Island Dynamics Leave Distinct Imprints on Evolutionary Trees

The geological history of volcanic archipelagos, characterized by the successive emergence and submergence of islands, leaves a profound and measurable imprint on the evolutionary trees of the species that inhabit them. A new study published in the Frontiers of Biogeography demonstrates that these geo-environmental dynamics significantly influence the shape of phylogenies, specifically altering the rate of lineage accumulation and the degree of tree imbalance. By utilizing a neutral, stochastic, individual-based model, researchers have been able to simulate the evolution of insular communities over time, revealing how changes in island area and connectivity drive diversification patterns that persist in the phylogenetic structure of extant species.

The study employs a model where four islands emerge and submerge consecutively, each following a hump-shaped curve of carrying capacity that reflects the General Dynamic Model of island ontogeny. This dynamic environment induces continuous changes in connectivity between islands and with the neighboring mainland. By tracking every birth, death, and immigration event, the researchers constructed complete phylogenies at various time points, allowing them to extract the phylogeny of extant species and analyze its structural properties. This approach provides a detailed view of how landscape dynamics drive insular species diversification rates and influence the resulting phylogenies of island species.

The results indicate that the rate of lineage accumulation and tree imbalance vary according to a hump-shaped curve, primarily driven by variations in total area and inter-island connectivity. As the archipelago expands in area, opportunities for cladogenesis increase, leading to a faster accumulation of lineages. Conversely, as islands enter their erosion and submergence phases, the reduction in area and population size leads to a slowdown in lineage accumulation and a decrease in tree imbalance. This pattern suggests that the current geographical configuration of an archipelago is a primary determinant of phylogenetic shape, with larger areas promoting more imbalanced trees due to the uneven distribution of species abundances among colonist lineages.

Inter-island connectivity plays a crucial role in modulating these effects. In compact archipelagos where islands are closely spaced, high levels of inter-island dispersal favor frequent inter-island speciation events. This leads to faster accumulation of lineages and larger evolutionary radiations, which in turn produce highly imbalanced phylogenies. The study highlights that a small number of colonist lineages in such configurations can give rise to a substantial portion of the archipelago’s biodiversity, creating a dominant lineage structure. This imbalance is a direct consequence of the priority effect, where early-arriving lineages monopolize resources and hinder the expansion of later arrivals, a process amplified by high connectivity.

In contrast, scattered archipelagos with reduced inter-island dispersal show softened changes in the speed of lineage accumulation and undetectable signals of geo-environmental dynamics in tree imbalance. The lack of connectivity inhibits inter-island speciation, resulting in phylogenies that are more balanced and less influenced by the cumulative effects of island emergence. This finding underscores the importance of spatial configuration in interpreting phylogenetic patterns, as the same geo-environmental dynamics can produce vastly different evolutionary outcomes depending on the distance between islands. The study suggests that phylogenies in scattered archipelagos may reflect single-island in situ speciation rather than archipelago-wide diversification processes.

Mainland connectivity also significantly affects phylogenetic shape. Archipelagos located near the mainland experience higher rates of immigration, which increases species turnover and slows down the rate of lineage accumulation. The frequent arrival of new mainland lineages reduces the time lag between colonizers, leading to more balanced phylogenies and erasing the geo-environmental signal in tree imbalance. On the other hand, distant archipelagos with lower mainland dispersal exhibit phylogenies with few successful immigration events but large evolutionary radiations. This isolation promotes endemic radiation, maintaining the effect of landscape dynamics on the rate of lineage accumulation while keeping tree imbalance consistent with the baseline model.

The study further reveals that past abrupt changes, such as island emergence, can leave persistent imprints on the rate of lineage accumulation. These imprints are detectable in phylogenies as distinct spikes in speciation rates following the rapid expansion of carrying capacity. However, tree imbalance is determined primarily by the current geographical configuration, with no detectable imprint of past geographical changes. This distinction highlights the different temporal scales at which area and connectivity influence phylogenetic metrics. The growth phase of island ontogeny is identified as a crucial moment for promoting evolutionary opportunities, leaving a lasting signature in the phylogenetic structure of island communities.

The findings have significant implications for the interpretation of phylogenetic patterns in island biogeography. The study demonstrates that it is difficult to disentangle the temporal and spatial effects of geographical structure from lineage accumulation and tree imbalance patterns based solely on phylogenetic topologies. The confounding effect of archipelago geography can substantially impact the reconstruction of biogeographic history, especially in dynamic contexts. Researchers are urged to account for both archipelago-level and island-level spatial scales when analyzing phylogenies, as the spatial configuration of an archipelago modulates the effects of geo-environmental dynamics on evolutionary outcomes.

Furthermore, the study acknowledges that sea-level fluctuations, which occur on a faster timescale than geological changes, may introduce additional complexity to phylogenetic reconstruction. These climatic factors alter island area and connectivity, potentially leaving further imprints on phylogeny structure. The neutrality of the model used in this study constrains the results, and future research should incorporate non-neutral models to account for species-specific effects. Nevertheless, the study provides a robust framework for understanding how geological and environmental histories shape the evolutionary trees of island species, emphasizing the need to integrate geo-environmental dynamics and geographic structure in biogeographic studies.

Subject of Research: Effects of archipelago geo-environmental dynamics on phylogenetic tree shape

Article Title: Effects of archipelago geo-environmental dynamics on phylogenetic tree shape

Article References: Vidal-Hosteng, A., Thébaud, C., Etienne, R. S., & Aguilée, R. (2025). Effects of archipelago geo-environmental dynamics on phylogenetic tree shape. Frontiers of Biogeography, 18, Article e146650. https://doi.org/10.21425/fob.18.146650

Image Credits: AI Generated

DOI: 10.21425/fob.18.146650

Keywords: Effects, archipelago, geo-environmental, dynamics, phylogenetic, tree, shape, scientific research