"It was a sight no one could ever forget. Over the range of hills, as far as eye could see, crept a darkening hem, ever longer and broader, until the shadow spread across the slope from east to west, then downwards, downwards, uncannily swift, and all the green herbiage of that wide vista was being mowed down as if by a giant sickle, leaving only the vat moving shadow, extending, deepening, and moving rapidly nearer."
While not one of the "monsters of god" set out in Daniel Quamen's eponymous book, legions of army ants on the move can be terrifying, as illustrated in the quote about from Carl Stephenson's classic 1950's pulp short story, Leiningen versus the Ants. Recently, the genetic history of these insects has been used for more benign and academic purposes, wherein a team of researchers has used genetic evidence to enlighten our understanding of the geographic development of Central America.
The study, by Winston et al.[1] and entitled Early and dynamic colionization of Central America drives speciation in Neotropical army ants, assessed genomic DNA differences between nine Eciton species (E. vagans, E. quadriglume, E. rapax, E. dulcium, E. mexicanum, E. lucanoides, E. burchellii, E. drepanophorum and E. hamatum) for sequence diversity over spatial habitats ranging from Brazil to Mexico. Phylogeny was inferred from 146 specimens comprising 419,804 loci and 6,700,494 nucleotide sites therein. It was motivated by suggestive evidence that the conventional explanation of how and when the Isthmus of Panama was formed (and the biological consequences thereof) was incomplete. Until recently, the Isthmus was believed to have formed about 3 million years ago (Ma) and to have resulted in an intermingling of species that had evolved independently in North and South America (called the Great American Biotic Exchange). However, physical evidence inconsistent with this scenario has been detected, including difference in salinity and benthic carbon between the Pacific Ocean and the Caribbean arising between 4.2 and 4.7 million years ago; Panamanian fluvial deposits in South America that suggest closure of the Isthmus between 13-15 million years ago; and tectonic reconstructions suggesting formation of a volcanic arc about 12 million years ago and development of a series of islands in the area of the Isthmus about 6 million years ago. There has also been a suggestion that "corridors" arose in the area of the Isthmus surrounding a volcanic area in modern-day Nicaragua and Costa Rica.
In the face of this uncertainty, the authors chose to make genomic DNA comparisons of Eciton species because these nomadic ants have wingless queens and cannot swim or use foliage or flotsam to bridge water barriers by rafting, indicating that the results of allopatric speciation events could illustrate the history of how the Isthmus arose in the relevant paleogeographic time period. The paper describes two alternative hypotheses for this speciation: the Full Closure Colonization (FCC) model, which depends on Eciton colonization after closure of the Isthmus about 3 million years ago; and the Early Dynamic Colonization (EDC) model, which posits episodic, earlier colonization as land bridges, islands, and other land corridors arose (and sometimes retreated) over a much longer time period. With regard to the EDC model the authors state:
[D]ispersal into Central America presents both a novel ecological opportunity for colonization and an opportunity for clade diversification across a spaiotemporally complex landscape. Specifically, rather than a single migration across a suitable corridor into Central America, the complex landscape provided a series of patches of suitable habitat analogous to steppingstones with respect to the eventual colonization of Central America. In this model, we would likely infer in situ divesification/speciation before full formation of the isthmus (c. 3 Ma) as a result of dispersing across a complex geological substrate.
The results of genomic DNA comparisons of these ant species provide evidence that the EDC model is far superior to the FCC model and indeed that the FCC model is inconsistent with the speciation events that can be inferred from the genomic DNA comparisons. Divergence times for three specie coincident across the Nicaraguan-Costa Rican volcanic range (north of the Isthmus) are earlier (E. burchellii, 4.27 Ma; E. vagans, 5.5 Ma; E. mexicanum, 6.61 Ma) than predicted by the FCC model, for example.
The authors summarize their findings as follows:
As the early timing (4 –7 Ma) of Central American colonization by Neotropical army ants was robust to multiple calibrations, this provides strong evidence for ephemeral land bridges prior to the full closure of the {Isthmus]. We suggest that these early, inter-continental land connections played an important role in the dispersal of many taxa during the GABI, and likely functioned as a mechanism of speciation through alternating bouts of dispersal and vicariance. Considering that the majority of lineages in lower Central America exhibit genetic structure in this region—including other Neotropical army ant genera—we predict that support for cryptic diversification and the EDC model will only increase as more research is conducted on weakly dispersing Central American species. Lastly, our research suggests that the spatial and temporal complexity of the emerging isthmus needs to be seriously evaluated when invoking the commonly used biogeographic phenomenon of sweepstakes dispersal.
This paper once again illustrates how the ability to elucidate and compare genomic DNA in naturally occurring species can supply important evidence to validate -- or rebut -- hypotheses regarding subjects as diverse and evolutionary biology and paleogeography, providing a better understanding of the Earth's deep past, the organisms living at those times and how the modern world developed from those beginnings.
[1] Winston, Kronauer and Moreau, Molec. Ecol., doi: 10.1111/mec.13846 (October 25, 2016)
