Abstract:This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
“…Gene flow may thereby continue between the different habitats, preventing radiation into locally endemic species in different niches, and instead creating the conditions for a single more generalist and widespread species to evolve that can tolerate a wide range of environmental conditions. This idea is consistent with recent observations among oceanic island plants that lineages undergoing spectacular radiations yield species that are rare and with narrow niches: "evolutionary winners are ecological losers" (Fernández-Palacios et al, 2021). Thus, we propose that radiation-or lack thereof-may arise more from dispersal limitation or genetic factors rather than the existence of the niches themselves.…”
Some lineages radiate spectacularly when colonizing a region, but others do not. Large radiations are often attributed to species’ adaptation into niches, or to other drivers, such as biogeography including dispersal ability and spatial structure of the landscape. Here we aim to disentangle the factors determining radiation size, by modeling simplified scenarios without the complexity of explicit niches. We build a spatially structured neutral model free from niches and incorporating a form of protracted speciation that accounts for gene flow between populations. We find that a wide range of radiation sizes are possible in this model depending on the combination of geographic isolation and species’ dispersal ability. At extremely low rates of dispersal between patches, each patch maintains its own endemic species. Intermediate dispersal rates foster larger radiations as they allow occasional movement between patches whilst sufficiently restricting gene flow to support further speciation in allopatry. As dispersal rates increase further, a critical point is reached at which demographically identical lineages may vary greatly in radiation size due to rare and stochastic dispersal events. At the critical point in dispersal frequency, some lineages remain a single species for a comparatively long time, whilst others with identical characteristics produce the largest radiations of all via a new mechanism for rapid radiation that we term a ‘radiation cascade’. Given a single species covering many patches connected with gene flow, a radiation cascade is triggered when stochastic dispersal is unusually low for a period, leading to an initial speciation event. This speciation means there are fewer individuals per species and thus further reduced gene flow between conspecifics. Reduced gene flow in turn makes it easier for further speciation to occur. During a radiation cascade, dispersal of individuals between patches continues at the same rate as before, but due to the increasing diversity it primarily introduces novel species that will later speciate, rather than adding to gene flow of existing species. Once a radiation cascade begins, it continues rapidly until it is arrested by a new equilibrium between speciation and extinction. We speculate that such radiation cascades may occur more generally and are not only present in neutral models. This process may help to explain rapid radiation, and the extreme radiation sizes of certain lineages with dispersing ancestors. Whilst niches no doubt play a role in community assembly, our findings lead us to question whether diversification and adaptation into niches is sometimes an effect of speciation and rapid radiation, rather than its cause.
“…Gene flow may thereby continue between the different habitats, preventing radiation into locally endemic species in different niches, and instead creating the conditions for a single more generalist and widespread species to evolve that can tolerate a wide range of environmental conditions. This idea is consistent with recent observations among oceanic island plants that lineages undergoing spectacular radiations yield species that are rare and with narrow niches: "evolutionary winners are ecological losers" (Fernández-Palacios et al, 2021). Thus, we propose that radiation-or lack thereof-may arise more from dispersal limitation or genetic factors rather than the existence of the niches themselves.…”
Some lineages radiate spectacularly when colonizing a region, but others do not. Large radiations are often attributed to species’ adaptation into niches, or to other drivers, such as biogeography including dispersal ability and spatial structure of the landscape. Here we aim to disentangle the factors determining radiation size, by modeling simplified scenarios without the complexity of explicit niches. We build a spatially structured neutral model free from niches and incorporating a form of protracted speciation that accounts for gene flow between populations. We find that a wide range of radiation sizes are possible in this model depending on the combination of geographic isolation and species’ dispersal ability. At extremely low rates of dispersal between patches, each patch maintains its own endemic species. Intermediate dispersal rates foster larger radiations as they allow occasional movement between patches whilst sufficiently restricting gene flow to support further speciation in allopatry. As dispersal rates increase further, a critical point is reached at which demographically identical lineages may vary greatly in radiation size due to rare and stochastic dispersal events. At the critical point in dispersal frequency, some lineages remain a single species for a comparatively long time, whilst others with identical characteristics produce the largest radiations of all via a new mechanism for rapid radiation that we term a ‘radiation cascade’. Given a single species covering many patches connected with gene flow, a radiation cascade is triggered when stochastic dispersal is unusually low for a period, leading to an initial speciation event. This speciation means there are fewer individuals per species and thus further reduced gene flow between conspecifics. Reduced gene flow in turn makes it easier for further speciation to occur. During a radiation cascade, dispersal of individuals between patches continues at the same rate as before, but due to the increasing diversity it primarily introduces novel species that will later speciate, rather than adding to gene flow of existing species. Once a radiation cascade begins, it continues rapidly until it is arrested by a new equilibrium between speciation and extinction. We speculate that such radiation cascades may occur more generally and are not only present in neutral models. This process may help to explain rapid radiation, and the extreme radiation sizes of certain lineages with dispersing ancestors. Whilst niches no doubt play a role in community assembly, our findings lead us to question whether diversification and adaptation into niches is sometimes an effect of speciation and rapid radiation, rather than its cause.
“…The high habitat diversity of the Canary Islands provides ample opportunities for species that, together with inter-island dispersal, promotes speciation and drives the high functional diversity of the Canary endemics. Conversely, Tenerife endemics are functionally quite similar; these species occupy similar island habitats, have a low degree of niche differentiation, and have likely emerged via allopatric speciation 45 .…”
Section: Main Textmentioning
confidence: 99%
“…The dominance of shrubby species on Tenerife is consistent with the idea that small herbaceous colonisers gradually evolve into taller plants with increased stem density to avoid competition with other species 44 . Yet, the evolution of several Tenerife shrub species and the subsequent increase in shrubbiness has mostly occurred in steep canyons and high-elevation ecosystems 45 , where conditions are unfavourable for trees and competition with taller plants is low. The underrepresentation of tall plants in Tenerife's trait space reflects the comparatively low number of tree species (mainly in laurel and pine forest, and thermophilous woodland ecosystems) compared to shrubs.…”
Section: An Oceanic Island Flora Faces Similar Functional Constraints...mentioning
confidence: 99%
“…Conversely, Tenerife endemics are functionally quite similar; these species occupy similar island habitats, have a low degree of niche differentiation, and have likely emerged via allopatric speciation 45 .…”
Section: An Oceanic Island Flora Faces Similar Functional Constraints...mentioning
Oceanic island floras are well-known for their morphological peculiarities and exhibit striking examples of trait evolution. These morphological shifts are commonly attributed to insularity and thought to be shaped by biogeographical processes and evolutionary histories of oceanic islands. However, the mechanisms through which biogeography and evolution have shaped the distribution and diversity of plant functional traits remain unclear. Here, we describe the functional trait space of an oceanic island flora (Tenerife, Canary Islands, Spain) using extensive field and laboratory measurements, and relate it to global trade-offs in ecological strategies. We find that the island trait space is concentrated around a functional hotspot dominated by shrubs with a conservative life-history strategy. By dividing the island flora into species groups with distinct biogeographical distributions and diversification histories, our results reveal that long-distance dispersal, and the interplay between inter-island dispersal and archipelago-level speciation processes drive functional divergence and expand trait space. Conversely, speciation via cladogenesis has overall led to functional convergence, densely packing trait space around shrubbiness. Our approach combines ecology, biogeography and evolution and opens avenues for new trait-based insights into how dispersal and speciation shape the assembly of native island floras.
“…A considerable, but as yet undocumented, number of described and candidate Lepidodactylus species are known only from small islands and atolls (e.g., Zug et al 2003 ; Stubbs et al 2017 ; Karin et al 2018 ). Prior work on the Lepidodactylus also suggests that, like some other diverse insular lineages (Fernández‐Palacios et al 2021 ; Richmond et al 2021 ), this clade shows evidence of evolutionary displacement, with taxa concentrated away from species-rich lowland rainforests and into ‘marginal’ open, coastal or montane habitats, especially on the fringes of continental areas and larger islands like New Guinea and Borneo (Oliver et al 2018a , 2020 ). Fig.…”
Cryptic ecologies, the Wallacean Shortfall of undocumented species’ geographical ranges and the Linnaean Shortfall of undescribed diversity, are all major barriers to conservation assessment. When these factors overlap with drivers of extinction risk, such as insular distributions, the number of threatened species in a region or clade may be underestimated, a situation we term ‘cryptic extinction risk’. The genus Lepidodactylus is a diverse radiation of insular and arboreal geckos that occurs across the western Pacific. Previous work on Lepidodactylus showed evidence of evolutionary displacement around continental fringes, suggesting an inherent vulnerability to extinction from factors such as competition and predation. We sought to (1) comprehensively review status and threats, (2) estimate the number of undescribed species, and (3) estimate extinction risk in data deficient and candidate species, in Lepidodactylus. From our updated IUCN Red List assessment, 60% of the 58 recognized species are threatened (n = 15) or Data Deficient (n = 21), which is higher than reported for most other lizard groups. Species from the smaller and isolated Pacific islands are of greatest conservation concern, with most either threatened or Data Deficient, and all particularly vulnerable to invasive species. We estimated 32 undescribed candidate species and linear modelling predicted that an additional 18 species, among these and the data deficient species, are threatened with extinction. Focusing efforts to resolve the taxonomy and conservation status of key taxa, especially on small islands in the Pacific, is a high priority for conserving this remarkably diverse, yet poorly understood, lizard fauna. Our data highlight how cryptic ecologies and cryptic diversity combine and lead to significant underestimation of extinction risk.
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