Genetic study uncovers five distinct groups within this widely distributed songbird.
One of most studied bird species is a taxonomic mess. The widely distributed Great Tit complex (Parus major) has been divided into 43 subspecies and ornithologists are still debating where to draw species limits between all these subspecies.
In the Handbook of Birds of the World, you will find only one species, while the IOC World Bird List recognizes three distinct species: Great Tit (P. major), Japanese Tit (P. minor) and Cinereous Tit (P. cinereus).
These taxonomic disputes are interesting to follow, but I prefer to focus on the evolutionary history of these birds (as I have explained in previous blog posts, such as here and here).
However, an unstable taxonomy often indicates that something interesting is going on. A recent study in the Journal of Biogeography took a closer look at the Great Tit species complex and uncovered some peculiar patterns.
A Great Tit in Italy © Banellino | Wikmedia Commons
Five Groups
The researchers collected no less than 340 samples from 67 geographic populations.
They sequences several genes: the mitochondrial cytochrome b (Cytb) and NADH dehydrogenase subunit 2 (ND2) genes, as well as the nuclear β-fibrinogen intron 5 (Fib5) and the transforming growth factor beta 2 intron 5 (TGFB2).
Analyses of the mitochondrial genes revealed five main groups. The Northern and Western Eurasia group contains individuals from western Europe, the Iberian Peninsula and North Africa, as well as populations from across the north part of Eurasia all the way to the Russian Far East. T
axonomically, this large group corresponds to the nominate major. The remaining four groups are scattered across Asia.
The Central Asia group consists of individuals from Uzbekistan, Tajikistan and western China. The Eastern Asia group includes birds from East Asia and Southeast Asia, including China, South Korea, Japan, the Indochinese Peninsula, Malaysia and Java.
The Eastern Himalaya group spans the eastern portion of the Himalayas and corresponds to the subspecies tibetanus and subtibetanus. Finally; the Southern Asia group houses individuals from Sri Lanka, the Indian Subcontinent, and parts of Afghanistan.
The contrast between Western Europe (one group) and Asia (four groups) probably reflects the topographic complexity around the Himalayas with a diverse selection of habitats, providing ample opportunity for populations to diversify.
Distribution of the five main mitochondrial groups within the Great Tit species complex. From: Song et al. (2020) Journal of Biogeography
Taxonomic Grey Zone
Interestingly, analyses of the nuclear genes could not resolve the phylogenetic relationships between the populations.
This suggests that the mitochondrial groups diverged recently – about 1.5 million years ago according to their calculations – and that the nuclear variation has not been divided over these groups yet (a phenomenon known as incomplete lineage sorting).
Moreover, some populations might still be connected by occasional gene flow. For example, birds of the major and minor groups are known to hybridize in the Amur River area in the Russian Far East and China.
This pattern of clear mitochondrial groups and lack of nuclear population structure is common in Eurasian bird species that originated in the last few million years.
In my own work, I have found similar patterns in the Bean Goose complex, where Taiga (Anser fabalis fabalis) and Tundra Bean Goose (A. f. serrirostris) started diverging about 2.5 million years ago and occasionally interbreed.
These cases provide evolutionary biologists with the exciting opportunity to study speciation in action, but complicate taxonomic decisions.
Because speciation is still ongoing, these populations end up in a taxonomic grey zone, often resulting in subjective decisions and a proliferation of subspecies.
A Japanese Tit in Osaka © Laitche | Wikimedia Commons
An Afterthought
While reading this paper, I realized that the authors used four very common genetic markers (the mitochondrial Cytb and ND2, and the nuclear Fib5 and TGFB2). If you browse through the ornithological literature on phylogenetics and taxonomy, you will often come across these markers.
This suggests that a large part of our knowledge of avian evolutionary history is based on a tiny fraction of their genomes. Although these findings are sound and insightful, who knows how much we are missing?
My feeling that there is still much to discover with genomic data was strengthened when I read the book “Who We Are and How We Got Here” by David Reich.
This book provides an overview of the recent progress in research on human evolution using genomics and ancient DNA.
The findings are mind-blowing, especially the discovery of so many ghost populations that went extinct but left their genetic signatures in present-day populations (a fascinating concept that I recently covered in this BioEssays paper). It makes you wonder how many Great Tit ghost populations have wandered across Eurasia.
Lähde: A common species with a complex history: The evolutionary story of the Great Tit | Avian Hybrids
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