Ornithologists compiled a large genetic and morphometric data set to answer this question.
Here is a fun fact about the history of population genetics: one of the most important papers in this field was published in the Journal of Eugenics. The journal does not exist anymore (for obvious reasons), but the papers are still accessible for “scholarly use”. Luckily, because in this paper, Sewall Wright introduced the still widely used F-statistics for quantifying population structure. Another landmark paper by Wright appeared a few years earlier under the simple title “Isolation by Distance”, introducing another fundamental concept in population genetics. The idea behind isolation by distance (or IBD) is simple: it describes the process of increasing genetic differentiation correlated with increasing geographic distance. It makes intuitive sense: the farther apart two populations are, the less likely they will be connected by gene flow.
IBD, IBA or IBE?
However, genetic differentiation can also be due to other processes, such as isolation by adaptation (IBA) or isolation by environment (IBE). The difference between these two processes is subtle, but a recent study in the journal Ecology and Evolution explained it nicely:
Isolation by adaptation is defined as the effect of environmental gradients that results in divergent natural selection that can lead to adaptive phenotypic divergence between populations, resulting in a positive correlation between genetic divergence and adaptive phenotypic differentiation. Isolation by environment is defined as the occupation of two populations in different points on the ecological gradient. This process is observed when the phenotypic target of selection is unknown or is not easily measured, and then, the environmental variation can be used as a proxy and a positive correlation between genetic divergence and environmental dissimilarity is expected.
This study focused on the Red‐crowned Ant-tanager (Habia rubica), a bird species that can be found from central Mexico to northeastern Argentina and southeastern Brazil. What processes can explain the genetic patterns in this passerine: IBD, IBA or IBE?
Sandra Ramírez‐Barrera and her colleagues collected 124 mitochondrial DNA (mtDNA) sequences across the range of the Red‐crowned Ant-tanager. These sequences could be divided into seven phylogroups (based on previous analyses) The researchers correlated the genetic data with various morphological and environmental variables. They used a Multiple Matrix Regression with Randomization (MMRR) approach which estimates the independent contributions of environment and geography on genetic and phenotypic variation.
The results from the regression analyses were clear: only geographic distance had a significant relationship with genetic distance, indicating that isolation by distance (IBD) is the main process driving genetic differentiation in the Red-crowned Ant-tanager. Does this mean that the environment has no effect on the genetic structure of this tropical species? Not necessarily, the researchers might have missed some environmental factors that shaped the genetic patterns in Red-crowned Ant-tanager. More research is clearly warranted here.
The importance of isolation by distance highlights the potential role of geographic barriers in this particular species. Indeed, five Mesoamerican phylogroups are located in the northern (NP) and southern (SP) regions of the Mexican Pacific coast, on the slope of the Gulf of Mexico (GM), from Southeastern Mexico to Costa Rica (SE), and Panama (PA). This region is known as a highly fragmented topographic complex area that has been strongly influenced by climatic and geological events. Moreover, the other two phylogroups occur in Western (WS) and Eastern/Northwestern (ES) South America where they are separated by a diagonal strip of dry vegetation (I have written about this Open Vegetation Corridor before). Whether isolation by distance has rendered these phylogroups so genetically distinct that they should be considered separate species remains open for debate…
Ramírez‐Barrera, S. M., Velasco, J. A., Orozco‐Téllez, T. M., Vázquez‐López, A. M., & Hernández‐Baños, B. E. (2019). What drives genetic and phenotypic divergence in the Red‐crowned Ant tanager (Habia rubica, Aves: Cardinalidae), a polytypic species?. Ecology and Evolution, 9(21), 12339-12352.