A new study on henna-tailed jungle-flycatcher and golden whistler might hold the answer to this question.
Most research on the Pleistocene ice ages has been focused on the Northern hemisphere where huge packs of ice covered the land. But what happened in the tropics? When large parts of the world’s seawater is locked up in ice, the worldwide sea levels will drop. In some regions, such as Australasia, this led to the formation of land bridges between islands. These bridges might enable previously isolated populations to come into contact and exchange genes. But do land bridges always lead to gene flow?
To answer this question, Kritika Garg and colleagues traveled to two Sulawesi islands: Peleng and Taliabu. These islands belong to different archipelago’s but have been connected by land bridges during the Pleistocene. The researchers looked for two bird species: the henna-tailed jungle-flycatcher (Cyornis colonus) and the golden whistler (Pachycephala pectoralis).
Introducing the Study Species
The golden whistler comprises a large radiation of numerous taxa across Melanesia. This study focuses on two subspecies: clio on Taliabu and pelengensis on Peleng. When it comes to diet, the golden whistler is a generalist that can easily switch between insects and seeds.
The henna-tailed jungle-flycatcher is divided into two subspecies which live on separate islands: one subspecies (colonus) occurs on Taliabu, while you can find the other subspecies (pelengensis) on Peleng. In contrast to the golden whistler, the henna-tailed jungle-flycatcher is specialized on catching insects in the air (hence the name flycatcher) and does not readily switch to a seed diet.
Land Bridge = Genetic Bridge?
The researchers collected DNA from henna-tailed jungle-flycatcher and golden whistler to reconstruct their evolutionary history. There was little evidence of genetic exchange between the jungle-flycatcher subspecies. Island populations of the golden whistler, however, have been exchanging genes during the Pleistocene. Genes mainly flowed from Peleng to Taliabu.
The differences in gene flow dynamics probably depend on the ecology of the species. The specialized jungle-flycatcher is known for its poor dispersal capacities. Because of its strictly insectivorous diet, it does not venture outside forests often. So, chances of crossing a land bridge are slim. The golden whistler, however, is a generalist that has little problems exploring new territories, including land bridges.
This study nicely shows that the existence of a land bridge does not automatically lead to genetic bridge between previously isolated populations. Other factors, such as ecology, should be taken into account.
Garg, K.M., Chattopadhyay, B., Wilton, P.R., Prawiradilaga, D.M. & Rheindt, F.E. (2018) Pleistocene land bridges act a semipermeable agents of avian gene flow in Wallacea. Molecular Phylogenetics and Evolution, 125:196-203.
The paper has been added to the Pachycephalidae page.