By: Katelyn Foley, Eileen Mello, Erica Shepherd, and Joe Varney (Stonehill College, BIO323 Evolution, Spring 2018)
When a species extends across large geographic ranges, it is often found that the individual populations of the species differ physically and genetically from one another, as these subgroups are exposed to a wide range of environmental conditions within the range. Typically, populations found at the edges of the geographic range, or peripheral populations, are more likely to have adaptations or evidence of new traits as a result of their environment. One peripheral population of interest, the elephants of the Namib Desert, demonstrate unique behaviors and physical adaptations in comparison to other African savanna elephant populations. Knowing this, researchers from the Department of Animal Sciences at the University of Illinois were interested in how these elephants might differ genetically, as they show different behaviors compared to populations that are nearby.
The elephants of the Namib Desert represent one of the few populations of African savanna elephants that thrive in the desert. Elephants have distinctive memory of their location and surroundings, as well as what niches they are close to that may have resources. This aptitude of desert elephants to be able to learn and adapt to new environments does equate to changes in their behavior, such as traveling for long periods of time without water, or for impressively far distances. Although previous research indicated that no migration occurred by the Namib elephants into or out of their northern Namib Desert habitat, a later study revealed that seasonal movement was detected in populations of desert elephants. Due to this potential gene flow between desert regions and Etosha National Park, where the migration was noted, certain genes could have been introduced into the gene pool that caused differing traits than other populations. Overall, this peripheral population of elephants exhibits behavioral changes which have not yet been explained by genetic similarities. These researchers were interested in the degree to which genetic mutations had occurred as a result of exposure to these desert environments, and they set up an experiment to investigate this.
To do so, the researchers first obtained blood, dung, or tissue samples from one elephant per herd at six geographically defined locations within Namibia: the Caprivi Strip, Hoanib and Hoarusib Desert, Central Kunene, Etosha, Huab River, and Ugab River [Figure 1]. They chose to work with the gene MT-ND5, mitochondrial DNA (mtDNA) marker, due to a higher mutation rate that lets it serve as a better genetic marker to compare diversity among the elephants. Nuclear DNA (nDNA) markers were also compared to examine the gene flow among populations. Only nDNA and mtDNA extracted from blood and tissue samples were used because the quality of DNA extracted from dung was determined to be too low for an accurate analysis. The collected DNA was then tested for significant variance to see if there was deviation from the Hardy-Weinberg equilibrium and linkage disequilibrium, which could indicate genetic differentiation between localities (desert vs. non-desert).
From this experiment, eight groups of genes (haplotypes) from mtDNA were commonly found amongst the subspecies of the study. These haplotypes were designated in a previous study and the researchers then categorized their elephants into these existing groups to see if similarities or trends of genes could be found. Even through utilizing the previous data, desert elephants were not found to be genetically different from non-desert elephants. The only differences they found were with Caprivi elephants compared to all other Namibia elephants. Nuclear DNA was also analyzed, and significant gene flow was found amongst the elephants. It was previously thought that this gene flow was from males only because they are the more migratory sex, but a simple test showed no migratory differentiation, indicating gene flow is from both males and females. Regardless, this gene flow works against genetic differentiation, explaining the insignificant results found.
There was also no correlation between genetic distance and geographic distance in Namibia. Possible reasons for this are discussed in the history of Namibia and the wars and over-hunting experienced over the last century. While desert elephants were not found to be genetically different than non-desert sub-populations (with the exception of Caprivi), the need for conservation of the desert-dwelling elephants was still addressed. Because there was likely no genetic component to the ability to survive in the desert, the researchers stated that it must be a behavioral adaptation. Using this ability to live in the desert, these elephants are essential for the arid ecosystem, as they are capable of clearing paths, producing water pools, and progressing seedling growth via seed dispersal. Due to the lack of behavioral adaptation, the non-desert elephants would be incapable of succeeding the desert elephants if extinction of the desert elephants were to occur. This is the essential argument the researchers make for the purpose of continued protection of elephants, especially the desert-dwellers.
Link to access the article: https://onlinelibrary.wiley.com/doi/full/10.1002/ece3.2352
Citation: Ishida, Y., P.J. Van Coeverden de Groot, K.E.A. Legget, A.S. Putnam, V.E. Fox, J. Lai, P.T. Boag, N.J. Georgiadis, and A.L. Roca. 2016. Genetic connectivity across marginal habitats: the elephants of the Namib Desert. Ecology and Evolution 6:6189–6201.