization in this region.

Feel free to email me for a PDF of the full publication.

Published September 8th in Molecular Ecology:

Irwin, D.E., M. Alcaide, K.E. Delmore, J.H. Irwin, and G.L. Owens. 2016. Recurrent selection explains parallel evolution of genomic regions of high relative but low absolute differentiation in a ring species. Molecular Ecology, online Early View: doi:10.1111/mec.13792 Link

The Abstract: Recent technological developments allow investigation of the repeatability of evolution at the genomic level. Such investigation is particularly powerful when applied to a ring species, in which spatial variation represents changes during the evolution of two species from one. We examined genomic variation among three subspecies of the greenish warbler ring species, using genotypes at 13 013 950 nucleotide sites along a new greenish warbler consensus genome assembly. Genomic regions of low within-group variation are remarkably consistent between the three populations. These regions show high relative differentiation but low absolute differentiation between populations. Comparisons with outgroup species show the locations of these peaks of relative differentiation are not well explained by phylogenetically conserved variation in recombination rates or selection. These patterns are consistent with a model in which selection in an ancestral form has reduced variation at some parts of the genome, and those same regions experience recurrent selection that subsequently reduces variation within each subspecies. The degree of heterogeneity in nucleotide diversity is greater than explained by models of background selection, but is consistent with selective sweeps. Given the evidence that greenish warblers have had both population differentiation for a long period of time and periods of gene flow between those populations, we propose that some genomic regions underwent selective sweeps over a broad geographic area followed by within-population selection-induced reductions in variation. An important implication of this ‘sweep-before-differentiation’ model is that genomic regions of high relative differentiation may have moved among populations more recently than other genomic regions.

Congrats to coauthors Dave Toews, Alan Brelsford, Christine Grossen, and Borja Milá on our publication of patterns of genomic variation within and among four forms of yellow-rumped warblers:

Toews, D.P.L., A. Brelsford, B. Milá, C. Grossen, and D.E. Irwin. 2016. Genomic variation across the yellow-rumped warbler species complex. The Auk: Ornithological Advances 133: 698-717. Link

Abstract:
Populations that have experienced long periods of geographic isolation will diverge over time. The application of high-throughput sequencing technologies to study the genomes of related taxa now allows us to quantify, at a fine scale, the consequences of this divergence across the genome. Throughout a number of studies, a notable pattern has emerged. In many cases, estimates of differentiation across the genome are strongly heterogeneous; however, the evolutionary processes driving this striking pattern are still unclear. Here we quantified genomic variation across several groups within the Yellow-rumped Warbler species complex (Setophaga spp.), a group of North and Central American wood warblers. We showed that genomic variation is highly heterogeneous between some taxa and that these regions of high differentiation are relatively small compared to those in other study systems. We found that the clusters of highly differentiated markers between taxa occur in gene-rich regions of the genome and exhibit low within-population diversity. We suggest these patterns are consistent with selection, shaping genomic divergence in similar genomic regions across the different populations. Our study also confirms previous results relying on fewer genetic markers that several of the phenotypically distinct groups in the system are also genomically highly differentiated, likely to the point of full species status.

Congrats to Saminda!

(Saminda Fernando is a PhD student at the University of Colombo who is advised by lab alumnus Dr. Sampath Seneviratne and co-advised by Darren Irwin)

Fernando, S.P., D.E. Irwin, and S.S. Seneviratne. 2016. Phenotypic and genetic analysis support distinct species status of the Red-backed Woodpecker (Lesser Sri Lanka Flameback: Dinopium psarodes) of Sri Lanka. The Auk: Ornithological Advances 133: 497-511. Link

Abstract:  Hybridization has challenged taxonomy, since hybridizing forms could be stable evolutionary entities or ephemeral forms that are blending together. The island of Sri Lanka has 2 subspecies of the flameback woodpecker D. benghalense: D. b. jaffnense in the north and D. b. psarodes in the south. Red plumage separates the endemic phenotype D. b. psarodes from other subspecies of D. benghalense. Despite these differences, intermediate phenotypes in north-central Sri Lanka discouraged the elevation of D. b. psarodes into a full species. The recent HBW and BirdLife International checklist, however, has elevated D. b. psarodes to a full species (D. psarodes), primarily based on its plumage. To objectively evaluate whether this taxonomic elevation is warranted, we examined the phenotypic and genetic affinities of D. psarodes within the D. benghalense cluster. In doing that we provide the first quantitative phenotypic and genetic analysis across a hybrid zone for an Old World woodpecker group. We sampled woodpeckers along a line transect across the island and measured body shape/size, plumage, and genetic variation in a mitochondrial gene (Cytb). Plumage color ranged from red in the south to yellow in the north, with varying proportions of orange in north-central Sri Lanka (an area of ~66 km). Morphology (body shape/size) and plumage characters showed a clear separation. There are 2 mitochondrial haplotype groups, one in the north and one in the south. A mixture of north and south haplotypes were seen in north-central Sri Lanka. Width of the hybrid zone suggests that some form of selection limits the spread of hybrids into the range of parental forms. Morphological, plumage, and genetic traits are all indicative of limited hybridization in a narrow zone between the 2 taxa, supporting the treatment of D. psarodes as a distinct species. This study provides an illustrative example of extensive hybridization between stable taxonomic entities, discouraging the practice of merging hybridizing forms as single species.

We wrote this review as an invited contribution to the Encyclopedia of Evolutionary Biology. The full citation:

Irwin, D.E., and D.B. Wake. 2016. Ring species. Vol. 3, Pages 467-475 in Encyclopedia of Evolutionary Biology, edited by R. M. Kliman. Oxford: Academic Press.

To read, you have three options (I recommend number 3):

1. Purchase the article from Science Direct for $31.50:    http://www.sciencedirect.com/science/article/pii/B9780128000496000779
2. Purchase the Encyclopedia from Elsevier for only $1,260.00    😉    http://store.elsevier.com/product.jsp?isbn=9780128000496
3. Email me a request to send you the PDF, and I will gladly do so.

The Abstract:
A ring species is a ring of populations in which there is only a single species boundary. Two contacting forms behave as distinct species yet are connected by a long chain of populations through which there is gradual or stepwise change. Such situations provide an illustration of how the process of speciation, by which one species splits into two, can occur. Ring species are rare, but two cases provide good examples of how ring species can teach us about speciation: greenish warblers and Ensatina salamanders.