An extensive review paper attempts to answer this intriguing question.
Seabirds travel thousands of kilometres to find foraging and breeding areas. Gliding over the largely homogeneous oceans, there seem to be no obvious barriers for these birds. You would expect that this high vagility is reflected in the genetics of the populations, namely no genetic structure. Interestingly, many studies report clear genetic differentiation between different seabird colonies. For example, a recent genetic study of the white-chinned petrel (Procellaria aequinoctialis), which breeds on New Zealand and islands in the Indian and Atlantic Ocean, uncovered clear population boundaries using genomic data. What factors could be responsible for the origin of genetic differentiation in highly mobile species, such as seabirds? A paper in the journal Biological Reviews tried to solve this conundrum.
Anicee Lombal and her colleagues dove into the scientific literature and collected as many studies on the population genetics of seabirds as they could find. For each study, they extracted the genetic data and noted down biological factors that could explain genetic differentiation between colonies. Let’s have a look at the most important factors on this list.
Seabirds are often philopatric, consistently returning to the same breeding area. This behavior might lead to isolation between colonies and consequently genetic differences. Spatial segregation can also occur outside of the breeding season. Birds from distinct colonies might use different foraging areas and are thus less likely to mix at sea. Eventually, they will return to different colonies. There can also be morphological or phenological differences between colonies that affect mate choice and culminate in genetic differentiation.
The biological factors above – philopatry, spatial segregation and morphological or phenological differences – make intuitive sense. However, the analyses found no consistent support these factors driving genetic differentiation in seabirds. Indeed, one section in the paper is entitled “Biotic factors do not predict genetic differentiation among seabird populations.” So, what factors are responsible for the observed genetic differences?
Perhaps the genetic patterns we quantify now are the outcome of past events? To test this idea, the researchers performed additional genetic analyses on the collected data. They calculated several population genetic statistics to quantify past changes in population size, including Tajima’s D, Fu and Li’s F* and Fu’s Fs statistic. The combination of these statistics can be used to deduce demographic events. For example, when Fs is significant and D and F* are not, there was probably a range expansion.
The genetic analyses revealed the strong genetic legacies of past demographic changes. However, the underlying events were slightly different for particular geographic regions. Northern temperate species were clearly influenced by the glacial cycles of the Pleistocene when populations retreated to different refugia, such as the southern edge of the Bering Land Bridge, the Newfoundland Bank and the Spitsbergen Bank, where they accumulated genetic differences. Species on the southern hemisphere, on the other hand, expanded their ranges after the Last Glacial Maximum (ca. 20,000 year ago) and colonized areas that became free of ice. This expansion resulted in geographic isolation and consequently genetic differentiation.
In summary, the seabird paradox can be partly resolved by the genetic legacy of past demographic changes. Additional biological factors might strengthen these patterns over time, but they are mostly species-specific.
Lombal, A. J., O’dwyer, J. E., Friesen, V., Woehler, E. J., & Burridge, C. P. (2020). Identifying mechanisms of genetic differentiation among populations in vagile species: historical factors dominate genetic differentiation in seabirds. Biological Reviews, 95(3), 625-651.