It brought to mind a few things that I have been thinking about on the "back burner" for several years, the inter-island effect and hybridization; however, I am not an expert in theories of speciation and adaptive radiation (especially the more ecological oriented ones related to priority effects and community assembly). A collaborative project on Cottus hybrids (Stemshorn et al. 2011) and learning about the rate of speciation in some groups when first moving to Hawaiʻi started me on this road. Hawaiʻi is very isolated from continents. Once the first two of the large islands formed (Kauaʻi and O‘ahu) I strongly suspect a situation was set up that promoted rapid speciation from intermittent movement between the islands (in addition to other effects related to novel ecological opportunities for newly established species (there is evidence for multiple fitness peaks in an "adaptive landscape" for some species, e.g., Martin and Wainwright 2013 and as evidenced indirectly by convergent multiple ecomorphs, e.g., Muschick et al. 2012 and references therein) in the diverse climates and new species compositions of the islands). A system of parapatry existed where populations could diverge and build (partial) barriers to gene flow yet still remain in contact. There is evidence in the Hawaiian honeycreeper phylogeny that the major speciation events took place after Kauaʻi and O‘ahu (Lerner et al. 2011). It would be interesting to see if this holds up in the timing of other adaptive radiations and the degree of isolation and inter-island distance in other archipelagos to map out the geographic Goldilocks zone for inter-island effects on rates of speciation for different taxa. The other part of this is the role of hybridization (and there is evidence of extensive hybridization in many of these adaptive radiations, e.g., Richards and Martin 2017; Jay et al. 2018; Poelstra et al. 2018;  see also Pfennig 2007 for a natural case of adaptive hybridization). Hybridization between closely related species breaks a lot of the standard rules and assumptions in population genetics related to the rate of adaptation. We usually think of a new rare allele resulting from a single mutation that is adaptive. It might become established in the population but in fact, counter-intuitively, it is overwhelmingly likely to be lost to drift (despite being adaptive) in the first few generations. We can write down predictions for the rate of adaptive evolution from population genetic theory like $4 N \mu_a s$, where $N$ is the population size, $\mu_a$ is the adaptive mutation rate (let's say per gene or group of genes), and $s$ is the relative fitness advantage (see Patwa and Wahl 2008 for review). Rough guess values like $N = 10,000$$\mu_a = 10^{-10}$ and $s = 0.05$ gives an average rate of a genetic adaptation event once every 5,000,000 generations.  Long story short, the rate of adaptation is predicted to be painfully slow, especially in small populations such as those that might be found on remote islands. However, hybridization doesn't work like this. In parapatry species hybrids can be constantly generated and the alleles are (so long as the parental species do not go extinct) never lost by drift. Thus it is essentially certain that adaptive differences will become established (giving a evolutionary boost over their single homogeneous species relatives on the mainland). Even better, these are variants that have been pre-tested in the parental species to work well in certain genomic backgrounds rather than new mutations that are more likely to either be unconditionally neutral or deleterious. And, once the process proceeds to the $F_2$ generation a huge diversity of combinations appear for selection to act upon (although the majority of rare hybrid offspring will be back-crosses to one of the parental species) and explore variants that may be near neutral in the parental configuration but novelly adaptive when they interact. Also, note that as two species diverge, assuming that they proceed to complete genetic isolation, they will transition through the full range of levels of divergence up until the point hybrids are sterile or inviable, testing the widest range of levels of genetic differentiation possible. As new species become established, beyond the first two that may be largely allopatric, (and as new islands form) the range of potential crosses and combinations is also increased.