Did it arise in situ or due to secondary contact? And did rivers play a role in the process?
There is more than one way to get a hybrid zone. Over the years, several models have been put forward to explain the formation of hybrid zones. The easiest model to envision is the origin of a secondary hybrid zone: two populations diverge in separate regions (i.e. allopatry) and establish secondary contact at some point. I have written about such hybrid zones (see for example here and here), which seem to be the most common type.
Hybrid zones can also arise in situ in response to environmental gradients. Imagine a population distributed across a landscape that changes from very wet to very dry . Over time, some individuals adapt to the wet areas whereas others learn to cope with dry conditions. The middle of the range contains a mixture of wet-adapted and dry-adapted individuals. This would be a primary hybrid zone. A possible example of this scenario is the Little Greenbul (Andropadus virens). You can read the whole story here.
Atlantic Forest Spinetails
In a recent Heredity paper, Henrique Batalha-Filho and his colleagues set out to test whether a hybrid zone between two Atlantic Forest spinetail species (Synallaxis) is of primary or secondary origin. The two species – the Rufous-capped Spinetail (S. ruficapilla) and the Bahia Spinetail (S. cinerea) – hybridize in a contact zone along the Doce and Jequitinhonha Rivers in Brazil. The location of this hybrid zone suggests that the rivers might play a role.
The researchers collected samples from more than 100 birds and sequenced a handful of molecular markers. The genetic analyses confirmed the location of the hybrid zone and indicated gene flow mainly from the Bahia Spinetail into the Rufous-capped Spinetail. Moreover, these species diverged about one million years ago.
But is the hybrid zone primary or secondary? Ecological niche modelling provided some clues. This technique predicts the distribution of a species across geographic space and time using environmental data. The analysis indicated that both species were geographically isolated in the past and established secondary contact at some point. This suggestion is further supported by the genetic data, which showed signatures of population expansion.
This observation leads to another question: did the rivers play a role? The answer seems to be no. The researchers found genetic material of the two species on both sides of the river. If the river was a barrier, the species would be restricted to separate banks. Moreover, the ecological niche modelling indicated range fragmentation in the past, unrelated to the rivers. So, they conclude that “isolation and divergence due to forest fragmentation during the last ca. 2 mya is the most likely scenario explaining the evolution of S. ruficapilla and S. cinerea in the Atlantic Forest.”
Batalha-Filho, H., Maldonado-Coelho, M., & Miyaki, C.Y. (2019). Historical climate changes and hybridization shaped the evolution of Atlantic Forest spinetails (Aves: Furnariidae). Heredity, 1.
This paper has been added to the Furnariidae page.