Genetic study uncovers rampant introgression between distantly related bird species.
How long does it take before two species cannot hybridize any longer? In 1975, Ellen Prager and Allan Wilson estimated that ” the average hybridizable species pair diverged
from a common ancestor about 22 million years ago.” That is a long time! In comparison, mammals lose the ability to hybridize after about 3 million years. It is thus not so surprising that hybrids between distantly related bird species have been documented. A recent study in the Biological Journal of the Linnean Society reports gene flow between two species of African Barbets that diverged more than four million years ago.
Yellow-fronted Tinkerbird (Pogoniulus chrysoconus extoni) and Red-fronted Tinkerbird (P. pusillus pusillus) meet along a contact zone in Southern Africa. Ornithologists didn’t know whether these species hybridized, although Graham Ross documented orange-colored birds in this area. He wrote that this observation “strongly suggested that intergrading between the two species had occurred.”
Emmanuel Nwankwo and his colleagues collected birds within and outside the contact zone. Genetic analyses of these birds led to two striking findings. First, the Yellow-fronted Tinkerbird and Red-fronted Tinkerbird were not even sister species. In fact, the Red-fronted Tinkerbird is more closely related to another Yellow-fronted Tinkerbird: Pogoniulus chrysoconus chrysoconus. Hybridization between non-sister species occurs, but it less common than hybridization between sister species (see this review).
Second, the contact zone housed a lot of hybrids. The researchers write that “almost all individuals in the contact zone [showed] some evidence of introgression, as do some individuals in near allopatry.” It is no surprise that they mention “rampant introgression” in the title of their paper. The genetic analyses also showed that backcrossing mostly occurred in the Yellow-fronted Tinkerbird. This indicates that females of both species might have a preference for red-fronted males.
Gholamhosseini, A., Vardakis, M., Aliabadian, M., Nijman, V., & Vonk, R. (2013). Hybridization between sister taxa versus non-sister taxa: a case study in birds. Bird Study, 60(2):195-201.
Nwankwo, E.C., Mortega, K.G., Karageorgos, A., Ogolowa, B.O., Papagregoriou, G., Grether, G.F., Monadjem, A. & Kirschel, A.N.G. (2019). Rampant introgressive hybridization in Pogoniulus tinkerbirds (Piciformes: Lybiidae) despite millions of years of divergence. Biological Journal of the Linnean Society, 127(1):125-142.
Prager, E.M., & Wilson, A.C. (1975). Slow evolutionary loss of the potential for interspecific hybridization in birds: a manifestation of slow regulatory evolution. Proceedings of the National Academy of Sciences, 72(1):200-204.
Ross, G.J.B. (1970). The specific status and distribution of Pogoniulus pusillus (Dumont) and Pogoniulus chrysoconus (Temminck) in southern Africa. Ostrich, 41(3):200-204.
This paper has been added to the Piciformes page.