The Question of Speciation within the African Elephants
The African elephant, genus Loxodanta, has recently been under scientific scrutiny to determine whether the populations make up two separate species, or solely one. The populations that showed evidence for speciation are the elephants that inhabit the savanna (Loxodanta africana), and those that inhabit the forests of Africa (Loxodanta cyclotis). Although the question of speciation has only been debated recently, morphological differences along with nuclear DNA testing, mitochondrial DNA testing, and microsatellite loci analyses have been used to determine that the rift between the African forest elephants and the African savanna elephants was established more than three million years ago (Roca et al., 2007).
The methods for determining speciation in African elephants have evolved over the years. Grubb and his group of researchers (Grubb et al., 2000) were the some of the first to gather relevant and significant evidence in analyzing morphological differences. They measured and recorded the skull measurements from 295 elephants ranging over all of Africa south of the Sahara. They concluded based on significant differences in morphology and behavior that the savanna and forest elephants are different species (Grubb et al., 2000).
While the morphological analyses were invaluable to future researchers, it was not enough to persuade many. As technology developed, genetic testing became more developed and more easily performed. DNA analysis methods became invaluable for researchers who studied speciation in the African elephants. Rohland et al. (2010) used shotgun sequencing on nuclear DNA on Asian elephants, the tooth of a Mammoth, African savanna elephants and African forest elephants. Shotgun sequencing is a method used for sequencing long strands of DNA by sequencing seemingly random smaller fragments one at a time. The fragments overlap and from the fragments, a whole sequence can be pieced together. Rohland et al. (2010) used the DNA sequences pieced together from the most similar portions of the data, and then compared the four groups. Getting significant differences from these results demonstrated that even the most similar genes between forest and savanna elephants are too different for them to be considered the same species (Table 1). They found that the genetic divergence between forest and savanna elephants was great and that it supported the hypothesis that they are two distinct species.
Table 1 (Rohland et al., 2010)
A more recent study performed by Ishida et al. (2011) placed more significance on mitochondrial DNA (mtDNA). They claimed that it was important to look at mtDNA since recombination does not occur in mtDNA, and because there is little migration of genes since the herds rarely leave their geographic locations. These are all reasons why previous researchers decided not to analyze mtDNA. One of the biggest arguments in choosing to include mtDNA is that since the females don’t leave the herd, but the males migrate and interbreed, the nuclear DNA and mtDNA would have different evolutionary trajectories (Ishida et al., 2011).
Ishida et al. (2011) determined genotypes at 15 nuclear microsatellite locations in samples from across the continent of Africa and evaluated the results with a Bayesian clustering analysis which is simply a population distribution model algorithm. Based on this analysis, they were able to divide the elephants into two distinct groups, the Savannah and the Forest (Figure 2). There was an extremely low percentage of overlap between the two groups, and only really in one intermediate area in Garamba. However, even the intermediate genotypes of the Garamba elephants could not be considered true hybrids (Figure 2A).
Figure 2 (Ishida et. al., 2011)
When they partitioned the elephants based on the microsatellite genotypes they had 99.5% assignment of forest elephants to forest elephant clusters and 99.4% assignment of savannah elephants to savannah elephant clusters with only a few hybrid individuals found in Garamba and Cameroon. An important distinction they made was that these were only hybrid individuals, and not hybrid populations. Ishida et al. (2011) found that the differences between the forest and savanna genotypes to be significant (p<10^-4) and concluded that they were in fact different species. Even after they went back and evened out the sample numbers or increased the number of microsatellite loci, they continued to obtained significant results, indicating that their results were statistically robust. Ishida et al (2011) also looked into the correlation between the results from the nuclear DNA microsatellites and the mtDNA. They were able to assign the nuclear DNA for savanna or forest elephants based on the mtDNA. They were also able to predict the geographic location based on the mtDNA, and found that the discrepancies between the results from nuclear loci versus mtDNA haplotypes could be resolved by looking at populations instead of individual elephants. By looking at either the nuclear loci or the mtDNA, they were able to determine the population location of the elephants.
Another important paper concerning the speciation of the African elephants was Roca et al (2007). They took data sets from previous studies and analyzed the information. They found that all the evidence favored a two-species concept, and not a one-species concept (Figure 3).
Figure 3 (Roca et al., 2007)
In summary, all three studies support the two species hypothesis. One of the reasons for studying this topic is to aid in the conservation of the African elephant. As many know, they are hunted and poached illegally for the ivory in their tusks. Conservationists wanted to be able to determine the origin of ivory that is confiscated to know where they needed to increase protection. Ishida et al. (2011) were able to determine the region the ivory came from, although it was not exact. Another reason from the conservationist point of view for wanting to know if there are two species is because each species would then be entitled to its own conservation plan, and these plans could be more specialized for each species. With all the evidence supporting the two species concept, it would appear as though the forest elephants and savanna elephants are being recognized as two separate species, and hopefully will be better protected.
Ishida Y, Oleksyk TK, Georgiadis NJ, David VA, Zhao K, et al. (2011) Reconciling Apparent Conflicts between Mitochondrial and Nuclear Phylogenies in African Elephants. PLoS ONE 6(6): e20642. doi:10.1371/journal.pone.0020642
Grubb P, Groves CP, Dudley JP, Shoshani J (2000) Living African elephants belong to two species: Loxodonta africana (Blumenbach, 1797) and Loxodonta cyclotis (Matschie, 1900). Elephant 2: 1–4.
Roca AL, Georgiadis N, O’Brien SJ (2007) Cyto-nuclear genomic dissociation and the African elephant species question. Quaternary International 169–170: 4–16.
Rohland N, Reich D, Mallick S, Meyer M, Green RE, et al. (2010) Genomic DNA Sequences from Mastodon and Woolly Mammoth Reveal Deep Speciation of Forest and Savanna Elephants. PLoS Biol 8(12): e1000564. doi:10.1371/journal.pbio.1000564