An evolutionary journey across South America, culminating in secondary contact.
The Chinese philosopher Lao Tzu wrote that “the journey of a thousand miles begins with a single step.” This perspective can also be applied to the evolution of life on our planet. Countless species have spread across the globe – or particular parts of it – one step or one wingbeat at a time. Occasionally, these evolutionary journeys would culminate in the meeting of closely related populations that diverged thousands or millions of years ago. If reproductive isolation between these populations is still incomplete, hybridization and consequent introgression might occur. On this blog, I have mostly focused on the final part of this process: introgression and its consequences (see for example here and here). But it is definitely worthwhile to change perspectives and explore the evolutionary trajectories that different species followed before they established secondary contact. A recent study in the Biological Journal of the Linnean Society provides the ideal opportunity to do just that. Time to delve into the evolutionary story of two Antshrike species (genus Cymbilaimus)!
Spreading across South America
Let me start by introducing the main characters in our story: the Bamboo Antshrike (C. sanctaemariae) and the Fasciated Antshrike (C. lineatus). These two cryptic species have recently been split based on subtle morphological, vocal and ecological differences. For our ensuing journey across South America, however, it is not that relevant whether taxonomists classify them as species or subspecies.
Using molecular analyses and species distribution models, Leonardo Miranda and his colleagues reconstructed the evolutionary history of these two species. A good place to start the story of the Cymbilaimus Antshrikes is at the beginning. Between 1.85 and 0.6 million years ago, the Bamboo Antshrike and the Fasciated Antshrike went their separate ways. The ancestors of the Bamboo Antshrike probably followed their preferred habitat, which is – you guessed it – bamboo-dominated forests. This type of vegetation could be found in the lowlands of Amazonia during the Pleistocene, allowing the Bamboo Antshrikes to move into southern part of the Amazon region (blue and yellow regions in the figure below). The ancestors of the Fasciated Antshrike, on the other hand, resided in the northeastern part of South America and spread across the Guiana Shield (pink region) between 0.86 and 0.42 million years ago.
The story of the Fasciated Antshrike does not stop there. The researchers indicate that “our data also support a pattern of sequential dispersal episodes, almost simultaneous between 0.6 and 0.2 mya.” Similar to the Bamboo Antshrike following bamboo-dominated forests, the different populations of the Fasciated Antshrike probably tracked their favorite habitat across South America. These dispersal events established new populations in Central America and the southern section of the Amazon. In the latter region, the Fasciated Antshrike came into secondary contact with the Bamboo Antshrike which it had left behind more than one million years ago. Both species hybridized, resulting in the exchange of genetic material. The consequence of these introgression events remain to be elucidated. So, that is where our story ends (for now).
Although the taxonomy of these Antshrikes has little impact on our evolutionary story, it is interesting to briefly explore the relevance of the genetic patterns for classifying these birds. Currently, there are two species of Cymbilaimus with three subspecies within the Fasciated Antshrike. Based on the molecular analyses in this study, the authors noted that “current subspecific limits with C. lineatus are inconsistent with its evolutionary history.” Indeed, the genetic results point to three main groups that correspond to:
- The Bamboo Antshrike (C. sanctaemariae)
- The Guinana group of the Fasciated Antshrike (C. lineatus, subspecies lineatus)
- The remaining populations of the Fasciated Antshrike (C. lineatus, subspecies, intermedius and fasciatus)
The taxonomy of these birds might thus have to be updated. But that is a different story.
Miranda, L. S., Prestes, B. O., & Aleixo, A. (2021). Molecular systematics and phylogeography of a widespread Neotropical avian lineage: evidence for cryptic speciation with protracted gene flow throughout the Late Quaternary. Biological Journal of the Linnean Society, 132(2), 431-450.
Featured image: Fasciated Antshrike (Cymbilaimus lineatus) © Brian Gratwicke | Wikimedia Commons