Three fixed differences between type A and the two ice-inhabiting forms is congruent with previous suggestions of separate species status. The genetic pattern suggests that for females, at least, there is no gene flow between them. However, the genetic evidence for reproductive isolation between types B and C (one fixed difference) is notably weaker; the variation within ecotypes is equivalent to the differences between them. This single fixed difference is therefore also consistent with simple geographical variation within a single species, especially considering that types B and C were sampled at geographically disparate locations. Determining the relationship between types B and C, and indeed among all the types, will require more extensive sampling, especially from areas where they co-occur (i.e. east Antarctica), as well as data from independent genetic markers.
It seems paradoxical that the types B and C can appear very different in external morphology, with no recorded intermediates, and yet be nearly identical in their control region sequences, a gene noted for its high levels of variation and rapid evolution. One could argue that unless intermediate phenotypes are found, the most parsimonious explanation is that these nevertheless represent different species, arising from a very recent divergence. However, caution dictates that any conclusion about reproductive isolation also requires data from other genetic markers and morphology, and on seasonal movements and breeding patterns; these are not currently available.
Whether or not interbreeding among the different types occurs, the pattern of genetic variation may say something about the relative plasticity of non-molecular characters. The largest degree of sequence differentiation, seen between types A and B/C, coincides with the most obvious morphological differences (body size and coloration), as well as the differences in habitat preference (open water versus ice). The body size differences alone are substantial: type C reaches a maximum of 6
m while type A attains 9
m, 50% larger (Pitman et al. 2007
). Among all three, the differences in prey preferences and morphology are all fairly dramatic regardless of the degree of genetic differentiation. There are some parallels between the Antarctic killer whales and the better studied north Pacific killer whales. In both, there are three known largely sympatric ecotypes (resident, transient and offshore in the north Pacific), and divergence in both communities seems to focus on specializations in habitat (inshore versus offshore) and prey (fishes versus mammals). However, morphological divergence among Antarctic forms is much more dramatic than among the north Pacific ecotypes. For both regions, osteological comparisons are needed, but efforts are hindered by a paucity of specimens.
also provides a context for examining this variation relative to samples from other geographical regions. Despite low levels of variation within the types, types B and C contribute to killer whale variation on a global scale as the sole representative of an entire clade. By contrast, type A killer whales from Antarctica are interspersed throughout the rest of the tree among those from other regions. In spite of the small sample size, type A killer whales thus show the highest level of sequence variation. This may be due to multiple populations being sampled. If type A killer whales represent seasonal visitors that follow migrating minke whales into Antarctic waters (Pitman & Ensor 2003
), it is possible that this sample set represents multiple breeding populations from low latitudes of the different ocean basins.
How distinct then are Antarctic types B and C in a global context? If one were to categorize killer whales from outside Antarctica according to the coloration designations used herein, all the killer whales in from outside Antarctica for which we have documentation would be considered as type A. Furthermore, all these haplotypes from other areas have CATT at the four diagnostic sites discussed above; the same pattern as in type A killer whales from Antarctica. In this sense, the diagnostic sites for types B and C are not area specific; the appearance and the genetic pattern are congruent on a global scale. In other words, the mutations that resulted in the diagnostic differences for types B and C appear not to have occurred in killer whales anywhere else. The congruence of these lines of evidence suggests that the divergence between these types represents a species boundary.
Although the existence of sympatric killer whale ecotypes has been recorded in different regions, on a global scale there does not appear to be congruence between ecotype and genotype (Hoelzel et al. 2002
); specific prey specializations apparently had multiple independent origins. Whether or not any of these scenarios represent speciation events (either complete or incipient) remains an open question; some specialization may merely reflect behavioural preferences of particular family groups, and therefore perhaps reversible. Although data are lacking from many killer whale populations around the world, ecological specialization and divergence does seem to be more prevalent at high latitudes, which may reflect higher densities of schooling fishes and mammals. The divergence from the more widespread type A to types B and C ecotypes seems to have occurred only once, as evidenced by the monophyly of the B/C clade, and seems to have taken place in Antarctica. Some uncertainty about the biogeography of this clade will persist until more is known about the distribution and seasonal movements of the two types.
Killer whales are arguably one of the best-studied and most eminently recognizable large animals on Earth, making the discovery of one or perhaps two new species a bit surprising. Recently, however, a number of studies have detected the presence of other previously unrecognized cetacean species based on morphological and genetic data (e.g. Dalebout et al. 2002
; Wada et al. 2003
; Beasley et al. 2005
). Taken together, these findings indicate that cetacean diversity in the world's oceans may be substantially underestimated, and that there is still much to learn about even the largest of mammals.
Biopsy collection was conducted under the guidelines of the US Marine Mammal Protection Act.