Revealed: Why song sparrows are larger in colder climates

In a new study, scientists have unraveled the genetic underpinnings that account for the significant variation in body size among song sparrows, a songbird species well-recognized across North America.

This research not only sheds light on the intriguing differences observed in these birds but also contributes to our understanding of their potential adaptability to the impending shifts in climate.

Song sparrow size from Mexico to Alaska

The study hones in on eight genetic variants that are predominantly responsible for the body size disparity in song sparrows, which can range nearly threefold from Mexico to Alaska.

For example, individuals inhabiting the Aleutian Islands remain year-round and can be triple the size of their relatives found in California’s coastal marshes. This substantial size difference aligns with Bergmann’s rule, which suggests that larger sizes are favored in colder climates to aid in heat retention.

Katherine Carbeck, a doctoral candidate at the University of British Columbia, explains that while “locally adapted” populations are known to exist, the precise genetic mechanisms that enable such adaptations have been elusive until now.

“The existence of ‘locally adapted’ populations implies that natural selection has shaped the genetic makeup of song sparrow populations across their range, enabling individuals to survive and reproduce in drastically different climatic conditions,” said Carbeck. “However, the genetic mechanisms underlying Bergmann’s rule have remained elusive until now.”

Methodology and results

In collaboration with researchers from the Cornell Lab of Ornithology, University of Alaska, and Ouachita Baptist University, Carbeck used whole-genome sequencing to decode the song sparrow’s genome.

The team studied genetic samples from both the largest subspecies in the Aleutian Islands and smaller subspecies. They included those that migrate to warmer climates and those that reside in the milder winters of the British Columbia coast.

Genomic insights

Through their comprehensive analysis, the researchers pinpointed candidate genes associated with body mass. The identification of these eight specific genetic variants not only corroborates Bergmann’s rule but also enhances our understanding of evolution, natural selection, and climate interactions throughout the history of species.

Dr. Jen Walsh, a co-author from the Cornell Lab of Ornithology, highlights the rarity of identifying a limited number of genes with a substantial impact on body size variation. The song sparrows’ diverse phenotypic range provides a promising avenue for identifying genes that govern eco-geographic principles.

Walsh added, “From a genomic perspective, identifying a small number of candidate genes with an apparently large impact on variation in body size is really interesting. The magnificent range of phenotypic diversity seen in song sparrows suggest they offer exciting opportunities to identify genes underlying a host of well-known and generally accepted eco-geographic rules.”

Song sparrow habitat conservation

The study emphasizes the importance of habitat conservation for maintaining gene flow between populations, which is crucial in the face of ongoing environmental changes. Carbeck stresses the significance of facilitating gene exchange for future adaptability.

“Our results highlight the potential role habitat conservation plays in enabling the continued exchange of genes between populations — which is important in the face of ongoing change,” said Carbeck.

Resilient future for song sparrows

Dr. Peter Arcese from UBC suggests that the findings point to a potentially resilient future for song sparrows. The adaptability of these populations to climate change is conceivable, provided that habitat conditions support the free movement of individuals and their genetic material.

“Our findings imply that some, if not all, locally adapted song sparrow populations may continue to adapt to climate change, as long as we maintain habitat conditions that facilitate the movement of individuals and genes between populations,” he said.

In summary, this study on song sparrows is a step forward in understanding how species adapt to their environment. It underscores the intricate link between genetics and adaptation strategies, paving the way for future research in the field of conservation genetics and climate change biology.

The full study was published in the journal Nature Communications.

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