Island floras as model systems for studies of plant speciation: Prospects and challenges

Abstract

Oceanic islands have long been called natural laboratories for studying evolution because they are geologically young, isolated, dynamic areas with diverse habitats over small spatial scales. Volcanic substrates of different ages permit the study of different stages of divergence and speciation within plant lineages. In addition to divergence, the dynamic island setting is conducive to hybridization. Discussion will focus on the potential of systematic/ecological studies, in combination with genomic data from high throughput sequencing and an ever-increasing array of analytical techniques, for studying evolution in island plants. These studies may include: generation of highly resolved phylogenies to clarify the biogeography of speciation and whether divergence has occurred with or without gene flow; identification of the barriers to gene flow (extrinsic vs. intrinsic) of importance during divergence; documentation of historical and current hybridization events within island lineages; and elucidation of the genomic composition and ecology of hybrid populations in order to infer the evolutionary consequences of hybridization, such as the origin of stabilized homoploid hybrid species.Key words: genomics, hybridization, islands, plants, reproductive isolation, speciation.Oceanic archipelagos represent only about 5% of the land surface of the Earth, yet nearly 25% of the described species of vascular plants occur on islands (Kreft et al., 2008). However, many insular species occur in a few, small populations, and are especially vulnerable; it has been suggested that 5% to 10% of the insular endemics globally are highly threatened and that 3% to 4% are in critical danger of extinction (Caujape-Castells et al., 2010). It was formerly assumed that oceanic lineages were the "end of the line," but it is now accepted that this is not inevitably the case (Bellemain & Ricklefs, 2008). Examples have been found of island plants dispersing back to continents or to other archipelagos with subsequent divergence and speciation (e.g., Carine et al., 2004;Harbaugh & Baldwin, 2007). From a purely pragmatic perspective, the study of island plant evolution is justified to aid conservation of some of the most rare and exotic plants on Earth.Island plants have several attributes that make them particularly good model systems for studying divergence and speciation. The isolation of archipelagos surrounded by vast oceans make them largely closed systems, although factors such as the proximity to continental source areas and the sizes of the islands influence levels of immigration. This isolation distinguishes island lineages from many continental groups, excepting those on "sky islands" and some other types of "virtual" islands on continents (Weller et al., 2007). Furthermore, the boundaries of islands in an archipelago often provide a reasonably accurate picture of the current as well as historical distribution of species (Warren et al., 2015). The ages of volcanic islands can be dated fairly accurately (Carracedo, 2011), making...

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