Land bridges can promote gene flow. But do the birds cross them?
The genetic patterns in present-day bird populations can often be explained by the glacial dynamics of the ice ages (during the Pleistocene, between 2.5 million and 11,000 years ago). On the Northern Hemisphere, huge ice sheets covered large parts of North America and Eurasia. These glaciers formed formidable barriers between plant and animal populations, which could not exchange genes any longer and started to diverge genetically. Hence, divergence times often align with glacial maxima on the northern half of our planet.
On the Southern Hemisphere, however, conditions were quite different, especially on Southeast Asian islands. The growth of expansive ice sheets in the north requires a lot of water, leading to a significant decrease in sea level. In the western part of the Indonesian archipelago, this drop in sea level resulted in the formation of land bridges between islands. These connections might allow previously isolated island populations to exchange genetic material. A recent study in the journal Molecular Ecology took a closer look at five bird species (two babblers and three bulbuls) on Singapore, Sumatra and Borneo to determine whether land bridges promoted gene flow between these island populations.
The Indonesian archipelago now comprises several islands, but during glacial maxima these islands were connected by land bridges (indicated in grey). Did this result in gene flow between bird populations on the different islands? From: Cros et al. (2020) Molecular Ecology
Five Species
To infer whether gene flow occurred during the ice ages, Emilie Cros and her colleagues generated DNA sequences for the following five species:
- Chestnut-winged Babbler (Cyanoderma erythropterum)
- White-chested Babbler (Trichastoma rostratum)
- Olive-winged Bulbul (Pycnonotus plumosus)
- Asian Red-eyed Bulbul (Pycnonotus brunneus)
- Cream-vented Bulbul (Pycnonotus simplex)
Genetic analyses indicated that the island populations of these species diverged well before the Last Glacial Maximum (ca. 20,000 years ago). However, two species did show signatures of recent gene flow, namely the Asian Red-eyed Bulbul and the Olive-winged Bulbul. What can explain these different gene flow patterns?
Each circle is an individual bird and the color represents its genetic make-up. There is clear genetic divergence between the islands populations (most obvious in C. erythropterum here). In some species, however, there are signatures of recent gene flow. This is nicely illustrated by P. plumosus where different islands share genetic variation. Adapted from: Cros et al. (2020) Molecular Ecology.
Ecology Matters
A closer look at the ecology of these species reveals a striking pattern. The babbler species reside in the understory of the forest, while the bulbuls can be found in the canopy. A study in South America showed that canopy species have lower genetic divergence values compared to understory birds. The researchers attributed this difference to higher dispersal propensity of the canopy species. The same reasoning applies to the birds in Southeast Asia: the understory babblers might not have dispersed far during the ice ages and rarely crossed the land bridges between the islands.
In addition to the position in the forest (canopy vs. understory), other habitat features explain the gene flow patterns. Forest specialists, such as the Cream-vented Bulbul, do not venture outside the forest and thus need forested areas to disperse. The land bridges probably consisted of open habitats, including swamps, woodlands and savannas. Not the ideal habitats for forest specialists. More generalist species, such as the Olive-winged Bulbul, could survive in these non-forested areas and travel between islands. This ecological explanation is reflected in genetic patterns: the generalist (Olive-winged Bulbul) showed higher levels of gene flow compared to the habitat specialist (Cream-vented Bulbul). Ecology matters!
References
Cros, E., Chattopadhyay, B., Garg, K. M., Ng, N. S., Tomassi, S., Benedick, S., Edwards, D. P. & Rheindt, F. E. (2020). Quaternary land bridges have not been universal conduits of gene flow. Molecular Ecology, 29(14): 2692-2706.
Featured image: Red-eyed Bulbul (Pycnonotus brunneus) © Lip Kee | Wikimedia Commons
This paper has been added to the Pycnonotidae page.
Avian Hybrids 