This wide-ranging seabird can be divided into three distinct groups.
The Southern Ocean is a vast expanse of water without any obvious barriers for widely wandering seabirds. So, you would expect to find little or no genetic differentiation between different island populations around Antarctica. The White-chinned Petrel (Procellaria aequinoctialis), for example, breeds on several subantarctic islands and is generally considered as one global population. However, some studies (based on isotopes and tracking data) found some differences in diet and foraging locations between birds from islands in the Atlantic and Indian Ocean. Could there be more population structure than meets the eye? A recent paper in the journal Molecular Ecology investigated the situation in greater detail.
Kalinka Rexer‐Huber and her colleagues collected samples from 220 white-chinned petrels, covering all known breeding populations. Next, they characterized the birds in two ways: (1) by sequencing two mitochondrial markers and (2) by sequencing more than 60,000 genome-wide markers with a genotyping-by-sequencing approach. Comparing the results from both data sets revealed some interesting patterns.
First, the mtDNA pointed to two distinct groups, namely birds breeding in New Zealand and birds breeding in the Indian and Atlantic Ocean. This subdivision confirmed previous studies using single markers. However, the genome-wide data uncovered more fine-scale population structure, discriminating between three groups: New Zealand, the Atlantic populations (South Georgia and Falklands) and the Indian populations (Prince Edward, Crozet and Kerguelen). This finding shows the importance of genomic data in pinpointing subtle population boundaries (you can check my recent book chapter for more details).
What can explain the genetic population structure of the White-chinned Petrel in a largely undifferentiated Southern Ocean? One possibility is isolation-by-distance: although the birds travel widely when foraging, they are very loyal to their birthplace (experts call this natal philopatry). Hence, there is little exchange between breeding colonies that diverge over time.
A second reason could be the Antarctic Polar Front, a thermal barrier for some marine species. However, this seems unlikely because White-chinned Petrels can just fly over this barrier. Moreover, there is gene flow between islands on either side of the Antarctic Polar Front (Falklands and South Georgia).
The results from this study have important implications for the conservation of the White-chinned Petrel, which is listed as “Vulnerable” by the IUCN. Based on mtDNA, this species would be split into two evolutionary significant units (ESUs): New Zealand and the Atlantic-Indian populations. But this arrangement would ignore the subtle – but significant – genetic differences between populations in the Atlantic and Indian Oceans. Hence, the authors argue that three distinct units will need to be taken into account to devise conservation strategies for the White-chinned Petrel.
Rexer‐Huber, K. et al. (2019). Genomics detects population structure within and between ocean basins in a circumpolar seabird: The white‐chinned petrel. Molecular Ecology, 28(20), 4552-4572.