There Are More Giraffe Species Than You Think

tags: researchblogging.org, giraffe species, Giraffa camelopardalis, speciation, evolution, conservation, molecular phylogeny

How many species of giraffes are there? Well, it may surprise you to learn this, but some people have actually thought about this throughout the decades, and they decided that there is only one species, Giraffa camelopardalis. However, a paper published today in BMC Biology convincingly demonstrates that giraffes are actually comprised of at least six, and possibly as many as eleven separate species instead of just one, as originally thought.

According to findings published by a research team led by David Brown, a geneticist at University of California Los Angeles (UCLA), these giraffe species live in different regions of sub-saharan Africa and show distinct and easily visible differences in their patched markings that are so different that these populations had previously been classified into separate subspecies.

However, mitochondrial DNA (mtDNA) and nuclear microsatellite loci DNA analyses of six of the nine subspecies reveal that these populations are more distinct than previously thought; for example, the reticulated giraffe (Currently: Giraffa camelopardalis reticulata) in North Kenya, which has reddish round spots; and the Masai giraffe (Currently: Giraffa camelopardalis tippelskirchi) in South Kenya -- genetically separated from each other between 0.5 and 1.5 million years ago (Figure 1A, below);

"Using molecular techniques we found that giraffes can be classified into six groups that are reproductively isolated and not interbreeding," in the wild, said Brown. A biological species is defined as a group of organisms capable of interbreeding and producing fertile offspring. However, because this is a very imprecise definition, there are other, more precise, species concepts. With the advent of modern technologies, more precise definitions have been formulated based on similarities in DNA, morphology or song, or some combination of these.

This amount of genetic differentiation is unique in large-bodied and highly mobile African mammals, whose different populations are likely to meet in the wild. This implies that there are environmental and behavioral mechanisms that limit gene flow between these populations.

"There are no rivers or forests to prevent breeding, but some evolutionary process is keeping the two groups reproductively separated," Brown pointed out.

These evolutionary processes likely include sexual selection as well as behavioral isolation, particularly distinct breeding seasons that could result in offspring that are born in the inappropriate season and demonstrating reduced fitness. Additionally, there could be differences in local habitats that each species has adapted to which prevents hybridization.

But where did all these giraffe species originally come from? To determine the likely geographic origin of giraffes, the team analyzed the mtDNA diversity in all giraffes sampled and found that Masai and reticulated giraffes were most centrally placed using the minimum-spanning DNA network, suggesting that East Africa could represent the geographic origin for giraffes (Figure 1B, below);

These DNA data are consistent with the fact that the earliest fossil remains of Giraffa camelopardalis have been found in East Africa.

How are these giraffe species genetically related to each other? To investigate the relationships between the different populations of giraffes sampled, the researchers used a neighbor-joining analysis of shared genetic differences. This analysis revealed that giraffe genotypes were strongly clustered into subspecific groups (Figure 2, below);

These data suggest that the divergence times between the seven giraffe clades ranged from 0.13-0.37 million years (MY) between Masai and South African clades, to 0.54-1.62 MY between the southern clade (Masai, Angolan and South African giraffes) and the northern clade (West African, Rothschild's and reticulated giraffes). Divergence values for the northern giraffe grouping were intermediate, with West African and Rothschild's giraffes diverging about 0.16-0.46 MY ago, with these two species splitting from reticulated giraffes about 0.18-0.54 MY ago. These dates argue for a mid- to late- Pleistocene radiation of giraffes. This was a time of intense climatic change in sub-Saharan Africa.

Taken together, these data have important implications for conservation planning because different species of giraffes present different conservation priorities and challenges.

"Lumping all giraffes into one species obscures the reality that some kinds of giraffe are on the brink," said Brown. "Some of these populations number only a few hundred individuals and need immediate protection."

Sadly, there has been a 30% drop in the giraffe population, with total numbers less than 100,000, during the previous decade. By providing giraffe subspecies with full species status, this will help conservation organizations make plans to preserve the most threatened populations. These (sub)species include the Nigerian giraffe (Currently: Giraffa camelopardalis peralta), which numbers only 160 individuals, and the Rothschild giraffe (Currently: Giraffa camelopardalis rothschildi), which numbers only several hundred.

The International Giraffe Working Group (IGWG) is currently reviewing these animals' status and their decision will influence the status of giraffes on the IUCN Red List of threatened and endangered species.

Sources

Brown, D.M., Brenneman, R.A., Koepfli, K., Pollinger, J.P., Milá, B., Georgiadis, N.J., Louis, E.E., Grether, G.F., Jacobs, D.K., Wayne, R.K. (2007). Extensive population genetic structure in the giraffe. BMC Biology, 5(1), 57. DOI: 10.1186/1741-7007-5-57 [free PDF].

Luciano B. Beheregaray and Adalgisa Caccone. MiniReview: Cryptic biodiversity in a changing world. (2007) Journal of Biology 6:9 | doi:10.1186/jbiol60 [free PDF]. (further reading).

BBCNews (quotes).