Genomic analyses reveal population fluctuations during the Pleistocene.
A few weeks ago, scientists announced that they almost completed a sequence of the human genome. This might come as a surprise: did we not sequence the human genome in 2003 when the results from the Human Genome Project were published? That genome sequence was actually incomplete, about 15% was missing (especially stretches of repetitive DNA are difficult to assemble). Similarly, many avian genome assemblies contain significant gaps (see this blog post for more details). However, scientists can still extract a lot of information from incomplete genome assemblies. For instance, you can reconstruct the demographic history of a species from one genome using a pairwise sequentially Markovian coalescent (PSMC) analysis. This technique – developed by Heng Li and Richard Durbin – is nicely explained by David Reich in his book Who We Are and How We Got Here:
A 2011 paper by Heng Li and Richard Durbin showed that the idea that a single person’s genome contains information about a multitude of ancestors was not just a theoretical possibility, but a reality. To decipher the deep history of a population from a single person’s DNA, Li and Durbin leveraged the fact that any single person actually carries not one but two genomes: one from his or her father and one from his or her mother. Thus it is possible to count the number of mutations separating the genome a person receives from his or her mother and the genome the person receives from his or her father to determine when they shared a common ancestor at each location. By examining the range of dates when these ancestors lived—plotting the ages of one hundred thousand Adams and Eves—Li and Durbin established the size of the ancestral population at different times. In a small population, there is a substantial chance that two randomly chosen genome sequences derive from the same parent genome sequence, because the individuals who carry them share a parent. However, in a large population the chance is far lower. Thus, the times in the past when the population size was low can be identified based on the periods in the past when a disproportionate fraction of lineages have evidence of sharing common ancestors.
A recent study in the journal BMC Genomics applied this PSMC analysis to the genome of a Chinese Grouse (Tetrastes sewerzowi). This forest-dwelling species can be found in the mountains east of the Qinghai-Tibet Plateau. It is currently considered “Near Threatened” because of population declines due to ongoing deforestation and fragmentation of its habitat.
The genomic analyses revealed that the population size of the Chinese Grouse has fluctuated over time. Populations decreased during early to middle Pleistocene but showed an expansion during late Pleistocene (between 30,000 and 40,000 years ago), followed by a sharp decline during the last glacial maximum (about 20,000 years ago). Similar patterns have been found in other bird species, highlighting the influence of the climatic cycles during the Pleistocene (see for example this study by Krystyna Nadachowska-Brzyska and her colleagues).
Next, the researchers focused on the underlying mechanisms of these population fluctuations. Why did the number of Chinese Grouse wax and wane during the Pleistocene? To answer this question, the researchers turned to Ecological Niche Modelling and reconstructed the distribution of the Chinese Grouse throughout the Pleistocene. This exercise showed that the population expansion during the late Pleistocene (30,000–40,000 years ago, also known as the Greatest Lake Period) can be explained by the warmer weather which allowed conifer forests, the primary habitat for Chinese Grouse, to reach their greatest extent. Later on, during colder periods, the coniferous habitat shrunk and the Chinese Grouse populations moved westwards into higher altitudes.
These findings indicate that the distribution of the Chinese Grouse is strongly dependent on the coniferous forest cover. It is thus essential to protect these fragmented forests and safeguard the future of this beautiful bird.
Song, K., Gao, B., Halvarsson, P., Fang, Y., Jiang, Y. X., Sun, Y. H., & Höglund, J. (2020). Genomic analysis of demographic history and ecological niche modeling in the endangered Chinese Grouse Tetrastes sewerzowi. BMC genomics, 21(1), 1-9.
Featured image: A drawing of Chinese Grouse (Tetrastes sewerzowi) © Ornithological Miscellany. Volume 2 | Wikimedia Commons