Ecological gradients might drive speciation in an African songbird
Ever heard of Wakwa, Ngaoundaba Ranch or Betare Oya? These are sites in Central Africa where Ying Zhen and colleagues collected samples of a small passerine bird: the Little Greenbul (Andropadus virens). You can find this songbird in the rainforest, but also as you walk towards the savannah. This transition from rainforest to savannah is characterized by less trees and less rainfall compared to the rainforest habitat. Biologists refer to such transitions as ecotones and think that they can play a crucial role in the origin of new species.
Many authors have speculated that rainforest-savannah ecotone in Central Africa has been driving speciation in the Little Greenbull by means of natural selection. However, previous genetic studies (based on “old-school” mtDNA and microsatellites) could not differentiate between ecotone and rainforest populations. Now, in a paper published in Molecular Ecology, researchers use the newest genomic tools to revisit this African species. Based on almost 50,000 genetic markers (Single-Nucleotide Polymorphisms or SNPs), they were able to discriminate between different Greenbul populations.
More Than Distance Alone
A first look at the data revealed that more distant populations were genetically more different. This pattern – known as isolation-by-distance – points to a neutral model of evolution, contrary to the expectation that natural selection is driving genetic divergence in Greenbuls. However, further analyses revealed that geographic distance alone cannot explain the observed genetic differentiation. Other processes, such as local adaptiation by means of natural selection, are also at work here.
Comparing SNPs between rainforest and ecotone populations uncovered several outliers that are probably under selection. These outliers include EDIL3, a calcium-binding protein that is involved in the formation of eggshels and MLXIPL, a protein that plays a role in fat deposition. It is tempting to speculate about possible roles for these genes in the different habitats, but one quickly threads into the dangerous territory of ‘Just-so-stories’. The message to take away here is that several genes seem to be under divergent natural selection, possibly driven by ecological differences.
In the end, these patterns are in line with a model of ecological speciation, where ‘natural selection caused by shifts in ecology can promote speciation.’ As evolutionary biologists interested in speciation, we surely live in interesting times.
Zhen Y, Harrigan RJ, Ruegg KC, Anderson EC, Ng TC, Lao S, Lohmueller KE, Smith TB. (2017). Genomic divergence across ecological gradients in the Central African rainforest songbird (Andropadus virens). Molecular Ecology. 26:4966-4977.
This paper has been added to the Pycnonotidae page.