Using evolution in conservation: Genomic vulnerability of the Little Greenbul

Taking into account future climatic changes.

You are probably familiar with the words of Theodosius Dobzhansky: “Nothing in biology makes sense except in the light of evolution.” The statement has been applied to many different disciplines, including conservation biology. If we want to prevent species from going extinct, we need to understand how they evolve. This knowledge is especially relevant in the context of current climate change where species are forced to adapt, move or die. By studying how certain species coped with past climatic changes, we can gain insights into possible future responses and take appropriate conservation measures. A recent study in the journal Evolutionary Applications focused on the Little Greenbul (Andropadus virens) in Central Africa. How will this passerine deal with future environmental changes?

Genomic Vulnerability

The researchers tried to estimate the “genomic vulnerability” of the Little Greenbul. This concept refers to the mismatch between current and predicted genomic variation based on current relationships between genotypes and environmental variables. If this sounds like jibber jabber to you, let me walk you through the different steps. First, the researchers captured the genetic variation across populations of the Little Greenbul (using RADseq). Next, they correlated the uncovered genetic variants with environmental variables, such as temperature and precipitation. The resulting correlations give an indication which genotypes are associated with particular environmental conditions. For example, a genetic variant (or SNP) on chromosome 11 might correlate strongly with local rainfall, suggesting it is involved in local adaptation to wet or dry conditions. Finally, the researchers modelled future climatic changes (in 2080) and other disturbances, such as logging and mining, and correlated the current genomic variation with these future conditions. The degree of mismatch in genotype-environment correlations for the current and the future situation gives an indication of the genomic vulnerability of a species. If a correlation remains similar, there is no problem. But if a correlation weakens or disappears in the future, the Little Greenbul is in trouble.

Rapid Evolution Required

The analyses revealed that populations in coastal and central areas of Cameroon will require large changes in genomic variation, while populations in forest-savannah transitions are relatively safe. The situation is nicely illustrated in the map below where green areas correspond to low genomic vulnerability and red areas to high genomic vulnerability. The authors note that “in the most genomically vulnerable areas of Cameroon, they will need to evolve at a rate faster than they have done since the LGM [i.e. last glacial maximum] – a magnitude of change in 50 years likely beyond the limits of biological reality.” That is bad news for the Little Greenbul.

A map of Cameroon showing the genomic vulnerability of the Little Greenbul based on the genomic mismatch between current and future conditions. From: Smith et al. (2021).

Future Distributions

This study nicely shows how we can use evolutionary information in conservation research. In particular, these results highlight that we should not only focus on the current distributions of endangered species, but also take into account future range shifts. Conservation often gives the impression of preserving a static situation, but the reality is that the world is constantly and rapidly changing. And we have to keep up.

References

Smith, T. B., Fuller, T. L., Zhen, Y., Zaunbrecher, V., Thomassen, H. A., Njabo, K., … & Harrigan, R. J. (2021). Genomic vulnerability and socio‐economic threats under climate change in an African rainforest bird. Evolutionary Applications, 14(5), 1239-1247.

Featured image: Little Greenbul (Andropadus virens) © Francesco Veronesi | Wikimedia Commons

Should I stay or should I go? Patterns of gene flow across land bridges in Southeast Asia

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.