Island biogeography therefore provides an opportunity to compare adaptive radiation and phylogenetic niche conservatism as drivers of speciation, in a system where changes in elevational range are not confounded by other (e.g. However, terrestrial biota of isolated islands and archipelagos have limited opportunities for latitudinal shifts, restricting responses to shifts in elevation. Depending on continental geography, such latitudinal shifts can lead to population fragmentation or range expansion. ![]() A key issue in assessing the role of phylogenetic niche conservatism for climate-driven speciation is that continental species may alter their latitudinal distributions in response to climate, rather than only elevation. ĭistinct shifts in climate since the beginning of the Quaternary enable the relative roles of adaptive radiation versus phylogenetic niche conservatism to be explored as instigators of speciation. This model is particularly relevant to lineages with narrow climatic niches, especially those that are spatially fragmented across existing landscapes or constrained to isolated refugia during climate extremes, promoting allopatric speciation. Phylogenetic niche conservatism provides an alternative, whereby the inability of a lineage to adapt to new or changing environments promotes speciation when populations become isolated. A predominant paradigm in evolution is that species arise from adaptive radiation into new niche spaces, with gene flow between the new and ancestral populations subsequently inhibited, eventually leading to speciation. We argue that phylogenetic niche conservatism is an important driver of tropical island bee biodiversity but that this phylogenetic inertia also leads to major extinction risks for tropical ectotherms under future warming climates.Īt its heart, evolutionary biology attempts to explain how new species arise and evolve to occupy a myriad of niches. Climate cycles, topographical complexity, and niche conservatism could interact to shape island biodiversity. Our results indicate that elevational niches have been conserved across most speciation events, contradicting expectations from an adaptive radiation model but concordant with phylogenetic niche conservatism. Using genetic (mitochondrial DNA (mtDNA) and single-nucleotide polymorphism (SNP)) and morphological data, we show high species diversity (22 species) in an endemic clade of Fijian Homalictus bees, with most species restricted to highlands and frequently exhibiting narrow geographical ranges. Adaptive radiation and phylogenetic niche conservatism therefore both predict that topographical complexity should encourage speciation, but they differ strongly in their inferred mechanisms. However, recent studies have argued that phylogenetic niche conservatism, combined with topographical complexity and climate change, could also promote speciation if populations are episodically fragmented into climate refugia that enable allopatric speciation. The topographical complexity of islands can drive speciation by providing a diversity of niches that promote adaptive radiation and speciation. ![]() ![]() Island biogeography explores how biodiversity in island ecosystems arises and is maintained.
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