Abstract Understanding how and why rates of evolutionary diversification vary is a key issue in evolutionary biology, ecology, and biogeography. Evolutionary rates are the net result of interacting processes summarized under concepts such as adaptive radiation and evolutionary stasis. Here, we review the central concepts in the evolutionary diversification literature and synthesize these into a simple, general framework for studying rates of diversification and quantifying their underlying dynamics, which can be applied across clades and regions, and across spatial and temporal scales. Our framework describes the diversification rate ( d ) as a function of the abiotic environment ( a ), the biotic environment ( b ), and clade‐specific phenotypes or traits ( c ); thus, d ~ a,b,c . We refer to the four components ( a – d ) and their interactions collectively as the “Evolutionary Arena.” We outline analytical approaches to this framework and present a case study on conifers, for which we parameterize the general model. We also discuss three conceptual examples: the Lupinus radiation in the Andes in the context of emerging ecological opportunity and fluctuating connectivity due to climatic oscillations; oceanic island radiations in the context of island formation and erosion; and biotically driven radiations of the Mediterranean orchid genus Ophrys . The results of the conifer case study are consistent with the long‐standing scenario that low competition and high rates of niche evolution promote diversification. The conceptual examples illustrate how using the synthetic Evolutionary Arena framework helps to identify and structure future directions for research on evolutionary radiations. In this way, the Evolutionary Arena framework promotes a more general understanding of variation in evolutionary rates by making quantitative results comparable between case studies, thereby allowing new syntheses of evolutionary and ecological processes to emerge.
Summary The mid-Cretaceous is an important time for the diversification of forests globally, including the rise to dominance of the angiosperms and the beginning of the isolation of Zealandia. In New Zealand, little information is available on the mid-Cretaceous xyloflora. New specimens of fossil wood from the mid-Cretaceous Tupuangi Formation were collected from Waihere Bay, Pitt Island, Chatham Islands, of which 16 well-preserved samples were identified, representing Araucariaceae ( Agathoxylon , 5 samples), Cupressaceae ( Taxodioxylon and Cupressinoxylon , one sample each), Podocarpaceae ( Protophyllocladoxylon , one sample), and the ‘Group B’ and ‘Group C’ Mesozoic conifers (four samples each) defined in Bamford & Philippe (2008). Of these, only Taxodioxylon had been identified previously from the Tupuangi Formation. Two new species are erected, Cupressoxylon dianneae sp. nov. and Protophyllocladoxylon jacobusii sp. nov. These records are important for understanding the mid-Cretaceous flora of New Zealand and the history of the unique modern flora of New Zealand.
Biome conservatism is often regarded as common in diversifying lineages, based on the detection of low biome shift rates or high phylogenetic signal. However, many studies testing biome conservatism utilise a single-biome-per-species approach, which may influence the detection of biome conservatism. Meta-analyses show that biome shift rates are significantly lower (less than a tenth), when single biome occupancy approaches are adopted. Using New Zealand plant lineages, estimated biome shifts were also significantly lower (14–67% fewer biome shifts) when analysed under the assumption of a single biome per species. Although a single biome approach consistently resulted in lower biome shifts, it detected fewer instances of biome conservatism. A third of clades (3 out of 9) changed status in biome conservatism tests between single and multiple biome occupancy approaches, with more instances of significant biome conservatism when using a multiple biome occupancy approach. A single biome approach may change the likelihood of finding biome conservatism because it assumes biome specialisation within species, falsely recognises some biome shift types and fails to include other biome shift types. Our results indicate that the degree of biome fidelity assumed has a strong influence on analyses assessing biome shift rates, and biome conservatism testing. We advocate analyses that allow species to occupy multiple biomes.
Abstract There are two prominent and competing hypotheses that disagree about the effect of competition on diversification processes. The first, the bounded hypothesis, suggests that species diversity is limited (bounded) by competition between species for finite ecological niche space. The second, the unbounded hypothesis, proposes that innovations associated with evolution render competition unimportant over macroevolutionary timescales. Here we use phylogenetically structured niche modelling to show that processes consistent with both of these diversification models drive species accumulation in conifers. In agreement with the bounded hypothesis, niche competition constrained diversification, and in line with the unbounded hypothesis, niche evolution and partitioning promoted diversification. We then analyse niche traits to show that these diversification enhancing and inhibiting processes can occur simultaneously on different niche dimensions. Together these results suggest a new hypothesis for lineage diversification based on the multi-dimensional nature of ecological niches that can accommodate both bounded and unbounded evolutionary processes.
Abstract Understanding how and why rates of evolutionary diversification vary is a key issue in evolutionary biology, ecology, and biogeography, and the metaphorical concepts of adaptive radiation and evolutionary stasis describe two opposing aspects causing variation in diversification rates. Here we review the central concepts in the evolutionary diversification literature and synthesize these into a simple, general framework for studying rates of diversification and quantifying their underlying dynamics, which can be applied across clades and regions and across spatial and temporal scales. Our framework describes the diversification rate ( d ) as a function of the abiotic environment ( a ), the biotic environment ( b ) and clade-specific phenotypes or traits ( c ); thus d ∼ a,b,c . We refer to the four components ( a – d ) and their interactions collectively as the ‘Evolutionary Arena’. We outline analytical approaches to this framework and present a case study on conifers, for which we parameterise the general model. We also discuss three conceptual examples: the Lupinus radiation in the Andes in the context of emerging ecological opportunity and fluctuating connectivity due to climatic oscillations; oceanic island radiations in the context of island formation and erosion; and biotically driven radiations of the Mediterranean orchid genus Ophrys . The results of the conifer case study are consistent with the long-standing scenario that low competition and high rates of niche evolution promote diversification. The conceptual examples illustrate how using the synthetic Evolutionary Arena framework helps to identify and structure future directions for research on evolutionary radiations. In this way, the Evolutionary Arena framework promotes a more general understanding of variation in evolutionary rates by making quantitative results comparable between case studies, thereby allowing new syntheses of evolutionary and ecological processes to emerge.
Abstract Aim To investigate species and clade biome occupancy patterns of Australian Acacia to test for within‐biome diversification, which indicate biome conservatism. Location Australia. Taxon Acacia (Fabaceae). Methods Species distributions were predicted for 481 Australian Acacia using the Thornley Transport Resistance Species Distribution Model and mapped across four biome typologies. Within Acacia 19 clades were identified. The number of biomes occupied and niche size was quantified for every species and clade using the range area projected by the distribution model. Relationships between clade species richness, niche size and biomes occupied were tested using phylogenetic least squares regression models. Results Only 9% of the Acacia 481 species and no clades were biome specialists. There were most specialist taxa in the Crisp Biome classification (8.7%), followed by WWF Biomes (6.2%), González–Orozco Biomes (5.0%) then Functional Biomes (1.2%). On average Acacia species occupied four WWF Biomes, seven Functional Biomes, three Crisp Biomes and three González–Orozco Biomes (out of 7, 13, 5 or 6 biomes respectively). Clades were also distributed across multiple biomes (2–13) with a significant positive relationship between clade species richness and the number of biomes occupied for all biome typologies. Species richness had positive linear relationships with biome area for all biome concepts except the González–Orozco Biomes. Larger clades had larger niche sizes. Main conclusions Acacia diversification occurred across biome boundaries and was not associated with biome specialization. Species and clades mainly occurred in multiple biomes, and there were typically few biome specialists. Diversification in Acacia appears to be decoupled from biome conservatism, associated with expanding niche size across biome boundaries. Major ecological–environmental units such as biomes may constrain adaptive radiation processes via biome conservatism in many groups, but this study leads us to hypothesize that for some lineages biome boundaries are permeable.
Abstract Aim How mountains accumulate species diversity remains poorly understood, particularly the relative role of in situ cladogenesis compared with colonization from lower elevations. Here, we estimated the contributions of in situ cladogenesis and colonization in generating biodiversity of a large mountain plant radiation and determined the importance of niche adaptation and divergence in these processes. We expected cladogenesis would accompany novel habitats formed by mountain uplift, but colonization would become more important with time as dispersal opportunities accrue. Location New Zealand, Southern Alps. Taxon Veronica sect. Hebe (Plantaginaceae). Methods We estimated the most complete time‐calibrated phylogeny to date for Veronica sect. Hebe to quantify rates of in situ cladogenesis and colonization of mountain habitat based on historical biogeographical models. We used environmental niche modelling to quantify species' climate niches and estimate niche disparity and divergence over time. Results In situ cladogenesis generated more species in the mountains than colonization from lowlands. Whereas cladogenesis slowed over time, colonization increased, especially in the alpine zone. Both adaptive ecological speciation along climate niche axes and non‐adaptive, vicariant speciation contributed to cladogenesis. However, climate niche disparity through time became saturated, suggesting competition for niche space was important. Colonization brought more divergent species into mountain niches. Main Conclusions We suggest mountain diversity accumulates through three main stages: high cladogenesis after initial colonization, decreasing cladogenesis with increasing competition and increasing colonization after niches saturate, likely promoted by niche divergence. Combining lineage and mountain uplift trajectories, these stages provide a conceptual model to understand how diversity accumulates elsewhere. Assuming these deep‐time findings apply to anthropogenic conditions, alpine specialists could struggle to outcompete colonizers facilitated by climate change, especially from generalist clades. Considering novel competitive interactions alongside niche traits and biogeographical processes will be crucial for predicting the fate of alpine biodiversity in a changing world.
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