Published: Assortative mating and hybrid zones


Darren’s simulation study of the effects of assortative mating and low hybrid fitness on hybrid zones was published this week in The American Naturalist.

The citation:
Irwin, D.E. 2020. Assortative mating in hybrid zones is remarkably ineffective in promoting speciation. American Naturalist, published online May 4, 2020: Link
(earlier version posted on bioRxiv: Link )

The Abstract:
Partial prezygotic isolation is often viewed as more important than partial postzygotic isolation (low fitness of hybrids) early in the process of speciation. I simulate secondary contact between two populations (species) to examine effects of assortative mating and low hybrid fitness in preventing blending. A small reduction in hybrid fitness (e.g., by 10%) produces a narrower hybrid zone than a strong but imperfect mating preference (e.g., 10 times stronger preference for conspecific over heterospecific mates). In the latter case, rare F1 hybrids find each other attractive (due to assortative mating), leading to the buildup of a continuum of intermediates. The weakness of assortative mating compared with reduced fitness of hybrids in preventing blending is robust to varying genetic bases of these traits. Assortative mating is most powerful in limiting blending when it is encoded by a single locus or is essentially complete, or when there is a large mate search cost. In these cases assortative mating is likely to cause hybrids to have low fitness, due to frequency-dependent mating disadvantage of individuals of rare mating types. These results prompt a questioning of the concept of partial prezygotic isolation, since it is not very isolating unless there is also postzygotic isolation.

To see videos of example simulations, see here.

Ethan Linck converted the HZAM code to a web app where you can play with the parameters and watch how the model responds.

Congrats to Dr. Silu Wang!


On Wednesday, Dec. 4th, Silu gave an outstanding presentation and defence of her PhD dissertation, titled “Genomic Architecture of Speciation in a Warbler Species Complex.”

It has been a great pleasure to serve as Silu’s advisor. We all wish her well in her next scientific adventure, as a postdoctoral research in Dr. Doris Bachtrog’s research group at UC Berkeley.

Congratulations Dr. Wang!!!

P.S. Silu’s first data chapter is published, and more are on the way. Here is the first from her PhD:

Wang, S., S. Rohwer, K. Delmore, and D.E. Irwin. 2019. Cross-decades stability of an avian hybrid zone. Journal of Evolutionary Biology 32: 1242-1251. Link

Silu Wang publishes JEB paper on stability of Townsend’s / Hermit warbler hybrid zone


Congratulations to Silu Wang, who has published her first paper on the Townsend’s / Hermit Warbler hybrid zone, showing surprising stability of the zone over a period of 25 years.

Silu and I are grateful to coauthor Sievert Rohwer, who collaborated on this project and allowed access to his many samples of warbler tissues from the 1987-1994 time period. We also thank coauthor Kira Delmore for her contributions to the genomic analysis.

Here is Silu’s painting inspired by the hybrid zone:

The full citation:
Wang, S., S. Rohwer, K. Delmore, and D.E. Irwin. Cross-decades stability of an avian hybrid zone. Journal of Evolutionary Biology 32: 1242-1251. Link

The Abstract:
Hybrid zones are particularly valuable for understanding the evolution of partial reproductive isolation between differentiated populations. An increasing number of hybrid zones have been inferred to move over time, but in most such cases zone movement has not been tested with long‐term genomic data. The hybrid zone between Townsend’s Warblers (Setophaga townsendi) and Hermit Warblers (S. occidentalis) in the Washington Cascades was previously inferred to be moving from northern S. townsendi southwards towards S. occidentalis, based on plumage and behavioural patterns as well as a 2000‐km genetic wake of hermit mitochondrial DNA (mtDNA) in coastal Townsend’s Warblers. We directly tested whether hybrid zone position has changed over 2–3 decades by tracking plumage, mtDNA and nuclear genomic variation across the hybrid zone over two sampling periods (1987–94 and 2015–16). Surprisingly, there was no significant movement in genomic or plumage cline centres between the two time periods. Plumage cline widths were narrower than expected by neutral diffusion, consistent with a ‘tension zone’ model, in which selection against hybrids is balanced by movement of parental forms into the zone. Our results indicate that this hybrid zone is either stable in its location or moving at a rate that is not detectable over 2–3 decades. Despite considerable gene flow, the stable clines in multiple phenotypic and genotypic characters over decades suggest evolutionary stability of this young pair of sister species, allowing divergence to continue. We propose a novel biogeographic scenario to explain these patterns: rather than the hybrid zone having moved thousands of kilometres to its current position, inland Townsend’s met coastal Hermit Warbler populations along a broad front of the British Columbia and Alaska coast and hybridization led to replacement of the Hermit Warbler plumage with Townsend’s Warbler plumage patterns along this coastline. Hence, hybrid zones along British Columbia and Alaska moved only a short distance from the inland to the coast, whereas the Hermit Warbler phenotype appears stable in Washington and further south. This case provides an example of the complex biogeographic processes that have led to the distribution of current phenotypes within and among closely related species.

Read about Jamie Clarke’s sapsucker adventures in Trek Magazine


Jamie Clarke, an undergraduate researcher in our group, teamed up with a reporter from Trek Magazine, Rachel Glassman, to write an article about Jamie’s summer research on sapsuckers. Jamie worked closely with PhD student Libby Natola on a study of 3 species of hybridizing sapsuckers (Red-breasted, Red-naped, and Yellow-bellied Sapsuckers) in central British Columbia.

Here’s a link to the article:

Silu Wang wins ASN poster award


Congratulations to PhD candidate Silu Wang for being awarded the Ruth Patrick Student Poster Award (awarded by the American Society of Naturalists) at the Evolution 2019 conference!

Here is Silu’s poster, which is beautiful both visually and in terms of scientific content:

Please note that the manuscript behind this poster is currently under review, thus the analysis/content might be subject to change.

Lab snowshoeing trip


On February 10th, we had a great time snowshoeing at Cypress Mountain. As a very experienced snowshoer, Silu led the way. Rashika in particular had fun, as it was her first time experiencing snow. And Maddie was enthusiastic, sort of a pre-celebration of her upcoming and now very successful MSc defence. Libby, Kenny, Ellen and I all had fun too.

Congrats to Madelyn Ore, M.Sc.!


Congrats to Madelyn (“Maddie”) Ore for her excellent M.Sc. defence! Maddie’s thesis is titled “Geographic variation in song and genetics in the Townsend’s Warbler (Setophaga townsendi).” Maddie defended her thesis on March 1, and will be entering the Ph.D. program at Cornell University in September 2019.

Armando interviewed by CBC Radio about Haida Gwaii goshawks


Dr. Geraldes did a great job with interviews on two CBC Radio stations, both on January 21st. You can listen here:

CBC Radio Victoria
CBC Radio Kelowna

These are both about our recent paper (open access!) on genomic variation in Northern Goshawks:

Geraldes, A.*, K. Askelson*, E. Nikelski, F.I. Doyle, W.L. Harrower, K. Winker, and D.E. Irwin. 2018. Population genomic analyses reveal a highly differentiated and endangered genetic cluster of northern goshawks (Accipiter gentilis laingi) in Haida Gwaii. Evolutionary Applications, online Early View: (*Shared first authorship)

For more about this paper, see:

Published: genomic differentiation of goshawks


This week, our paper on genomic differentiation of Northern Goshawks was published. Sylvia Heredia of UBC Biology produced this wonderful graphic that summarizes the paper:

The Abstract:

Accurate knowledge of geographic ranges and genetic relationships among populations is important when managing a species or population of conservation concern. Along the western coast of Canada, a subspecies of the northern goshawk (Accipiter gentilis laingi) is legally designated as Threatened. The range and distinctness of this form, in comparison with the broadly distributed North American subspecies (Accipiter gentilis atricapillus), is unclear. Given this morphological uncertainty, we analyzed genomic relationships in thousands of single nucleotide polymorphisms identified using genotyping‐by‐sequencing of high‐quality genetic samples. Results revealed a genetically distinct population of northern goshawks on the archipelago of Haida Gwaii and subtle structuring among other North American sampling regions. We then developed genotyping assays for ten loci that are highly differentiated between the two main genetic clusters, allowing inclusion of hundreds of low‐quality samples and confirming that the distinct genetic cluster is restricted to Haida Gwaii. As the laingi form was originally described as being based on Haida Gwaii (where the type specimen is from), further morphological analysis may result in this name being restricted to the Haida Gwaii genetic cluster. Regardless of taxonomic treatment, the distinct Haida Gwaii genetic cluster along with the small and declining population size of the Haida Gwaii population suggests a high risk of extinction of an ecologically and genetically distinct form of northern goshawk. Outside of Haida Gwaii, sampling regions along the coast of BC and southeast Alaska (often considered regions inhabited by laingi) show some subtle differentiation from other North American regions. These results will increase the effectiveness of conservation management of northern goshawks in northwestern North America. More broadly, other conservation‐related studies of genetic variation may benefit from the two‐step approach we employed that first surveys genomic variation using high‐quality samples and then genotypes low‐quality samples at particularly informative loci.

The citation and link:

Geraldes, A.*, K. Askelson*, E. Nikelski, F.I. Doyle, W.L. Harrower, K. Winker, and D.E. Irwin. 2018. Population genomic analyses reveal a highly differentiated and endangered genetic cluster of northern goshawks (Accipiter gentilis laingi) in Haida Gwaii. Evolutionary Applications, online Early View: (*Shared first authorship)

The UBC press release:

Media coverage:
CBC Radio interviews
CBC article by The Canadian Press

We thank the many sample contributors for their contributions to this study (see the Acknowledgements section of the paper).

This research was funded by grants from Genome British Columbia; British Columbia Ministry of Forests, Lands and Natural Resource Operations; Coast Forest Products Association; Western Forest Products; and the Natural Sciences and Engineering Research Council of Canada.