What selection pressure is acting on hybrids between Audubon’s and Myrtle Warbler?

Another attempt to characterize the selection pressure acting on warbler hybrids.

What form of selection is acting on hybrids between Audubon’s Warbler (Setophaga coronata auduboni) and Myrtle’s Warbler (S. c. coronata)? It is a question that is haunting several American ornithologists. Genetic analyses of a hybrid zone between these species indicate that there is some selection against hybrids, but the exact mechanism remains a mystery.

Some weeks ago I wrote about a paper that tested whether parasites might be involved. The answer was negative, so the search for the exact selection mechanism continues. The latest attempt was recently published in the journal The Auk.


An Audubon’s warbler (from: https://www.allaboutbirds.org/)


Comparing Classes

David Toews and his colleagues wanted to know if there are any differences in viability between male and female hybrids or between different age classes. If they uncover differences, this might provide important insights into what kind of selection is acting on the hybrids.

Females might be less fit compared to males. This pattern – known as Haldane’s Rule – has been observed in several bird species. If Haldane’s Rule also applies to these warblers, we would expect to find more early-generation male compared to female hybrids.

There might also be differences between age classes due to variation in migration strategies. If difference species follow distinct migration routes, hybrids might opt for an intermediate – and possibly sub-optimal – route. This could increase the mortality rate among hybrids. If this is the case, we would expect more early-generation hybrids before their migration.


A female Myrtle warbler (from: https://www.allaboutbirds.org/)



To test these two expectations, the researchers used triangle plots (I have written about these before). Here is the crystal-clear explanation from the paper:

“In these plots, individuals near the top of the triangle are first-generation hybrids (i.e. they are heterozygous at nearly all the highly divergent sites). Individuals falling along the right and left edges of the triangle are backcrosses to the parental forms; individuals within the center of the triangle are F2, F3, and subsequent hybrid classes.”


A triangle plot to deduce the genetic ancestry of hybrids (from Toews et al. 2018, The Auk)


And the selection pressure is …

These triangle plots were compared between the different classes (males vs. females and pre-migration vs. post-migration) and revealed…no differences. There seems to be no selection against a particular sex or age class. Hence, the quest for the hybrid warbler selection pressure will continue.



Toews, D.P.L., Lovette, I.J., Irwin, D.E. & Brelsford, A. (2018) Similar hybrid composition among different age and sex classes in the Myrtle-Audubon’s warbler hybrid zone. The Auk, 135: 1133-1145.


The paper has been added to the Parulidae page.


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