Uncovering the historical and recent processes shaping species' geographic ranges: Insights from a rocky intertidal gastropod

Background/Question/Methods

Spatial patterns of gene genealogy can expose the processes that have shaped the geographic range of a species over historic and recent time scales. Reciprocal monophyly between populations may imply cryptic vicariance over long historic time scales whereas more recent processes, such as range expansion, may spread a single allele over a broad range due to founder effects. However, the mere documentation of such patterns tells us little about their underlying physical and biological causes. Population genetic data should ideally be combined with contemporary and historical geographic distribution, rangewide abundance, and habitat and food availability to gain a better understanding of the processes that have shaped the geographic range of a species. Phylogeographic patterns shared by co-occurring species are especially noteworthy, as they may point to major events that have helped shape regional biotas, such as the common phylogeographic break found in the mid-peninsular region of Baja California.

Here, we test for a phylogeographic break in the mid Baja peninsula and for northern and southern range dynamism for the rocky intertidal gastropod Mexacanthina lugubris lugubris. We asses the underlying physical and biotic causes of phylogeographic patterns and range dynamism by complementing the genetic data with museum records, range wide field surveys of the distribution and abundance of M. l. lugubris and its primary prey (Chthamalus fissus) and spatial patterns of habitat availability.

Results/Conclusions

The geographic range of M. l. lugubris can be characterized by three different events in its history: a vicariant event creating a clear phylogeographic break in the mid-peninsular region of Baja and a more recent northern range expansion and southern range contraction. This common phylogeographic break is often attributed to a hypothesized late Miocene – middle Pleistocene seaway bisecting the peninsula, but for M. l. lugubris, it may also be a result from spatial variation in habitat availability. The northern range is conducive to expansion because habitat and food (C. fissus) is ample and individuals from source populations are highly abundant. Ultimately, the observed range dynamism may partially result from climate change, but is also a likely consequence of the same but opposite trending factors: leading edge abundances and habitat / food availability. Our study highlights the importance of taking an integrative approach to understanding the geographic range of species via combining data from population genetics, temporal geographic distributions, spatial patterns of habitat / food availability and comparative phylogeography.