Social cohesion among kin, gene flow without d…

Social cohesion among kin, gene flow without dispersal and the evolution of population genetic structure in the killer whale (Orcinus orca):


Killer whales are largely considered “xenophobic, that is, reluctant to interact with individuals outside of their own population. However, genetic studies have revealed low levels of gene flow between killer whale populations and even ecotypes!

In the above study, the authors analyzed genetic data from the following populations: Russian residents, Bering Sea residents, Southeast Alaska residents, offshores, Alaska transients, California transients, and Icelandic killer whales. They used various programs to determine the paternity and maternity of individuals, matings between populations, and matings between ecotypes. Some of their results were fascinating. They found: 

  • High rates of gene flow between Alaska transients and California transients (CA20 was identified as an F2 migrant, the result of an Alaska transient breeding with a California transient).
  • High rates of gene flow between Russian residents and Bering Sea residents.
  • Russian residents that migrated to the Southeast Alaska resident population. 
  • An individual that likely had an offshore mother and transient father.
  • Two Icelandic killer whales that had Pacific transient fathers (one of which was identified to likely be an Alaska transient).

While very rare, these results tell us there are some levels of inter-population and even inter-ecotype matings in killer whales. The transient/offshore matings likely happen in offshore habitats when the two ecotypes happen upon each other. The Icelandic/transient matings are harder to explain given the sheer geographic distance, but the authors propose a few ideas: “…the supposed mating between females from the IC population (the North Atlantic) and transient males (the North Pacific), suggested by detection of F2 immigrants between these populations, must have involved either the long-distance movement of mating individuals or entire pods, or gene flow through intermediary populations. Killer whale social groups are capable of migration over distances of several thousand kilometers in a season (Sternson & Simila, 2004; Dahlheim et al., 2008), so the possibility of long-distance associations cannot be excluded.”

Some of the identified migrants between populations are possibly not true migrants, but may have been the result of temporary associations between populations. For example, there were two proposed F1 migrants from the Russian resident population to the Southeast Alaska resident population. However, given that these whales were not identified as belonging to any identified Alaska resident pod, it’s likely they were temporarily associating with the Alaska residents rather than being permanent migrants.

 Regardless, it’s still fascinating to know some cases of breeding outside populations and ecotypes occur. We still have much to learn!