Founder effects are invisible in endemic species of oceanic islands

One of the axioms of island biology is the importance of the founder effect (Whittaker & Fernández-Palacios, 2007). There can be no question that founder effects occur, with successful immigrants arriving by chance dispersal to oceanic islands and bringing with them only a small portion of genetic variation that existed in the usually larger and more widely distributed parental continental populations. Genetic analyses covering both continental and island populations provide direct evidence of founder effects involved with recent immigration of island populations (e.g. Gugger & Cavender-Bares, 2011; Yamada & Maki, 2012). Founding events, acting in conjunction with selection, can also have an impact on morphological characteristics of colonizing populations (Kolbe et al., 2012). The founder effect has also been regarded as fundamental for stimulating genetic divergence that eventually leads to cladogenetic speciation and adaptive radiation (Carson & Templeton, 1984). The basic idea is that the dramatic reduction in genetic variation opens up opportunities for reorganization of the genome and this, acted upon by directional selection into diverse island environments, is what leads to many of the well-known spectacular examples of adaptive radiation, such as the silverswords of the Hawaiian Islands (Carlquist et al., 2003). Experimental and theoretical studies have attempted to refine understanding of the genetic effects of founding events (Barton & Charlesworth, 1984; Yeung et al., 2011). An alternative mode of island speciation, anagenesis, also develops after a founder effect, although the results are quite different. This type of speciation takes place in islands with relatively uniform environments, in which selection remains uniform (stabilizing) over the island and does not drive adaptive radiation (Stuessy et al., 2006). Genetic variation within populations accumulates over time via mutation and recombination, bringing about genetic and morphological change that eventually is recognized taxonomically as a distinct species. Genetic variation drifts and accumulates, presenting a different genetic profile. It has been estimated that at least one-fourth of plant species endemic to oceanic archipelagos have speciated in this fashion (Stuessy et al., 2006). With the advent of molecular techniques, a number of studies on endemic species of oceanic islands have reported low levels of genetic diversity that have been inferred as founder effects (Frankham, 1997). The difficulty with this interpretation is that speciation, either via cladogenesis or anagenesis (barring polyploidy, which is uncommon in oceanic islands; Stuessy & Crawford, 1998), takes considerable geological time to occur (Coyne & Orr, 2004). It stands to reason, therefore, that observed levels of genetic variation within and among populations of endemic oceanic island species will no longer bear any relationship to a founder effect. Simply too much time has elapsed since the arrival of the immigrant individuals that formed the initial founding population. It would be possible for a founding population to stay small and genetically greatly restricted, but in that case no cladogenetic or anagenetic speciation would occur. By the time that new genetic and morphological species are recognized, i.e. at the point they become regarded as endemics, the founder effect as measured by genetic diversity has long been obscured (Fig. 1). Founder effects might be detectable by considering qualitative allelic variation, but this is independent of quantitative genetic variation. The place to actually see a founder effect in island archipelagos is in populations of invasive and newly established species. Invasive species that have recently arrived on an oceanic island will often show a dramatic