A test of island biogeographic theory applied to estimates of gene flow in a Fijian bird is largely consistent with neutral expectations

Abstract

Archipelagos are exemplary systems for studying allopatric speciation (Mayr, 1942) and have been key to better understanding the biogeography of both island and mainland species (Losos & Ricklefs, 2009; MacArthur & Wilson, 1963, 1967). They are natural laboratories for comparative studies of the process of divergence in allopatry. Specifically, the long-standing hypothesis of island speciation posits that ocean barriers prevent gene flow and thus promote differentiation (Mayr, 1942; Slatkin, 1987). Insular faunas and floras provide classic examples of diversification and espe

PleistoDist: A toolbox for visualising and quantifying the effects of Pleistocene sea-level change on island archipelagos

“…where 𝑛 (1→2) is the number of propagules of a particular species successfully dispersing from island1 to island2, 𝑤 2 is the width of island2 relative to island1, 𝑑 is the distance between the two islands, 𝛼 is the population density of the propagule population on island1, 𝐴 1 is the area of island1, and 𝜆 is the mean overwater dispersal distance of the propagule (see Gyllenhaal et al 2020 for further discussion). While this equation is difficult to solve since it requires estimates for both 𝛼 and 𝜆, if we take the ratio of 𝑛 (1→2)…”

“…Comparison of empirical estimates of net inter-island migration with contemporary and time-corrected neutral expectations from MacArthur and Wilson (1967), figure adapted from Gyllenhaal et al 2020.…”

“…In addition to metrics of island shape and inter-island distance, PleistoDist includes two higher-level functions that calculate the expected equilibrium net migration between island pairs (MacArthur and Wilson 1967), as well as the visibility of an island relative to an observer on another. The net migration calculation is based on a model described by MacArthur and Wilson (1967) estimating the number of successful propagules dispersing from a source island (island1) to a sink (island2), as given by the equation: where n ( 1 → 2 ) is the number of propagules of a particular species successfully dispersing from island1 to island2, w 2 is the width of island2 relative to island1, d is the distance between the two islands, α is the population density of the propagule population on island1, A 1 is the area of island1, and λ is the mean overwater dispersal distance of the propagule (see Gyllenhaal et al 2020 for further discussion). While this equation is difficult to solve since it requires estimates for both α and λ , if we take the ratio of n ( 1 → 2 ) and n ( 2 → 1 ) —the net migration between islands 1 and 2—we cancel out both the α and exponential terms and reduce the relationship to: assuming that d, λ , and α are identical for both islands.…”

“…One of the predictions of the theory of island biogeography (MacArthur & Wilson, 1967) is that the relative rate of migration between island pairs can be predicted by the size, relative orientation, and distance of source islands. Gyllenhaal et al (2020) tested this expectation by estimating the rates of inter-island migration in the Fiji bush warbler ( Horornis ruficapilla ) between the four large islands of Fiji, concluding that rates of inter-island migration are largely consistent with neutral expectations. This neutral expectation can now be easily and accurately calculated using PleistoDist, while taking into account the effect of sea-level change over Pleistocene time.…”

Diversity patterns and speciation processes in a two-island system with continuous migration

“…In this aspect, models can help elucidate and differentiate processes from patterns. Furthermore, current advances in obtaining genome-wide datasets provide robust estimates of gene flow and demographic history [49,50], evincing that speciation with gene flow in insular systems is common and may rise even in the absence of ecological divergence [50]. Therefore, understanding how migration and diversification are related can provide a better interpretation of speciation events in these intermediary geographic configurations that are not fully sympatric or allopatric.…”