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Research review Process and pattern in cichlid radiations – inferences for understanding unusually high rates of evolutionary diversification Author for correspondence: Ole Seehausen Tel: +41 31 631 31 31 Email: [email protected]

Ole Seehausen1,2 1

Institute of Ecology and Evolution, University of Bern, Bern, Switzerland; 2EAWAG Centre for Ecology, Evolution and

Biogeochemistry, Kastanienbaum, Switzerland

Received: 23 December 2014 Accepted: 26 February 2015

Summary New Phytologist (2015) 207: 304–312 doi: 10.1111/nph.13450

Key words: adaptive radiation, biodiversity, cichlid fish, diversification, evolutionary ecology, phylogenetic lineage, speciation.

The cichlid fish radiations in the African Great Lakes differ from all other known cases of rapid speciation in vertebrates by their spectacular trophic diversity and richness of sympatric species, comparable to the most rapid angiosperm radiations. I review factors that may have facilitated these radiations and compare these with insights from recent work on plant radiations. Work to date suggests that it was a coincidence of ecological opportunity, intrinsic ecological versatility and genomic flexibility, rapidly evolving behavioral mate choice and large amounts of standing genetic variation that permitted these spectacular fish radiations. I propose that spatially orthogonal gradients in the fit of phenotypes to the environment facilitate speciation because they allow colonization of alternative fitness peaks during clinal speciation despite local disruptive selection. Such gradients are manifold in lakes because of the interaction of water depth as an omnipresent third spatial dimension with other fitness-relevant variables. I introduce a conceptual model of adaptive radiation that integrates these elements and discuss its applicability to, and predictions for, plant radiations.

Introduction Replicated species radiations in related lineages are excellent systems to investigate causes of the tremendous variation in diversity and diversification that is a hallmark of biological evolution. Classical examples include replicate radiations of plants and animals on oceanic island archipelagos, on mountains, and in lakes (Simpson, 1953; Schluter, 2000; Gavrilets & Losos, 2009). Environmental factors such as availability and diversity of underexploited resources, as well as lineage-specific traits and their interaction influence the occurrence and extent of radiations (Givnish, 2010; Wagner et al., 2012a,b; Onstein et al., 2014). Arguably the most informative approaches to investigating these questions couple evolutionary ecology studies of mechanisms with phylogenetic comparative studies of patterns of diversity (Schiestl & Schlueter, 2009). This integrative approach is facilitated in systems with replicate radiations varying in age from very recent to rather ancient. Among animals, cichlid fish radiations in African lakes provide precisely this. The youngest of their radiations exceed all other known animal radiations in rates of species origination and 304 New Phytologist (2015) 207: 304–312 www.newphytologist.com

species richness attained, resembling some of the most rapid and extensive species radiations in flowering plants (Hughes & Eastwood, 2006; Givnish, 2010). The classical radiations in the three largest African lakes, Tanganyika, Malawi, and Victoria, have received most attention, but the power of this system comes from the large number of radiations of various sizes and ages in lakes all across Africa, and even more cases where cichlids colonized lakes without radiating at all (Seehausen, 2006; Wagner et al., 2012a,b). On a first pass, the question of what cichlid radiations might have in common with plant radiations might seem a little far-fetched, begging the question, why have a review of cichlid radiations in this New Phytologist Special Issue on plant evolutionary radiations? However, on closer inspection, many interesting parallels are revealed. Just as for cichlid fish, much of the great species diversity of flowering plants is relatively young, the result of many nested and parallel radiations across disparate lineages, adaptive and nonadaptive (sensu Rundell & Price, 2009), and ranging from very recent (Hodges & Arnold, 1994; Drummond et al., 2012; Jabaily & Sytsma, 2013; Breitkopf et al., 2015) to many millions yr old (Linder, 2008; Hughes et al., 2013; Onstein et al., 2014; Schwery Ó 2015 The Author New Phytologist Ó 2015 New Phytologist Trust

New Phytologist et al., 2015). Importantly, students of both groups have been assembling increasingly large comparative data sets covering multiple replicate radiations to infer correlates of diversification and stasis, while also conducting extensive investigations of mechanisms. It is very timely, therefore, to compare and combine insights from studying radiations in these very different taxa and ecosystems. While a comprehensive synthesis of cichlid and plant radiations is beyond the scope of this article, I introduce major features of cichlid radiations in a comparative context to facilitate a fruitful exchange of ideas and insights amongst investigators of these different radiations.

Global distribution of cichlid fish diversity Cichlids are distributed in freshwaters of tropical and subtropical America, Africa, Madagascar, and southern India. With some 1300 valid species described, but at least 2200 known, they are the second most diverse family of freshwater fish. Species richness is geographically highly heterogeneous: 450 and 110 species are known from tropical South and Central America, respectively, 32 from Madagascar, three from India, four from the Levant and one from Iran. The exceptional species richness of cichlid fish only manifests in tropical Africa, with some 1600 species known and a pronounced latitudinal species richness gradient; 1400 of these are endemic to individual lakes close to the equator. Endemic species are much less common in lakes in subtropical Africa and the Levant (Wagner et al., 2014), and just 175 species are known from rivers all across Africa, again with a peak near the equator (Fig. 1). In Madagascar, India, and Central and South America, on the other hand, most cichlid species occur in rivers. Lake endemism is uncommon on these continents, probably because geologically older lakes (i.e. predating the Holocene) are rare. The lakes of Nicaragua are an exception (Elmer et al., 2013), but large paleolakes in the Amazon basin (Miocene Lake Pebas and older lakes) might have been important centers of neotropical cichlid diversification in the past (Anderson et al., 2010; Albert & Reis, 2011). A hallmark of replicated species radiations is that their exceptional diversity results from local evolutionary processes that operate within geographically confined island and island-like areas (Gavrilets & Losos, 2009). Only the most geologically stable of the African lakes, Lake Tanganyika, has functioned as a refugium for old lineages (Genner et al., 2007b), whereas both old and young lakes have functioned as cradles for the birth of new lineages. Many of these lineages subsequently escaped, spread across the continent, and seeded new radiations in lakes elsewhere (Joyce et al., 2005; Salzburger et al., 2005), spawning a series of phylogenetically nested radiations (Seehausen, 2006). Similar nested radiations have also been documented in birds (Burns et al., 2014) and several angiosperm families (Drummond et al., 2012; Koenen et al., 2013; Schwery et al., 2015), suggesting that local evolutionary processes have lasting effects on global biodiversity. Here I address possible reasons why cichlids, in particular, have radiated so often in geographically narrowly confined areas despite very limited opportunities for geographical isolation, conditions where most other animals – including most other fish – did not radiate at all. Ó 2015 The Author New Phytologist Ó 2015 New Phytologist Trust

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Cichlid radiations are characterized by many sympatric reproductively isolated species In order for a lineage to evolve large species numbers within a geographically confined area, populations have to quickly become sufficiently distinct to coexist and remain reproductively isolated in sympatry. In Lake Victoria alone, more than 500 phenotypically distinct putative species have originated, probably within the past 15 000 yr (Johnson et al., 2000); 100 or more can be found in sympatry or ecological parapatry within small sections of the lake. Most other fish groups with a known tendency for rapid speciation, exemplified by sticklebacks (McKinnon & Rundle, 2002), fail to evolve more than two or three reproductively isolated species that can coexist in sympatry. What allows for this striking difference? A high amount of sympatry among closely related species is a distinctive feature of adaptive radiation, in contrast to nonadaptive radiations characterized by rapid origin of many geographically isolated species that do not coexist locally (Rundell & Price, 2009). However, adaptive diversification can also involve intraspecific polymorphism without speciation (Smith & Skulason, 1996). Some have suggested that this might explain the phenotypic diversity of cichlids in Lake Victoria (Sage & Selander, 1975; Samonte et al., 2007). Bezault et al. (2011) investigated genetic diversity and differentiation of 105 mostly sympatric, putative species, defined as groups that differ in several functionally independent phenotypic traits. They found a pervasive genetic signature of speciation, that is, considerable genetic differentiation in sympatry. Newer investigations using next-generation sequencing of restriction site associated DNA (RAD tags) demonstrated complete reciprocal monophyly for each of 16 putative species from a single island community (Wagner et al., 2012b). Because species were sampled from narrow-sense sympatry, both studies demonstrate reproductive isolation and conclusively answer the speciation vs polymorphism question. Nonetheless, occasional gene flow between related species does occur and probably explains why sympatric populations of related species can be more similar at neutral markers than allopatric populations of the same species (Konijnendijk et al., 2011). It seems that the rapid evolution of strong reproductive isolation despite full sympatry is the key feature that sets cichlid radiations apart from other freshwater fish radiations studied to date (Seehausen & Wagner, 2014). We therefore need to consider ecological and behavioural mechanisms and genetic underpinnings of reproductive isolation and coexistence of closely related species.

Modes and mechanisms of speciation in cichlid adaptive radiations The evidence for monophyly of lacustrine cichlid species flocks suggests that the vast majority of speciation events must have happened within individual lake systems. Opportunities for modes of speciation that require geographical isolation within a lake scale with lake size and age. Ways by which allopatric speciation might occur in lakes are summarized in Box 1, but many radiations occur in lakes that are too small and recent for allopatric speciation New Phytologist (2015) 207: 304–312 www.newphytologist.com

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Fig. 1 Patterns of cichlid fish diversity in Africa. The map shows continental distribution of species richness plotted as approximate number of species occurring in 10 9 10 km squares. Data for riverine cichlid diversity are collated from Lamboj (2004) and FishBase (Froese & Pauly, 2014), and those for lacustrine diversity are from Genner et al. (2004) and Wagner et al. (2014). Lakes are visible as islands of elevated species richness (note that n species per 10 9 10 km square are reported also for lakes, rather than total species numbers of lakes). Cichlid images show representatives of typical ecotypes present in immigration-assembled river communities (left) and in speciation-assembled lake communities (right). Phylograms are schematic to illustrate the very different phylogeographic structure of immigration and speciation assemblages; topology and timescale (in millions of yr) are taken from Wagner et al. (2012a). In situ diversified clades are shown as blue triangles. Letters denote genera as follows (and typical diet): O, Oreochromis (detritus); Ti, Tilapia (plants); He, Hemichromis (large insects, fish); P, Pelvicachromis (Aufwuchs); Be, Benitochromis (small insects); Ty, Tylochromis (snails); A, Astatotilapia and Thoracochromis (insects); Se, Serranochromis (fish); Sa, Sargochromis (snails, insects); Ps, Pseudocrenilabrus (small insects). For the lake radiations, letters denote the following: a, pelagic zooplanktivore; b, rock-dwelling algae scraper; c, paedophage (absent from Lake Tanganyika); d, scale eater; e, snail crusher; f, reef-dwelling planktivore; g, lobe-lipped insect eater; h, pelagic piscivore; i = A, ancestral river-dweller that also occurs in the lakes alongside the radiation members (absent from Lake Tanganyika). Photographs are courtesy of: Ad Konings (Tanganyika, all; Malawi, a, c–h, i = A), Ole Seehausen (Victoria, a–g, i = A; Malawi, b), Frans Witte (Victoria, h).

(Schliewen et al., 1994; Barluenga et al., 2006; Wagner et al., 2012a,2014). Studying the radiations in Lakes Victoria, Edward, Kivu, and smaller lakes, Bezault et al. (2011) showed that individual species were genetically nearly as variable as the entire radiation. This implies large effective population sizes during and after speciation, or hybridization after speciation, or both. Analysis of whole genomes and single nucleotide polymorphism data revealed that very young Lake Victoria species carry large amounts of very old standing genetic variation (Brawand et al., 2014), unlikely to be New Phytologist (2015) 207: 304–312 www.newphytologist.com

maintained through many successive speciation events involving small geographical isolates. This all points to an important role of speciation in large populations involving selection and gene flow. Studies in recent plant radiations reveal a diversity of modes of speciation. Work on an island radiation in the Juan Fernandez Archipelago (Chile) found evidence of reduced genetic variation within individual species, more consistent with allopatric speciation (Takayama et al., 2015). Nonallopatric speciation has been suggested to contribute to the rapid radiations in taxa as diverse as sexually deceptive orchids (Breitkopf et al., 2015), Amazonian Ó 2015 The Author New Phytologist Ó 2015 New Phytologist Trust

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Box 1 Evidence for allopatric speciation

Box 2 Evidence for nonallopatric speciation in lakes

Palaeo-hydrological and phylogenetic data for the Lake Tanganyika radiation provide evidence for speciation in isolated lake basins. The distribution of mitochondrial haplotype lineages in several taxa reflects ancient shorelines of basins that were isolated during Pleistocene low water levels. Isolation lasted for a minimum of 20 000 yr (Ruber et al., 1999; Sturmbauer et al., 2001). In some cases, these haplotype lineages now coexist sympatrically as reproductively isolated species (Egger et al., 2007). These are strong cases for allopatric speciation. However, alone they cannot explain the origin of species richness. Evidence for speciation in smaller satellite lakes is limited. The strongest case is a dwarf Rhamphochromis in Lake Chilingali, a satellite of Lake Malawi (Genner et al., 2007a). Several authors suggested a role for restricted gene flow among isolated patches of rocky habitat to drive genetic differentiation among populations within a lake (Kocher, 2004; Koblmueller et al., 2011). However, reproductive isolation upon secondary contact among such allopatric populations is often weak. Laboratory tests found hybridization rates between 7% and 68% (Knight & Turner, 2004; Egger et al., 2008). The incipient species would probably often fuse upon secondary contact, unless there was strong selection against hybridization. Loss of genetic differentiation has indeed been shown in several cases of recent secondary contact (Streelman et al., 2004; Egger et al., 2012). True allopatric speciation, whereby reproductive isolation is already complete upon return to sympatry, is unlikely to be very common in lacustrine cichlids. It remains unclear how often species that now live in full sympatry originated as allopatric populations, and whether sometimes exclusive and mosaic distributions of closely related species (Allender et al., 2003) result from allopatric speciation or from parapatric speciation with competitive exclusion. Contrary to popular perception of cichlid radiations, endemism at individual islands is nearly entirely absent in the 500 species strong radiation of Lake Victoria itself (Seehausen, 1996).

Classical examples of sympatric speciation in cichlid fish come from crater lakes in Cameroon (Schliewen et al., 1994) and Nicaragua (Barluenga et al., 2006). Similar processes are likely to have contributed to the origins of species diversity in the large lake radiations. Several studies in these lakes indicate that even though geographically sympatric, the process typically involves fine-scale spatial structure in habitat conditions and resources. Divergent selection between spatially structured ecological niches may initiate speciation in primary contact (Gavrilets, 2004). Depth differentiation of feeding and breeding sites is a good predictor of reproductive isolation among geographically sympatric species, trophic morphs, and color morphs in Lake Victoria. Several studies found speciation was nonexistent or incipient in the absence of depth differentiation but there was increasingly strong neutral genetic differentiation as depth differentiation increased (Seehausen & Magalhaes, 2010). Such speciation, even though completely sympatric at geographical scales, is actually parapatric at finer spatial resolution (see Fig. 2). It is likely that even subtle differences in depth habitat facilitate the evolution of reproductive isolation through initializing linkage disequilibrium between genes involved in adaptation, and between these and mating genes (Seehausen et al., 2008). In taxa with strong behavioral mate choice, such initial deviations from linkage equilibrium may be maintained or reinforced by assortative mating as a consequence of the interaction of natural and sexual selection. The evolution of complete behavioral reproductive isolation could eventually permit return to complete sympatry, perhaps associated with ecological shifts along niche axes other than depth (Fig. 2).

lowland forest trees (Kursar et al., 2009), and different families of plants on Lord Howe Island (Papadopulos et al., 2013). However, in most cases, the mechanisms and geographical modes of speciation in recent plant radiations are largely unknown (Schwery et al., 2015). Cichlids have radiated almost exclusively in lakes (Seehausen, 2006), and while sympatric speciation is considered very uncommon in vertebrates in general, it has been reported in lake-dwelling fishes more often than in any other group (Bolnick & Fitzpatrick, 2007; Seehausen & Wagner, 2014). Lakes differ from many terrestrial and most river environments by the omnipresence of an additional spatial habitat dimension: depth (Seehausen & Magalhaes, 2010). Water depth mediates wide but steep gradients in light, temperature, and oxygen. These in turn affect resources, predators, parasites, sensory requirements, and signaling conditions. Such differences along continuous depth gradients allow sorting of adaptive variation by ecological (e.g. matching habitat choice; Edelaar et al., 2008) and evolutionary (selection) mechanisms. If mating then happens mostly within the foraging habitat, the process automatically leads to partial reproductive isolation (Seehausen & Magalhaes, 2010; Servedio et al., 2011). Such speciation may indeed characterize lacustrine cichlid radiations (Box 2). Very similar mechanisms have been suggested to explain Ó 2015 The Author New Phytologist Ó 2015 New Phytologist Trust

sympatric speciation in several plant genera on Lord Howe Island. In these cases, sympatric speciation is associated with divergence along altitudinal gradients where adaptation to altitude involves shifts in flowering time that may precipitate prezygotic isolation (Papadopulos et al., 2011). Besides such ecological mechanisms, spatial genetic structure and intermittent periods of geographical isolation, mediated by changes in water level, are nevertheless also likely to play important roles. By reducing gene flow between populations, they too generate deviations from linkage equilibrium among adaptation and mating genes which may subsequently facilitate speciation by divergent natural and sexual selection (Aguilee et al., 2013). It has been suggested that similar processes may play an important role in mountain and alpine plant radiations where climate fluctuations may drive cycles of isolation and reconnection among populations. For example, Schwery et al. (2015) demonstrated that plant speciation rates are consistently higher in mountainous landscapes than in lowlands in the species-rich Ericaceae. More tests in other groups will be exciting.

The macroecology of radiations: ecological opportunity facilitates radiation when reproductive isolation can evolve quickly Adaptive radiation is thought of as an evolutionary response to ecological opportunity (Simpson, 1953). Extrinsic factors that have been suggested to mediate such opportunity include an New Phytologist (2015) 207: 304–312 www.newphytologist.com

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abundance and diversity of resources and a paucity of competitors and antagonists (Schluter, 2000). Alternatively, opportunity may arise when new traits evolve that affect ecological versatility (key morphological (Liem, 1973; Givnish et al., 2014) or lifehistory traits (Drummond et al., 2012, but see Givnish, 2015)). Moreover, opportunity for evolutionary response to ecological opportunity may arise when traits evolve that affect gene flow and reproductive isolation, such as the prevalence of sexual selection (Kraaijeveld et al., 2011) or spatial vagility (Givnish, 2010; Kisel & Barraclough, 2010). Analyzing a large data set of lake characteristics, species traits, species richness, clade ages, and phylogenetic relationships, Wagner et al. (2012a) found that the best-supported positive predictor variables for cichlid fish radiation included environmental variables that predict ecological opportunity (lake depth, energy availability) and the lineagespecific trait of sexual dichromatism (indicating evolutionary response to sexual selection) (Wagner et al., 2012a). Additionally, richness of radiations was predicted by lake area, depth, and energy consistent with diversity limitation by these factors (Wagner et al., 2014). Interestingly, in African cichlid fish, adaptive radiation strongly modifies the species–area relationship, resulting in assemblages orders of magnitude richer in species than those assembled by immigration alone. It will be interesting to see if this is a general phenomenon across organismal groups. Several of these macroevolutionary continental-scale patterns in cichlid diversity follow intuitively from the mechanisms uncovered in speciation case studies. The latter have shown that intensity of sexual selection is a key influence on the probability that cichlids speciate (Knight & Turner, 2004; Seehausen et al., 2008), and that water depth, besides mediating divergent sexual selection, is an important axis of ecological and reproductive niche differentiation. Combined effects of intrinsic traits and ecological opportunity also consistently predict plant radiations (Givnish, 2010). Schwery et al. (2015) found that the combination of specific leaf area and invasion of oligotrophic mountain habitats predicted increased rates of speciation in Ericaceae (Schwery et al., 2015), and (Onstein et al., 2014) found similar interactions in other families of the Cape Flora. Drummond et al. (2012) found increased rates of speciation associated with evolution of perennial life history and invasion of island-like montane ecosystems in Lupinus. A widespread analog to the importance of mate choice in cichlids is the importance of floral isolation evolving through pollinator partitioning in many angiosperms. Increased diversification rates in clades that shifted to a diverse group of specialized pollinators and expanded their ecological niche ranges have been documented in several plant groups (Schiestl & Schlueter, 2009; Givnish, 2010; Breitkopf et al., 2015).

Radiation in lakes and stasis in rivers Every large cichlid radiation has re-evolved most of the different trophic levels and feeding types that characterize tropical freshwater fish assemblages more generally, ranging from phytoplankton feeders, detritus feeders, and filamentous algae scrapers to various kinds of specialized invertivores and a bewildering diversity of New Phytologist (2015) 207: 304–312 www.newphytologist.com

New Phytologist piscivores, associated with a spectacular range of morphological, physiological, and behavioral adaptations. No other freshwater fish group is known to have evolved such trophic diversity in such short a time. The resulting morphologically recognized ecotypes repeat themselves with modifications in each larger radiation (Fig. 1). What is less widely appreciated is that many of these classical radiation ecotypes are recasts of the major ecotypes that characterize comparatively species-poor cichlid assemblages in African rivers. The major differences between these communities are threefold. First, river communities are immigration-assembled, whereas lake communities are speciation-assembled. Whereas each ecotype in rivers is typically represented by a separate lineage of considerable age that has independently colonized each river, various ecotypes have re-evolved from within the same lineage in many lakes. Second, additional ecotypes have evolved in lakes that are unknown from rivers, mostly specialists on resources that are uncommon or temporally unavailable in rivers, which may not support such specialized species. Third, each ecotype niche is fine-partitioned among many species in the lake radiations, allowing an order of magnitude more species to coexist locally in lake communities than in rivers. That river cichlid assemblages differ from lake cichlid radiations in species richness and the absence of in situ speciation, but not necessarily in ecotypic disparity, suggests that species richness in rivers is limited by constraints to speciation and/or coexistence of closely related species. In those parts of central and western equatorial Africa where the diversity of old cichlid lineages is at its highest, quite a few more ecotypes can be present in any one river than in most other parts of Africa, where very few old lineages persist. This is striking in Africa east of the rift valley. River cichlid assemblages in these regions contain between one and (rarely) four species, each an independent colonist from a different old lineage, mostly representing just two ecotypes: small invertivores and larger detritivores (Fig. 1). Given that river habitats are not obviously less diverse in these parts of Africa, and that noncichlid fish assemblages are also depauperate as a consequence of the isolated biogeographical history of this region, the few cichlids that are present should encounter ecological opportunities to radiate into multiple niches in rivers, but no case is known where this happened. A low diversity of cichlids in rivers is better explained by constraints on speciation than by lack of ecological opportunity. I speculatively suggest that the explanation lies in the very different adaptive landscapes in lakes vs rivers. The phenotypes of African river cichlids are conserved and disparate among sympatric lineages, suggesting that cichlid niches in rivers, unlike lakes, are functionally distant, discrete, and represent few alternative trophic levels and body sizes (a large detritivore, a small invertivore, occasionally a medium-sized or larger predator). Temporal instability of river habitats and resources may make several otherwise distinct peaks within a trophic level collapse to a single peak separated from peaks at other trophic levels by deep fitness valleys that cannot easily be traversed. This would make ecological speciation in rivers difficult. The greater spatial dimensionality, diversity, and temporal stability of habitats and resources in lakes Ó 2015 The Author New Phytologist Ó 2015 New Phytologist Trust

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divergence, and regulation by novel microRNAs. This diverse enrichment of the ancestral genome suggests that the radiations were preceded by periods of relaxed constraint allowing accumulation of genomic variation, most of which would not have been advantageous at the time. The propensity of the East African cichlid lineage to adaptively radiate might have originated during these periods. Analysis of very young species pairs from the recent radiation in Lake Victoria revealed evidence of genomically widespread species divergence in coding and regulatory variants, some of which were recruited from ancient polymorphisms (Brawand et al., 2014). Although the cause of the relaxed constraint remains unknown, genetic bottlenecks seem an unlikely explanation because they would not explain the maintenance of the large amounts of old standing genetic variation. In several lakes, there is evidence that radiations have evolved from hybrid populations (Joyce et al., 2005, 2011). Whether and how such hybridization facilitated adaptive radiations, or was just incidental to it, are subject to ongoing investigation (Selz et al.,

might facilitate multiple functionally adjacent peaks that are connected by ridges of relatively high fitness that exist in some combinations of traits, habitat, and space (Fig. 2).

The genomic substrate for adaptive radiation While the adaptive landscape might explain why cichlid radiations happen in lakes and not in rivers, it does not explain why other fishes do not also radiate in these lakes. In an attempt to understand how the spectacular phenotypic diversity and evolvability of African lake cichlids is accommodated in their genomes, genomes and transcriptomes of five lineages have been analyzed (Brawand et al., 2014). This work identified a large excess of gene duplications in the radiating East African lineage compared with other teleosts, including an older riverine cichlid lineage (the Nile tilapia (Oreochromis niloticus)) and sticklebacks, accelerated coding sequence evolution, an abundance of noncoding element divergence, transposable element insertions and associated expression

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Fig. 2 Schematic model of adaptive radiation through cycles of clinal adaptation, speciation, and return to full sympatry. A population of ancestral phenotype x1z1 in habitat y1 may evolve phenotype x2z2 as it expands its range along a habitat gradient (i.e. water depth) and tracks the clinally changing fitness peak in phenotype space (blue arrow 1; adaptation). LD, linkage disequilibrium between unlinked genes underlying the trait axes. Frequency-dependent selection emerging from local resource competition could lead to displacement of the optimum phenotypes away from what would otherwise be the local fitness optimum in the middle of the cline (indicated by hatched cones of high fitness). The local disruptive selection that results from this facilitates character displacement and the evolution of behavioral reproductive isolation. The latter is facilitated if one (or several) of the dimensions in phenotype space also affect mate choice (e.g. adaptation of the visual system to depth-mediated light regimes). Species A splits into A and B with different habitat (e.g. depth) niches and trait values. Further character displacement, possibly involving ecological dimensions other than habitat axis y, subsequently allows return to complete sympatry among reproductively isolated species (blue arrow 2). When this is associated with speciation of species B into B and C, the process of clinal divergence along habitat axis y might repeat itself in species C, leading to the origin of species D, followed by another episode of character displacement (E). A bottom-dwelling (benthic) cichlid population can, for instance, expand from shallow water to greater depth along the lake bottom, and break into two different species. Facilitated by some adaptations to deep water, the new species might subsequently invade the open water habitat and expand its depth range within this (limnetic) niche to again include shallow water. Note that fitness peaks in phenotype space would collapse to ridges of high fitness in this model if habitat space were not shown as a separate dimension, but that peaks of high fitness would be isolated by valleys of low fitness if habitat space were limited to just a narrow slice through space axis y (perhaps because a species lacks the ecological versatility to occupy a large sector of this axis or because the habitat does not exist). Note that the real world often offers more highly dimensional phenotype space as a template for speciation processes (–, areas of low fitness; +, areas of high fitness). Ó 2015 The Author New Phytologist Ó 2015 New Phytologist Trust

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2014, J. Meier et al., unpublished data), but it is clear that occasional hybridization after the beginning of radiation has given rise to additional species in all large lakes (Salzburger et al., 2002; Schliewen & Klee, 2004; Genner & Turner, 2012; Keller et al., 2012). It is possible that the period of relaxed constraint in the ancient history of the radiation-prone lineage was also associated with hybridization. The origins of genomic variation in cichlids have notable parallels in plant radiations, where interspecific hybridization has also led to the accrual of important genomic variation (Givnish, 2010). A classical example is the Hawaiian silversword alliance, which was derived from an allopolyploid hybrid population between two mainland diploid tarweed species (Barrier et al., 1999). Among other effects, this permitted accelerated evolution of duplicated regulatory regions involved in floral morphology (Barrier et al., 2001). Interspecific hybridization facilitated the radiation into new and extreme niches in other plants as well, with and without changes in ploidy (Rieseberg & Willis, 2007; Givnish, 2010; Renaut et al., 2014), and whole-genome duplications have been suggested as important drivers of diversification across flowering plants as a whole (Schranz et al., 2012).

A model for the process of adaptive radiation Drawing on all these observations, I suggest a conceptual model of adaptive radiation that contains four key elements (Fig. 2):  Genomically anchored ecological and phenotypic versatility of an ancestral population allowing it to explore a range of habitats and the genetic variation to adaptively partition these habitats and resources among species.  Spatial habitat gradients with a range of accessible resources, facilitating clinal sorting of phenotypes and genotypes. This initiates genetic coupling between initially independently inherited traits, including adaptation and mating traits, with minimal loss of genetic variation. Such clines may represent ridges of relatively high fitness in an adaptive landscape, connecting fitness peaks in phenotype space that would be isolated by valleys of low fitness in a spatially homogeneous environment, permitting their colonization without crossing deep fitness valleys. Together with disruptive selection, which might arise from negative frequency dependence of resource competition along a cline, this may cause speciation, provided that reproductive isolation can readily evolve (Doebeli & Dieckmann, 2003).  Rapidly evolving behavioral mate choice or pollinator partitioning, both of which facilitate the origins of prezygotic reproductive isolation.  The ability of reproductively isolated species to reinvade each other’s spatial niches while undergoing further character displacement in ecological dimensions unrelated to the spatial dimension of the original cline (Fig. 2, arrow 2). This can happen once species have acquired a trait combination that allows them to utilize a new ecological resource within the ancestral spatial niche. This ‘return to sympatry’ allows for a renewed cycle of clinal divergence and speciation in the same place but in a different ecological niche. New Phytologist (2015) 207: 304–312 www.newphytologist.com

Adaptive radiations in this model build through cycles of adaptation, speciation, and niche shift. The model predicts that disparity builds not through big jumps in ecophenotype space, but rather stepwise during sequential speciation events. The model predicts that exceptional radiations with rapid origin of many sympatric species should occur when ecologically versatile populations with highly evolvable mate choice or floral traits encounter ecological opportunities in a spatially heterogeneous environment with multiple resource gradients. These variables given, the rate of diversification may then depend primarily on the spatial dimensionality of the adaptive landscape (with at least partially orthogonal orientation of resource gradients, allowing return to sympatry through character displacement) and the availability of genetic variation, while the diversity eventually obtained may depend on ecological versatility. The model also predicts that rapid adaptive radiation may be constrained – despite genetic variation and ecological opportunity – when reproductive isolation cannot evolve rapidly, or when limited ecological versatility, or low spatial dimensionality of adaptive landscapes, constrains ecological niche shifts. Cichlids may be unusual among animals because they are ecologically versatile, genetically variable, have rapidly evolving behavioral mate choice, and utilize fully threedimensional lake landscapes with fitness variation along orthogonal dimensions of space (shoreline, onshore–offshore, depth). Applying this model to plants would predict that large and rapid radiations happen when taxa that display considerable phenotypic versatility, and that also have access to a diverse assemblage of specializing pollinators, invade landscapes with gradients in climate, soils, and pollinators. Mountainous landscapes are among those most likely to provide such conditions, and some very young large radiations are indeed found in mountain ranges with the steepest and most extended terrestrial environmental gradients in the world (Hughes & Atchison, 2015). Radiations of epiphytes into the forest canopy (Givnish, 2010), and perhaps even of canopy trees in tropical rainforest mosaics of hydrology, soils, and biotic interactions, could be other examples (Kursar et al., 2009). There appears to be much common ground between the exceptionally rapid radiations documented for cichlid fishes and flowering plants, offering expanded scope to address questions about what drives radiations, including the role of ancestral hybridization and occasional hybridization during the radiation, and how ecological opportunity and opportunity for the evolution of reproductive isolation interact, and perhaps coevolve, during radiations.

Acknowledgements I thank Catherine Wagner, Colin Hughes, and two insightful reviewers for commenting on earlier versions of this paper.

References Aguilee R, Claessen D, Lambert A. 2013. Adaptive radiation driven by the interplay of eco-evolutionary and landscape dynamics. Evolution 67: 1291–1306. Albert JS, Reis ER. 2011. Historical Biogeography of neotropical freshwater fishes. Berkeley, CA, Los Angeles, CA, USA & London, UK: University of California Press.

Ó 2015 The Author New Phytologist Ó 2015 New Phytologist Trust

New Phytologist Allender CJ, Seehausen O, Knight ME, Turner GF, Maclean N. 2003. Divergent selection during speciation of Lake Malawi cichlid fishes inferred from parallel radiations in nuptial coloration. Proceedings of the National Academy of Sciences, USA 100: 14074–14079. Anderson LC, Wesselingh FP, Hartman JH. 2010. A phylogenetic and morphologic context for the radiation of an endemic fauna in a long-lived lake: Corbulidae (Bivalvia; Myoida) in the Miocene Pebas Formation of western Amazonia. Paleobiology 36: 534–554. Barluenga M, Stolting KN, Salzburger W, Muschick M, Meyer A. 2006. Sympatric speciation in Nicaraguan crater lake cichlid fish. Nature 439: 719–723. Barrier M, Baldwin BG, Robichaux RH, Purugganan MD. 1999. Interspecific hybrid ancestry of a plant adaptive radiation: allopolyploidy of the Hawaiian silversword alliance (Asteraceae) inferred from floral homeotic gene duplications. Molecular Biology and Evolution 16: 1105–1113. Barrier M, Robichaux RH, Purugganan MD. 2001. Accelerated regulatory gene evolution in an adaptive radiation. Proceedings of the National Academy of Sciences, USA 98: 10208–10213. Bezault E, Mwaiko S, Seehausen O. 2011. Population genomic tests of models of adaptive radiation in Lake Victoria Region cichlid fish. Evolution 65: 3381–3397. Bolnick DI, Fitzpatrick BM. 2007. Sympatric speciation: models and empirical evidence. Annual Review of Ecology Evolution and Systematics 38: 459–487. Brawand D, Wagner CE, Li YI, Malinsky M, Keller I, Fan S, Simakov O, Ng AY, Lim ZW, Bezault E et al. 2014. The genomic substrate for adaptive radiation in African cichlid fish. Nature 513: 375–381. Breitkopf H, Onstein RE, Cafasso D, Schl€ uter PM, Cozzolino S. 2015. Multiple shifts to different pollinators fuelled rapid diversification in sexually deceptive Ophrys orchids. New Phytologist 207: 377–389. Burns KJ, Shultz AJ, Title PO, Mason NA, Barker FK, Klicka J, Lanyon SM, Lovette IJ. 2014. Phylogenetics and diversification of tanagers (Passeriformes: Thraupidae), the largest radiation of Neotropical songbirds. Molecular Phylogenetics and Evolution 75: 41–77. Doebeli M, Dieckmann U. 2003. Speciation along environmental gradients. Nature 421: 259–264. Drummond CS, Eastwood RJ, Miotto STS, Hughes CE. 2012. Multiple continental radiations and correlates of diversification in Lupinus (Leguminosae): testing for key innovation with incomplete taxon sampling. Systematic Biology 61: 443–460. Edelaar P, Siepielski AM, Clobert J. 2008. Matching habitat choice causes directed gene flow: a neglected dimension in evolution and ecology. Evolution 62: 2462– 2472. Egger B, Koblmuller S, Sturmbauer C, Sefc KM. 2007. Nuclear and mitochondrial data reveal different evolutionary processes in the Lake Tanganyika cichlid genus Tropheus. BMC Evolutionary Biology 7: 137. Egger B, Obermuller B, Eigner E, Sturmbauer C, Sefc KM. 2008. Assortative mating preferences between colour morphs of the endemic Lake Tanganyika cichlid genus Tropheus. Hydrobiologia 615: 37–48. Egger B, Sefc KM, Makasa L, Sturmbauer C, Salzburger W. 2012. Introgressive hybridization between color morphs in a population of cichlid fishes twelve years after human-induced secondary admixis. Journal of Heredity 103: 515–522. Elmer KR, Lehtonen TK, Fan S, Meyer A. 2013. Crater lake colonization by neotropical cichlid fishes. Evolution 67: 281–288. Froese R, Pauly D. 2014. FishBase. World Wide Web electronic publication. Gavrilets S. 2004. Fitness landscapes and the origin of species. Princeton, NJ, USA: Princeton University Press. http://www.fishbase.org/ [accessed 2014]. Gavrilets S, Losos JB. 2009. Adaptive radiation: contrasting theory with data. Science 323: 732–737. Genner MJ, Nichols P, Carvalho G, Robinson RL, Shaw PW, Smith A, Turner GF. 2007a. Evolution of a cichlid fish in a Lake Malawi satellite lake. Proceedings of the Royal Society B-Biological Sciences 274: 2249–2257. Genner MJ, Seehausen O, Cleary DFR, Knight ME, Michel E, Turner GF. 2004. How does the taxonomic status of allopatric populations influence species richness within African cichlid fish assemblages? Journal of Biogeography 31: 93– 102. Genner MJ, Seehausen O, Lunt DH, Joyce DA, Shaw PW, Carvalho GR, Turner GF. 2007b. Age of cichlids: new dates for ancient lake fish radiations. Molecular Biology and Evolution 24: 1269–1282. Ó 2015 The Author New Phytologist Ó 2015 New Phytologist Trust

Research review

Review 311

Genner MJ, Turner GF. 2012. Ancient hybridization and phenotypic novelty within Lake Malawi’s cichlid fish radiation. Molecular Biology and Evolution 29: 195–206. Givnish TJ. 2010. Ecology of plant speciation. Taxon 59: 1326–1366. Givnish TJ. 2015. Adaptive radiation versus ‘radiation’ and ‘explosive diversification’: why conceptual distinctions are fundamental to understanding evolution. New Phytologist 207: 297–303. Givnish TJ, Barfuss MHJ, Van Ee B, Riina R, Schulte K, Horres R, Gonsiska PA, Jabaily RS, Crayn DM, Smith JAC et al. 2014. Adaptive radiation, correlated and contingent evolution, and net species diversification in Bromeliaceae. Molecular Phylogenetics and Evolution 71: 55–78. Hodges SA, Arnold ML. 1994. Columbines – a geographically widespread species flock. Proceedings of the National Academy of Sciences, USA 91: 5129–5132. Hughes C, Atchison G. 2015. The ubiquity of alpine plant radiations: from the Andes to the Hengduan Mountains. New Phytologist 207: 275–282. Hughes C, Eastwood R. 2006. Island radiation on a continental scale: exceptional rates of plant diversification after uplift of the Andes. Proceedings of the National Academy of Sciences, USA 103: 10334–10339. Hughes CE, Pennington RT, Antonelli A. 2013. Neotropical plant evolution: assembling the Big Picture. Botanical Journal of the Linnean Society 171: 1–18. Jabaily RS, Sytsma KJ. 2013. Historical biogeography and life-history evolution of Andean Puya (Bromeliaceae). Botanical Journal of the Linnean Society 171: 201– 224. Johnson TC, Kelts K, Odada E. 2000. The holocene history of Lake Victoria. Ambio 29: 2–11. Joyce DA, Lunt DH, Bills R, Turner GF, Katongo C, Duftner N, Sturmbauer C, Seehausen O. 2005. An extant cichlid fish radiation emerged in an extinct Pleistocene lake. Nature 435: 90–95. Joyce DA, Lunt DH, Genner MJ, Turner GF, Bills R, Seehausen O. 2011. Repeated colonization and hybridization in Lake Malawi cichlids. Current Biology 21: R108–R109. Keller I, Wagner CE, Greuter L, Mwaiko S, Selz O, Sivasundar A, Wittwer S, Seehausen O. 2012. Population genomic signatures of divergent adaptation, gene flow, and hybrid speciation in the rapid radiation of Lake Victoria cichlid fishes. Molecular Ecology 22: 2848–2863. Kisel Y, Barraclough TG. 2010. Speciation has a apatial scale that depends on levels of gene flow. American Naturalist 175: 316–334. Knight ME, Turner GF. 2004. Laboratory mating trials indicate incipient speciation by sexual selection among populations of the cichlid fish Pseudotropheus zebra from Lake Malawi. Proceedings of the Royal Society of London Series B, Biological Sciences 271: 675–680. Koblmueller S, Salzburger W, Obermueller B, Eigner E, Sturmbauer C, Sefc KM. 2011. Separated by sand, fused by dropping water: habitat barriers and fluctuating water levels steer the evolution of rock-dwelling cichlid populations in Lake Tanganyika. Molecular Ecology 20: 2272–2290. Kocher TD. 2004. Adaptive evolution and explosive speciation: the cichlid fish model. Nature Reviews Genetics 5: 288–298. Koenen EJM, de Vos JM, Atchison GW, Simon MF, Schrire BD, de Souza ER, de Queiroz LP, Hughes CE. 2013. Exploring the tempo of species diversification in legumes. South African Journal of Botany 89: 19–30. Konijnendijk N, Joyce DA, Mrosso HDJ, Egas M, Seehausen O. 2011. Community genetics reveal elevated levels of sympatric gene flow among morphologically similar but not among morphologically dissimilar species of Lake Victoria cichlid fish. International Journal of Evolutionary Biology 2011: 616320. Kraaijeveld K, Kraaijeveld-Smit FJL, Maan ME. 2011. Sexual selection and speciation: the comparative evidence revisited. Biological Reviews 86: 367–377. Kursar TA, Dexter KG, Lokvam J, Pennington RT, Richardson JE, Weber MG, Murakami ET, Drake C, McGregor R, Coley PD. 2009. The evolution of antiherbivore defenses and their contribution to species coexistence in the tropical tree genus Inga. Proceedings of the National Academy of Sciences, USA 106: 18073– 18078. Lamboj A. 2004. The cichlid fishes of Western Africa. Bornheim, Germany: Birgit Schmettkamp Verlag. Liem KF. 1973. Evolutionary strategies and morphological innovations – cichlid pharyngeal jaws. Systematic Zoology 22: 425–441. New Phytologist (2015) 207: 304–312 www.newphytologist.com

312 Review

Research review

Linder HP. 2008. Plant species radiations: where, when, why? Philosophical Transactions of the Royal Society B-Biological Sciences 363: 3097–3105. McKinnon JS, Rundle HD. 2002. Speciation in nature: the threespine stickleback model systems. Trends in Ecology & Evolution 17: 480–488. Onstein RE, Carter RJ, Xing Y, Linder HP. 2014. Diversification rate shifts in the Cape Floristic Region: the right traits in the right place at the right time. Perspectives in Plant Ecology Evolution and Systematics 16: 331–340. Papadopulos AST, Baker WJ, Crayn D, Butlin RK, Kynast RG, Hutton I, Savolainen V. 2011. Speciation with gene flow on Lord Howe Island. Proceedings of the National Academy of Sciences, USA 108: 13188–13193. Papadopulos AST, Price Z, Devaux C, Hipperson H, Smadja CM, Hutton I, Baker WJ, Butlin RK, Savolainen V. 2013. A comparative analysis of the mechanisms underlying speciation on Lord Howe Island. Journal of Evolutionary Biology 26: 733–745. Renaut S, Rowe HC, Ungerer MC, Rieseberg LH. 2014. Genomics of homoploid hybrid speciation: diversity and transcriptional activity of long terminal repeat retrotransposons in hybrid sunflowers. Philosophical Transactions of the Royal Society of London Series B, Biological Sciences 369: 20130345. Rieseberg LH, Willis JH. 2007. Plant speciation. Science 317: 910–914. Ruber L, Verheyen E, Meyer A. 1999. Replicated evolution of trophic specializations in an endemic cichlid fish lineage from Lake Tanganyika. Proceedings of the National Academy of Sciences, USA 96: 10230–10235. Rundell RJ, Price TD. 2009. Adaptive radiation, nonadaptive radiation, ecological speciation and nonecological speciation. Trends in Ecology & Evolution 24: 394–399. Sage RD, Selander RK. 1975. Trophic radiation through polymorphism in cichlid fishes. Proceedings of the National Academy of Sciences, USA 72: 4669–4673. Salzburger W, Baric S, Sturmbauer C. 2002. Speciation via introgressive hybridization in East African cichlids? Molecular Ecology 11: 619–625. Salzburger W, Mack T, Verheyen E, Meyer A. 2005. Out of Tanganyika: genesis, explosive speciation, key-innovations and phylogeography of the haplochromine cichlid fishes. BMC Evolutionary Biology 5: 1–15. Samonte IE, Satta Y, Sato A, Tichy H, Takahata N, Klein J. 2007. Gene flow between species of Lake Victoria haplochromine fishes. Molecular Biology and Evolution 24: 2069–2080. Schiestl FP, Schlueter PM. 2009. Floral isolation, specialized pollination, and pollinator behavior in Orchids. Annual Review of Entomology 54: 425–446. Schliewen U, Klee B. 2004. Reticulate sympatric speciation in Cameroonian crater lake cichlids. Frontiers in Zoology 1: doi: 10.1186/1742-9994-11811185. Schliewen U, Tautz D, P€a€a bo S. 1994. Sympatric speciation suggested by monophyly of crater lake cichlids. Nature 368: 306–308. Schluter D. 2000. The ecology of adaptive radiation. Oxford, UK: Oxford University Press. Schranz ME, Mohammadin S, Edger PP. 2012. Ancient whole genome duplications, novelty and diversification: the WGD Radiation Lag-Time Model. Current Opinion in Plant Biology 15: 147–153.

New Phytologist (2015) 207: 304–312 www.newphytologist.com

New Phytologist Schwery O, Onstein RE, Bouchenak-Khelladi Y, Xing Y, Carter RJ, Linder HP. 2015. As old as the mountains: the radiations of the Ericaceae. New Phytologist 207: 355–367. Seehausen O. 2006. African cichlid fish: a model system in adaptive radiation research. Proceedings of the Royal Society B, Biological Sciences 273: 1987–1998. Seehausen O, Magalhaes IS. 2010. Geographical mode and evolutionary mechanism of ecological speciation in cichlid fish. In: Grant P, Grant R, eds. In search of the causes of evolution. Princeton, NJ, USA: Princeton University Press, 282–308. Seehausen, O. 1996. Lake Victoria Rock Cichlids: taxonomy, ecology, and distribution. Zevenhuizen, Netherlands: Verduijn Cichlids. Seehausen O, Terai Y, Magalhaes IS, Carleton KL, Mrosso HDJ, Miyagi R, van der Sluijs I, Schneider MV, Maan ME, Tachida H et al. 2008. Speciation through sensory drive in cichlid fish. Nature 455: 620–626. Seehausen O, Wagner CE. 2014. Speciation in freshwater fishes. Annual Review of Ecology, Evolution and Systematics 45: 621–651. Selz OM, Lucek K, Young KA, Seehausen O. 2014. Relaxed trait covariance in interspecific cichlid hybrids predicts morphological diversity in adaptive radiations. Journal of Evolutionary Biology 27: 11–24. Servedio MR, Van Doorn GS, Kopp M, Frame AM, Nosil P. 2011. Magic traits in speciation: ‘magic’ but not rare? Trends in Ecology & Evolution 26: 389–397. Simpson GG. 1953. The major features of evolution. New York, NY, USA: Columbia University Press. Smith TB, Skulason S. 1996. Evolutionary significance of resource polymorphisms in fishes, amphibians, and birds. Annual Review of Ecology and Systematics 27: 111– 133. Streelman JT, Gmyrek SL, Kidd MR, Kidd C, Robinson RL, Hert E, Ambali AJ, Kocher TD. 2004. Hybridization and contemporary evolution in an introduced cichlid fish from Lake Malawi National Park. Molecular Ecology 13: 2471–2479. Sturmbauer C, Baric S, Salzburger W, Ruber L, Verheyen E. 2001. Lake level fluctuations synchronize genetic divergences of cichlid fishes in African lakes. Molecular Biology and Evolution 18: 144–154. Takayama K, Lopez-Sepulveda P, Greimler J, Crawford DJ, Penailillo P, Baeza M, Ruiz E, Kohl G, Tremetsberger K, Gatica A et al. 2015. Relationships and genetic consequences of contrasting modes of speciation among endemic species of Robinsonia (Asteraceae, Senecioneae) of the Juan Fernandez Archipelago, Chile, based on AFLPs and SSRs. New Phytologist 205: 415–428. Wagner CE, Harmon LJ, Seehausen O. 2012a. Ecological opportunity and sexual selection together predict adaptive radiation. Nature 487: 366–369. Wagner CE, Harmon LJ, Seehausen O. 2014. Cichlid species-area curves are shaped by adaptive radiations that scale with area. Ecology Letters 17: 583–592. Wagner CE, Keller I, Wittwer S, Selz O, Mwaiko S, Greuter L, Sivasundar A, Seehausen O. 2012b. Genome-wide RAD sequence data provides unprecedented resolution of species boundaries and relationships in the Lake Victoria cichlid adaptive radiation. Molecular Ecology 22: 787–798.

Ó 2015 The Author New Phytologist Ó 2015 New Phytologist Trust