Does migration promote or inhibit diversification?

A trans-Tasman collaboration by Chris Burridge and Jon Waters tested whether migratory behaviour results in greater numbers of species than non-migratory behaviour.

ldon's galaxias (Galaxias eldoni), an endangered species native to New Zealand. Stella McQueen/Wikimedia Commons (CC BY-SA 4.0)

Isolation is often considered ‘king’ with respect to speciation, allowing gene pools to diverge, and hence non-migratory behaviours, involving lower vagility, might be expected to produce more species. However, non-migratory species often have smaller ranges, which exposes them to both greater risk of extinction, and lower likelihood of encountering different environments—a promotor of adaptation and speciation.

Published recently in the journal Evolution, our research tested whether diversification rates (the difference between speciation and extinction) were higher in migratory (diadromous) or non-migratory galaxiid species. The study produced an updated phylogeny for galaxiid fishes, with divergences calibrated through time. Using this phylogeny, the research tested whether rates of diversification rates differed with respect to migratory status.

Our study suggested that non-migratory galaxiids experience faster diversification. The non-migratory lineages likely experience faster diversification owing to chance events that isolate strictly freshwater lineages. In contrast, the potentially lower extinction rates of migratory lineages and greater ability to colonise new environments did not appear to offset the impediment of speciation posed by greater gene flow among populations.

The common river galaxias (Galaxias vulgaris) is found only in Canterbury, New Zealand. Zureks/Wikimedia Commons (CC BY-SA 4.0)

This result contrasted with studies of other migratory/non-migratory taxa, which may relate to the relative dispersal abilities of taxa, and also the geographic context of studies. Diadromy in galaxiids may effectively homogenise gene pools within landmasses, but be insufficient for frequent colonisation of new environments, given the disjunct distribution of freshwater habitats at temperate Southern Hemisphere latitudes.

What was also interesting was that within the non-migratory lineages we also had evidence for two diversification rates. Lineages within geologically dynamic regions of New Zealand, and also those of the Galaxias olidus complex in mainland Australia, exhibited very fast diversification. We hypothesise that this reflects the role of geology in creating allopatric distributions in New Zealand, and morphological attributes of the olidus complex that could promote occasional crossing of otherwise insurmountable barriers, again creating allopatric distributions.

This research was published by Chris Burridge (University of Tasmania) and Jon Waters (University of Otago).