The sun bear (Helarctos malayanus) is the smallest and least studied of the eight living bear species. It inhabits the tropical forests of Southeast Asia, where it feeds mainly on fruits, honey, and insects. The sun bear has a distinctive orange-yellow crescent on its chest, which gives it its name. But how is this elusive bear related to other bears, such as the sloth bear (Melursus ursinus) and the polar bear (Ursus maritimus)?
The Ursidae Family Tree
To answer this question, we need to look at the phylogenetic tree of the Ursidae family, which shows the evolutionary relationships among living and extinct bear species. A phylogenetic tree is a diagram that represents the history of divergence and common ancestry of different organisms, based on their genetic or morphological similarities and differences.
There are different methods to construct a phylogenetic tree, such as using morphological characters, fossil records, or molecular data. In recent years, molecular data, especially from mitochondrial DNA (mtDNA), has been widely used to infer the phylogeny of bears, because mtDNA is inherited maternally and evolves faster than nuclear DNA.
However, different mtDNA sequences may yield different phylogenetic trees, depending on the evolutionary processes that affect them, such as incomplete lineage sorting (ILS) and gene flow. ILS occurs when ancestral polymorphisms persist through species divergences, resulting in discordant gene trees. Gene flow occurs when interbreeding between closely related species transfers genetic material across species boundaries, resulting in hybridization.
The Sun Bear’s Place in the Phylogeny
According to a recent study that analyzed complete mtDNA genome sequences from all living bear species, the sun bear is more closely related to the sloth bear than to the polar bear. The study found strong evidence that the spectacled bear (Tremarctos ornatus) diverged first from the common ancestor of all bears, followed by the giant panda (Ailuropoda melanoleuca). Within the genus Ursus, the sloth bear is the sister taxon of all the other five ursines: the brown bear (Ursus arctos), the polar bear, the American black bear (Ursus americanus), the Asian black bear (Ursus thibetanus), and the sun bear. The latter group is divided into two clades: one consisting of the brown bear and the polar bear, and another consisting of the two black bears and the sun bear.
This mtDNA-based phylogeny is consistent with another study that used multiple nuclear loci from several individuals per species. The study also found considerable heterogeneity among nuclear loci and discordance between nuclear and mtDNA phylogenies, suggesting that both ILS and gene flow are prominent evolutionary forces in ursid evolution. For example, gene flow was detected from Asian black bears into sloth bears, and from brown bears into American black bears.
However, not all studies agree on this phylogeny. Some studies based on single or partial mtDNA sequences have suggested different relationships among ursid species. For example, one study found that the American black bear and the Asian black bear are more closely related to each other than to any other ursine species. Another study found that the sun bear and the sloth bear do not share any common ancestors with other ursids.
These conflicting results may be due to sampling bias, methodological limitations, or evolutionary complexities. Therefore, more data and analyses are needed to resolve the phylogeny of bears with confidence.
Conclusion
The sun bear is a fascinating animal that deserves more attention and conservation efforts. According to the most comprehensive mtDNA-based phylogeny of bears, it is more closely related to the sloth bear than to the polar bear. However, this phylogeny may not reflect the true history of ursid evolution, as it may be affected by ILS and gene flow. More studies using multiple independent loci and coalescence-based methods are needed to fully understand the complex evolutionary patterns of bears.
