Abstract
Factors shaping the geographic range of a species can be identified when phylogeographic patterns are combined with data on contemporary and historical geographic distribution, range-wide abundance, habitat/food availability, and through comparisons with codistributed taxa. Here, we evaluate range dynamism and phylogeography of the rocky intertidal gastropod Mexacanthina lugubris lugubris across its geographic range – the Pacific coast of the Baja peninsula and southern California. We sequenced mitochondrial DNA (CO1) from ten populations and compliment these data with museum records, habitat availability and range-wide field surveys of the distribution and abundance of M. l. lugubris and its primary prey (the barnacle Chthamalus fissus). The geographic range of M. l. lugubris can be characterized by three different events in its history: an old sundering in the mid-peninsular region of Baja (∼ 417,000 years ago) and more recent northern range expansion and southern range contraction. The mid-peninsular break is shared with many terrestrial and marine species, although M. l. lugubris represents the first mollusc to show it. This common break is often attributed to a hypothesized ancient seaway bisecting the peninsula, but for M. l. lugubris it may result from large habitat gaps in the southern clade. Northern clade populations, particularly near the historical northern limit (prior to the 1970s), have high local abundances and reside in a region with plentiful food and habitat – which makes its northern range conducive to expansion. The observed southern range contraction may result from the opposite scenario, with little food or habitat nearby. Our study highlights the importance of taking an integrative approach to understanding the processes that shape the geographic range of a species via combining range-wide phylogeography data with temporal geographic distributions and spatial patterns of habitat/food availability.
Highlights
Spatial patterns of gene genealogy can expose patterns hinting at the processes that have shaped the geographic range of species over historic and recent time scales
We look for a potential southern range contraction and assess the underlying physical and biotic causes of range dynamism and phylogeographic patterns 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 (C. fissus), habitat availability, and comparative phylogeography
The geographic range of M. l. lugubris can be characterized by three different events in its history: an old vicariant event creating a phylogeographic break in the midpeninsular region of Baja California, a more recent northern range expansion and a southern range contraction
Summary
Spatial patterns of gene genealogy can expose patterns hinting at the processes that have shaped the geographic range of species over historic and recent time scales. Reciprocal monophyly between populations may imply vicariance over long historic time scales (say >100 kya, times will vary with effective population sizes and generation times), whereas more recent processes, such as range expansions, may spread a single allele over a broad range due to founder effects (Hellberg 2014). To better understand the processes that have shaped the geographic range of a species, phylogeographic patterns from a single species should ideally be combined with data on contemporary and historical geographic distribution, range-wide abundance, habitat and food availability, as well as comparisons to codistributed taxa. Phylogeographic patterns shared by codistributed species may point to major events that have helped shape regional biotas.
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