Abstract Modern phylogenetic evidence offers novel opportunities to test classic models in island biogeography. One of the earliest, E. O. Wilson’s Taxon Cycle, proposed that insular lineages progress through sequential and predictable phases, beginning with geographic expansion, followed by ecological specialization, and ultimately range contraction (and even extinction), shaped by dispersal and interspecific competition. However, despite renewed interest in the Taxon Cycle, the extent of empirical support for its predictions and its broader significance within contemporary evolutionary biogeography remain unclear. Here, we evaluated the strength of support across 50 studies of diverse archipelagos and taxonomic groups using an evidence-graded classification scheme based on data type and inferential strength. We found evidence for taxon cycles in 82% of these studies, with 54% supported by time-calibrated phylogenies and 32% also providing explicit tests of ecological and evolutionary mechanisms. Notably, one-third of studies supporting taxon-cycle dynamics were not designed as explicit tests of the Taxon Cycle yet recovered patterns consistent with its predictions. Support based on temporal or mechanistic evidence was concentrated in Oceanian, Australasian, and Neotropical systems, with lineage origins most frequently estimated at approximately 5–20 Ma. Trait-mediated dispersal and ecological interactions were identified as key mechanisms associated with stage transitions. However, we did not find strong support for continental examples (based on habitat mosaics). Together, these results show that the Taxon Cycle has evolved into an increasingly generalized predictive framework for understanding island diversification, with potential applications to biodiversity conservation, especially concerning invasive species.
Abstract Hybridization between species was once considered an uncommon occurrence but is now recognised to occur frequently across many different taxa. It can result in homogenization of previously distinct forms, a potential conservation issue, but can also act as a catalyst for diversification through introgression and sharing of favorable genes. Repeated rounds of island colonization followed by speciation result in secondary sympatry, with the potential for hybridization between early and late arrivers. In the southwest Pacific, this situation has arisen in the avian family Zosteropidae (the white-eyes). Here we use whole genome sequencing of live birds and historical specimens to characterise hybridization between three white-eye species on Norfolk Island: two island endemics, Zosterops tenuirostris and the now-extinct Zosterops albogularis, and Zosterops lateralis, which colonized the island in 1904. Despite over two million years of divergence between Z. lateralis and the two endemics, we provide genomic evidence of their hybridization. First, we confirm the identities of three Z. lateralis x Z. tenuirostris hybrids and additionally identify one Z. lateralis x Z. albogularis hybrid. We also report genome-wide introgression from both endemics into Z. lateralis, with introgressed regions enriched for a range of potential functions. However, despite this introgression, species boundaries have been maintained, and the extant endemic Z. tenuirostris does not appear to be at risk of genetic extinction. Our work additionally demonstrates introgression from the recently extinct Z. albogularis into Z. lateralis. This study sheds light on the genomic outcomes of secondary sympatry and its potential consequences for single-island endemics.
Abstract Clade–clade interactions are thought to be a major force in the changing representation of organisms with similar ecological niches over time and space. It has long been known that the global numbers of cheilostome bryozoan species have increased relative to cyclostome bryozoan species, due to higher rates of both origination and extinction in cheilostomes. However, the latitudinal differences in this evolutionary turnover, and their ecological mechanisms, have been underexplored. Here we test the hypothesis that the numerical dominance of cheilostome species is enhanced at low latitudes today, and quantify this dominance in the geological past. Our data come from 288 extant and 1257 fossil bryozoan assemblages reported in the published literature. We show using generalized linear mixed models that assemblages contain a greater proportion of cheilostome species in lower latitudes through the last 100 million years. We discuss why higher latitudes may have represented a stronghold for cyclostomes, even as cheilostomes have continued to outnumber cyclostomes in species richness over the Cenozoic. We hypothesize that a combination of lower average temperatures and greater seasonality could give cyclostome bryozoans with relatively slower metabolism an advantage in higher latitudes, even as cheilostome bryozoan species numbers dominate globally.
Abstract Phylogenetic studies of insular radiations have traditionally relied on mitochondrial DNA, yet the increasing availability of genomic datasets has revealed numerous cases of phylogenetic discordance between mitochondrial and nuclear markers across animal taxa. The present study focuses on the Aegean Dailognatha, a group of six predominantly insular darkling beetle species. The evolutionary history of the group was assessed using 1610 nuclear protein-coding loci and mitochondrial genomes obtained from low-coverage Whole Genome Sequencing (WGS), which supported the existence of mito-nuclear discordance in both major clades (Clades A and B). The phylogenetic conflict observed in Clade A was associated with pervasive incomplete lineage sorting (ILS), while that in Clade B with introgressive hybridization, both presumably resulting from the complex palaeogeographic history of the Aegean archipelago. Specifically, the formation of the Mid-Aegean trench coincided with three temporally clustered speciation events and likely played a key role in generating the observed pattern of ILS. An additional cladogenetic event coincided with the Messinian Salinity Crisis, during which hybridization between Dailognatha hellenica and Dailognatha quadricollis likely also occurred. Overall, this study highlights the role of complex palaeogeographic dynamics in promoting both ILS and introgression within insular radiations, suggesting that mito-nuclear discordance may be more widespread in such systems than currently appreciated.
Abstract Early stages in the adaptive radiation of Darwin’s finches are characterized by divergence (fission) and convergence (fusion) caused by natural selection and introgressive hybridization, respectively. The causes of hybridization are rare misimprinting of nestlings on a heterospecific neighbour’s song or extra-pair mating. The resulting hybrid offspring are at no detectable fitness disadvantage in terms of survival or reproduction. We studied the long-term consequences of hybridization in three species of ground finches on the small Galápagos island of Daphne Major. Over a 30-year period, heterospecific ancestry increased in Geospiza fortis and Geospiza scandens, linearly and without slowing or reversing. Increases imply greater genetic gains through introgression than losses through selection. The increases were due to continued interbreeding and accumulation of heterotypic alleles in backcrosses and later generations, and not due to increases in the rate of hybridization. To explain the increase in G. fortis ancestry in G. scandens we proposed a hypothesis of heterosis based on previous evidence of inbreeding depression in G. scandens. The hypothesis is supported by higher average heterozygosity at 14 microsatellite loci of the survivors of a strong environmental stress, and by an increase in the proportion of G. scandens in the finch community. The study provides an example of the erosion of differences between species, leading towards fusion into a single, variable, admixed population with an enhanced potential for evolutionary change in a new direction.
