Madagascar Speciation on Mountains: Pleistocene Glaciation Cycles Promote Genetic Divergence, Secondary Contact, and Morphological Diversification in Sympatric Lineages of the Pelican Spider <i>Eriauchenus workmani</i> (Archaeidae)

Habitat contraction and expansion around montane areas, caused by climatic cycles, can allopatrically isolate lineages and then put them back into secondary contact. When closely related lineages come back into coexistence, competition for similar resources may be strong, promoting niche partitioning via ecological character displacement. Eriauchenus workmani is an endemic spider species of the montane rainforest areas of Madagascar, with high morphological variation in somatic features. In this study, we address whether this variation is attributed to diverging populations, some of which co-occur in sympatry, and what role did mountains play in creating this divergence. Sampling individuals across the distribution range of this species, we gather hundreds of loci per specimen and then implement population genetic analyses, species delimitation methods, demographic models, ecological niche models, and morphological multivariate analyses. Findings show population contraction around montane areas during warm periods, when the distributions become fragmented, and population expansion during the last glacial maximum, when distributions become continuous. Further, we found four genetically distinct populations, three of which cohabitate in sympatry; however, gene flow is low considering populations are sympatric. There are significant differences in body size between populations. Within a population there are significant morphological differences, including a reduction in disparity when comparing allopatric with sympatric individuals. In conclusion, we hypothesise that the repeated cycles of isolation and secondary contact that occur in montane areas due to glacial cycles are fuelling speciation via allopatric divergence followed by ecological character displacement upon secondary contact.