Figure 4.

Three scenarios that could result in a majority-rule phylogeny consistent with sympatric speciation (adapted from Martin et al., 2015). (a) A single randomly mating population colonises the North Pacific and diverges during sympatry (i.e., sympatric speciation, sensu, Gavrilets, 2003). Under this scenario, the three North Pacific ecotypes would be expected to all share a similar proportion of their ancestry with outgroups. (b) colonisation of the North Pacific preceded by admixture with the outgroups followed by a period panmixia would also result in the three North Pacific ecotypes sharing a similar proportion of their ancestry with outgroups; alternatively, colonisation of the North Pacific by a structured meta-population or hybrid swarm (see, for example, Roy et al., 2015) would result in the amount of shared ancestry with outgroups differing among ecotypes. This second scenario would not satisfy the condition of Gavrilets (2003) for sympatric speciation. (c) Repeated colonisation of the North Pacific and episodic admixture upon secondary contact would also result in some North Pacific ecotypes sharing more of their ancestry with the outgroups most closely related to the source population of this secondary colonisation. This scenario would be consistent with the discordance of the mitochondrial and nuclear topologies if introgression was through male-mediated gene flow among matrilineal groups, as mitochondrial haplotypes would become fixed in the descendent lineage given that killer whale populations typically subdivide through matrilineal fission. These three examples are not meant to be exhaustive, but simply illustrative of how different evolutionary histories can result in the same majority-rule topology if evolutionary history is modelled as a single bifurcating tree.