Speciation in Cisco with Emphasis on Secondary Contacts, Plasticity, and Hybridization

The CiscoCoregonus artediis a planktivorous fish that is widely distributed in lakes across glaciated areas of North America. With retreat of Laurentide ice, Cisco dispersed from refugia into a vast network of meltwater (proglacial) lakes that eventually receded, stranding populations in depressions-today's lakes. Refugial populations also colonized lakes that fell outside of the footprint of a proglacial lake. These two types of Cisco lakes, those inside the footprint of a proglacial lake and those outside the footprint, though uncounted, number in the high hundreds or more. All 53 lakes reported to contain sympatric forms of Cisco occur in previously inundated lakes; no sympatric forms occur in lakes that were not inundated. We reviewed Laurentide zoogeography to assess whether secondary contacts in the proglacial lakes were extensive enough to account for the distribution of sympatric lakes. We concluded that secondary contacts had been more extensive in the proglacial lakes than detected in genetic studies of Cisco. We inferred that secondary contacts in the proglacial lakes resulted in more plastic, diverse phenotypes than were found in lakes not inundated and that these phenotypes were more capable of divergence than were phenotypes inhabiting lakes not inundated. Secondary contacts in the proglacial lakes appear to have been limited to shallow-water forms of Cisco. Deepwater forms likely did not evolve until after the proglacial lakes receded, as meltwater appears to have been trophically unsuitable for pelagic fishes like Cisco. Cisco are assumed to have colonized meltwater lakes by "hopscotching" between inlets, thereby avoiding prolonged existence in meltwater. Sympatric populations of Cisco are rare in comparison with the number of allopatric populations, even in inundated lakes, indicating that divergence occurs under specialized conditions. Canalization appears to have diminished the evolvability of contemporary forms as compared to their ancestors. Hybridization within Cisco species pairs appears to be the norm and is aggravated by anthropogenically induced events.