Speciation in seabirds: why are there so many species…and why aren’t there more?

Abstract

Speciation-the multiplication of species through the evolution of barriers to reproduction between populations-plays a central role in evolution since it enables two or more populations to adapt and evolve independently. However, mechanisms of speciation are notoriously difficult to study and poorly understood. Seabirds provide useful models to investigate factors that may promote or inhibit speciation because their ecology and evolutionary genetics are relatively well understood. Here I review population genetic studies of seabirds to test the importance of six factors with the potential to disrupt gene flow enough to result in speciation. Over 200 studies, including over 100 species, have been published to date. Most show evidence of restrictions in gene flow. Physical (geographic) barriers to dispersal are clearly important: conspecific populations that are separated by large expanses of land or ice show evidence of restricted gene flow, and sister species often are separated by physical barriers to gene flow. However, many species of seabirds show evidence of restrictions in gene flow in the absence of physical barriers to dispersal. Study results indicate that differences in ocean regimes, nonbreeding distributions, foraging distributions during the breeding season, and breeding phenology also can disrupt gene flow enough to lead to speciation. Of these, physical isolation and differences in ocean regime appear to be the most important. Philopatry alone may be sufficient to result in reproductive isolation, but usually it acts in combination with other barriers to gene flow. The effects of many other potential influences on gene flow need to be investigated more thoroughly, including colony distribution/location, wind, interspecific interactions, environmental stability/variability, variation in phenotypic traits associated with mate choice (morphology, behaviour, vocalisations) and intrinsic (genomic) incompatabilities. Recent advances in genome sequencing, especially if used in combination with ecological tools such as geolocators and new methods for data interpretation, are opening exiting new avenues to test the importance of various behavioural, ecological, demographic and genomic factors in reducing or promoting gene flow and so affecting speciation.

Seabird population changes following mammal eradications on islands

“…The results also emphasize that seabirds are perhaps less philopatric to their natal sites than is widely accepted (Weimerskirch, 2002;Brooke, 2004;Friesen, 2015). Further suggesting a lack of philopatry are observations of seabirds of several species and taxonomic groups arriving to colonize and breed on newly restored islands shortly after project completion (Bell, 1998;Ratcliffe et al, 2010;Rocamora & Henriette, 2015).…”

Widespread gene flow between oceans in a pelagic seabird species complex

“…Although many studies focus on the gene flow and genetic structure within single seabird species at different island colonies, the potential for gene flow between species at different spatial scales remains poorly understood (Friesen, ). Introgression between bird species is fairly common in nature (Grant & Grant, ; Mallet, ; McCarthy, ), but is usually prevented by physical (e.g., distance, mountain ranges, oceans) or biological barriers.…”

“…Biological barriers to interspecific gene flow have been documented in a wide range of bird species; for example, hybrid phenotypes may be unsuitable to changing environmental conditions, for example, in Darwin's finches (Grant & Grant, ), hybrids may suffer reduced reproductive fitness in one or both sexes, for example, in Flycatchers (Gelter, Tegelström, & Gustafsson, ; Svedin, Wiley, Veen, Gustafsson, & Qvarnström, ) and Spotted eagles (Helbig et al., ), or have a reduced adult survival rate, for example, in Larus gulls (Neubauer, Nowicki, & Zagalska‐Neubauer, ). Gene flow between distinct populations of a single seabird species is often restricted (Friesen, ); therefore, introgression of genes from one seabird species to another may be considered very unlikely. However, examples of naturally occurring seabird hybrids exist (murres, Friesen, Barrett, Montevecchi, & Davidson, ; boobies, Taylor, Zavalaga, & Friesen, ; Taylor, Anderson, & Friesen, ; giant petrels, Brown, Techow, Wood, & Phillips, ; Hunter, ).…”

No evidence of divergence at neutral genetic markers between the two morphologically different subspecies of the most numerous Arctic seabird

“…The apparent lack of genetic structure in the Little Auk is perhaps not surprising. High‐latitude seabirds typically have reduced levels of population structure relative to variation among populations sampled from lower latitudes (Adams & Hadley , Friesen ). Nevertheless, there can be surprising variation among and within species.…”