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Biol. Rev. (2015), 90, pp. 1248–1262. doi: 10.1111/brv.12154
Phylogenetic niche conservatism and the evolutionary basis of ecological speciation R. Alexander Pyron1,∗ , Gabriel C. Costa2 , Michael A. Patten3,4 and Frank T. Burbrink5,6 1
Department of Biological Sciences, The George Washington University, 2023 G Street NW, Washington, DC 20052, U.S.A. de Ecologia, Centro de Biociˆencias, Universidade Federal do Rio Grande do Norte, Campus Universit´ario Lagoa Nova, Natal, 59072-970, Rio Grande do Norte Brazil 3 Oklahoma Biological Survey, University of Oklahoma, 111 E. Chesapeake Street, Norman, OK 73019, U.S.A. 4 Department of Biology, University of Oklahoma, 730 Van Vleet Oval, Norman, OK 73019, U.S.A. 5 Department of Biology, The Graduate School and University Center, The City University of New York, 365 5th Avenue, New York, NY 10016, U.S.A. 6 Department of Biology, The College of Staten Island, The City University of New York, 2800 Victory Boulevard, Staten Island, NY 10314, U.S.A. 2 Departamento
ABSTRACT Phylogenetic niche conservatism (PNC) typically refers to the tendency of closely related species to be more similar to each other in terms of niche than they are to more distant relatives. This has been implicated as a potential driving force in speciation and other species-richness patterns, such as latitudinal gradients. However, PNC has not been very well defined in most previous studies. Is it a pattern or a process? What are the underlying endogenous (e.g. genetic) and exogenous (e.g. ecological) factors that cause niches to be conserved? What degree of similarity is necessary to qualify as PNC? Is it possible for the evolutionary processes causing niches to be conserved to also result in niche divergence in different habitats? Here, we revisit these questions, codifying a theoretical and operational definition of PNC as a mechanistic evolutionary process resulting from several factors. We frame this both from a macroevolutionary and population-genetic perspective. We discuss how different axes of physical (e.g. geographic) and environmental (e.g. climatic) heterogeneity interact with the fundamental process of PNC to produce different outcomes of ecological speciation. We also review tests for PNC, and suggest ways that these could be improved or better utilized in future studies. Ultimately, PNC as a process has a well-defined mechanistic basis in organisms, and future studies investigating ecological speciation would be well served to consider this, and frame hypothesis testing in terms of the processes and expected patterns described herein. The process of PNC may lead to patterns where niches are conserved (more similar than expected), constrained (divergent within a limited subset of available niches), or divergent (less similar than expected), based on degree of phylogenetic relatedness between species. Key words: phylogenetic niche conservatism, ecological speciation, allopatry, niche divergence, canalization, evolutionary constraint. CONTENTS I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1249 II. The pattern and process of PNC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1251 (1) PNC as a process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1251 (2) PNC as a pattern . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1255 (a) Low physical and environmental heterogeneity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1255 (b) Low physical heterogeneity and high environmental heterogeneity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1255 (c) High physical heterogeneity and low environmental heterogeneity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1255 (d) High physical and environmental heterogeneity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1255 (e) Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1256
I. INTRODUCTION Phylogenetic niche conservatism (PNC; Harvey & Pagel, 1991) is typically defined as the tendency for lineages to retain ancestral ecological characteristics over time (Holt & Gaines, 1992; Ricklefs & Latham, 1992; Peterson, Soberon & Sanchez-Cordero, 1999; Prinzing, 2001; Wiens & Graham, 2005). It is the notion that some biological process constrains niche divergence between closely related species, and therefore constrains populations from expanding into new niches (Losos, 2008a). Recent work highlights the significance of PNC as an important principle in several areas of ecology and evolution (Wiens et al., 2010; Peterson, 2011a; Condamine et al., 2012; Stegen, Ferriere & Enquist, 2012). For example, PNC may explain species-richness patterns, including latitudinal gradients and community diversity (Ricklefs & Latham, 1992; Wiens et al., 2007; Pyron & Burbrink, 2009a; Buckley et al., 2010; Stevens, 2011; Wiens, Pyron & Moen, 2011; Rosser et al., 2012). The presence of PNC is also a key (but often unacknowledged) assumption in many forms of species distribution modelling (Elith & Leathwick, 2009; Peterson, 2011b). Understanding PNC may help predict the impact of climate change on adaptation to novel environments as niches shift through time and space (Sinervo et al., 2010; Roberts & Hamann, 2012). The focus of this review is to distinguish between the pattern and process of PNC as it relates to speciation. Confusingly, the term PNC is often used to describe both a pattern of high niche similarity in related species, and a process comprising the evolutionary mechanisms responsible for that similarity (Losos, 2008a,b; Wiens, 2008). As a measurement of this pattern (high-similarity ecological niches between related species), a simple null model of Brownian Motion evolution produces a phylogenetic signal of closely related species having more similar traits than distantly related species (Revell, Harmon & Collar, 2008). Thus, a main question is how similar do niches need to be to qualify as conserved (the pattern of PNC)? By contrast, if PNC is a process (a biological mechanism promoting active retention of niche characteristics via selection), then how do the mechanisms responsible for this similarity promote speciation by constraining niche evolution? What patterns do this process generate, and what patterns do processes which are categorized as ‘niche conservatism’ generate? We recognize that all niche-related traits will be divergent in some ways and conserved in others. The geography of speciation and the mechanisms of evolutionary heritability ensure that there will be at least some similarity between newly diverged species. We thus wish to
Genetic constraint Fig. 1. Illustration of the evolutionary origin and ecological consequences of phylogenetic niche conservatism (PNC), from endogenous factors of constraint on phenotypes to the exogenous factors of ecological selection. This leads to the primary patterns of PNC, including conservatism, constraint, and divergence in niches between species.
widely within it (Holt, 2009), which we refer to as ‘niche constraint’ rather than conservatism (Fig. 1). In the broad sense, the aquatic niche of fish has been conserved for hundreds of millions of years, but this tells us nothing about the finer-scale processes that lead to ecological speciation in African rift-lake cichlids that occur in lakes only a few million years old (Seehausen, 2006). If traits are not under strong selection, then PNC (as a pattern of niche similarity) may arise simply via ‘evolutionary inertia,’ and divergence in niches and traits from drift. This may be particularly true if the timescale for divergence by drift is longer than that for the origin of the species under investigation, as in rapid evolutionary radiations where speciation occurs because of selection on a non-ecological trait such as sexually selected phenotypes (Wagner, Harmon & Seehausen, 2012). Thus, most discussions of PNC invoke evolutionary mechanisms beyond inertia and drift for how traits, and more specifically, ecological niches and climatic traits might be conserved through time (Blomberg & Garland, 2002; Losos, 2008a). Conservatism can be observed in any trait, although PNC is usually interpreted as the similarity of ecological niches in related species over recent evolutionary timescales (Ricklefs & Latham, 1992; Peterson et al., 1999). Generally, PNC could occur at any phylogenetic scale (Hadly, Spaeth & Li, 2009; Holt, 2009), but it is typically examined in terms of species and speciation processes (Wiens, 2004). When conservatism of ecological niches results in allopatric divergence in ecologically similar patches of geographically distant habitat, PNC is thought to be a major mechanism of speciation. Thus, species will track their ancestral niches spatially, resulting in interrupted gene flow between populations that have been separated geographically (Peterson et al., 1999; Wiens, 2004; Kozak & Wiens, 2006; Peterson & Nyari, 2008; Pyron & Burbrink, 2009b; McCormack, Zellmer & Knowles, 2010). We are also interested in the less-obvious role of PNC in ecological speciation, defined as the separation of lineages as a result of ecologically mediated divergent selection on standing genetic variation in populations (Schluter, 2001; Nosil, Harmon & Seehausen, 2009; Schluter & Conte, 2009). Ecological differentiation among related species is common, occurring significantly in 74% of surveyed cases in a recent review (Sexton, Hangartner & Hoffmann, 2014). The absolute case of ecological speciation is one in which lineages do not experience ecological selection for stasis in their current niche, PNC therefore does not occur, and species immediately track changing environments. Thus, populations in different areas experiencing ecological shifts in different directions or of different magnitudes are quickly separated geographically, leading to rapid ecological divergence and speciation. Nevertheless, PNC may influence ecological speciation in changing ecological environments if species track favourable ecological niches through time that slowly diverge from the ancestral niche (Ackerly, 2003; Kozak & Wiens, 2006). Such a process of local adaptation is posited as a key cause of subspecific differentiation (Belliure et al., 2000). In a
Niche conservatism and speciation changing environment, any force (e.g. stabilizing selection, genetic canalization) that promotes retention of ancestral niche characteristics (i.e. PNC) may actually enhance niche divergence due to increased adaptation to the local environment that most closely resembles the ancestral niche (i.e. the available niche with the highest fitness). If two allopatric populations experience different available ecological conditions, the closest analogue of the ancestral niche may be different for the two populations. Thus, the process of PNC may lead them to adapt to what are ultimately divergent ecological niches (Warren et al., 2008). This becomes evident when one considers that it is not the ‘ancestral’ niche per se that is being retained, but the instantaneous niche (see Section II.1). Thus, if populations are experiencing rapid ecological change, selection for stasis in their current niche (PNC) may actually result in more rapid niche divergence as the availability of the current niche is consistently shifting away from the ancestral niche (Figs 1–4). If two allopatric populations experience different environmental conditions, the closest analogue of the ancestral niche may be different in the different environments. As a result, the two populations will experience different paths of tracking the ‘ancestral’ niche (i.e. the ‘instantaneous’ niche), and therefore could present a pattern of niche divergence when compared to each other and the ancestral condition. The patterns (ecological similarity) and processes (ecological constraint) typically defined as PNC may arise at different phylogenetic scales for entirely different reasons and under entirely different conditions. Distinguishing between PNC as a pattern and process (and the mechanisms responsible) is crucial for evaluating historical evolutionary scenarios (Wiens et al., 2010). In order for PNC be considered a process driving observed patterns (Losos, 2008a), one must show higher similarity than expected from null models of drift, as might be expected under a model of ecological speciation in the absence of PNC. Several mechanisms have been proposed for the maintenance of this similarity (PNC as a process), including stabilizing selection, where individuals preserving ancestral niche properties are favoured (Holt & Barfield, 2008), gene flow swamping local adaptation where introgression from one part of a species range to another may prevent adaptations in niche traits to new habitats (Sexton et al., 2009), pleiotropic constraints on adaptation where a gene that allows niche expansion is linked with a gene that reduces fitness (Etterson & Shaw, 2001), and lack of standing genetic variation (Wiens & Graham, 2005; Saltz & Nuzhdin, 2014). Any of these may lead to failure to adapt (Futuyma, 2010), causing lineages to maintain the status quo in their ecological niches, and a tendency for sister species to inhabit highly similar ecological niches. The process of ecological constraint is a primary driver of PNC as an evolutionary mechanism. At the species level, the tendency to maintain a given niche (Tingley et al., 2009) may lead to allopatric divergence if a population is divided among multiple ecologically similar refugia due to PNC (Wiens, 2004).
1251 This tendency for stasis could be thought of as an extension of the idea of canalization, the suppression of phenotypic variation (Wagner, Booth & Bagheri, 1997). Thus, adaptive phenotypes for a given range of novel climatic conditions may be limited either by a lack of genetic potential for evolvability, or by limited plasticity of the ecologically relevant characters. We expand on this model in a population-genetic framework below. However, this model would seem to predict a relationship between the rate of climatic-niche evolution and niche breadth, which has not been found by some authors (Fisher-Reid, Kozak & Wiens, 2012). Finally, interpreting the pattern of PNC solely as the tendency for closely related species to exhibit similar climatic niches may obscure important mechanisms by which the process of PNC may promote speciation. Recent empirical and theoretical work has focused on the mechanisms by which speciation processes vary given different amounts of ecological and geographic heterogeneity (Kozak & Wiens, 2007; Connor et al., 2010; Hua & Wiens, 2010; Pyron & Burbrink, 2010; Anacker & Strauss, 2014). Importantly, ecological niches are not coupled among independent lineages (i.e. the niche of one species does not generally depend on the niche of another species) and in many cases, PNC may promote the retention of a single ancestral niche in related species if it is equally available to them. However, lineages that experience divergent ecological selection may nonetheless experience the process of PNC, leading each lineage to retain its current niches through time, meaning that the set of lineages ultimately end up in different niches. Thus, an evolutionary process of PNC may lead to allopatric speciation bearing the pattern of niche divergence (Fig. 1) when the current niche of species begins to differ from the ancestor. Given these ambiguities in the definition, delimitation, and interpretation of PNC, we aim to address three primary questions: (i) what is PNC (as a pattern and process); (ii) how do we identify and test for it; and (iii) how is PNC related to ecological speciation across populations? By attempting to clarify these issues, we hope to provide a more stable framework for future researchers investigating ecological speciation.
Ecophysical allopatry e.g. Montane tropics, Madrean sky islands
Soft allopatry e.g. Central plains, Cochise filter barrier
Non-ecological speciation (Polyploidy, Isolation by distance, etc.)
Hard allopatry e.g. Mississippi river, Isthmus of Panama River channel
Geography “Niche conservatism” (Constraint)
Physical heterogeneity Fig. 2. Illustration of the factors that can influence speciation via phylogenetic niche conservatism (PNC), including axes of physical and environmental heterogeneity. Speciation events occurring at different points along these axes may result in different patterns as described above the graphs, including conservatism (A), constraint (C, D), and divergence (B) in niches between species. In (A), curve I represents an environmental variable that changes slowly from a high to low value, with populations adapted to either extreme meeting in the middle. Curve II represents two ecologically similar areas separated by an intervening higher value for a given environmental variable, with two similarly adapted populations nearing the edge of their range limits in that area.
as populations move rather than adapt (Fig. 4). It is thus actually PNC that causes populations to become allopatrically separated. Were they to adapt fully or partially to changing environments, geographic separation would be minimized, and the selection for ecological speciation would be reduced (Figs 2 and 3). Thus, the primary driver of ecological speciation, the separation of populations in ecologically disjunct geographic areas, can in many ways be interpreted as the result of PNC. This is not to say that PNC drives all patterns of speciation, but rather that certain underlying evolutionary processes may result in a broad range of patterns. Why does PNC as an evolutionary process occur? How might these processes affect our interpretation of ecological speciation? Considering the potential factors that influence niche evolution leads to the recognition of a set of endogenous and exogenous influences leading to retention of niche characteristics through time (Figs 1–3). As noted above, previous authors have invoked four mechanisms by which instantaneous niches could be retained through time: stabilizing selection, gene flow swamping local adaptations, pleiotropic effects constraining adaptation, and lack of genetic variation. This covers much of what we term the endogenous factors that lead to PNC. These endogenous factors are those specific to the local population, and are hierarchically arranged based on their underlying cause (Fig. 1). The ultimate root cause of PNC is genetic constraint (Futuyma, 2010); all adaptations to any ecological niche are
Niche conservatism and speciation
Fig. 3. Formulating the process of phylogenetic niche conservatism (PNC) in terms of standing genetic variation of the population, as measured via niche width. The red curve (see online version for colour) shows the initial niche width of the ancestral population (A–F), which is centered at the fitness peak (red) in the environment, and is fairly broad, including a wide range of slightly suboptimal phenotypes near the margin. The second panel of each part of the figure shows that population separated by a physical (black) or ecological (grey dotted) barrier. Selection (indicated by an arrow) can then shift towards different optima (colours), if phenotypes adapted to those optima were already present in the ancestral population. If they were not (narrow ancestral population), then the population will either go extinct (E), or remain in the ancestral niche (F). Note that (A), (E), and (F), would all qualify as a pattern of ‘niche conservatism,’ while (B), (C), and (D) would typically be interpreted as ‘niche divergence.’
predicated on the existence of genetic variation sufficient for the evolution of traits necessary to persist in that niche. Similarly, physiological or developmental constraints, even in the presence of sufficient genetic variation, may limit adaptation or the expression of various traits (Beldade, Koops & Brakefield, 2002). Finally, ecological constraints, such as the impacts of biotic interactions (Hatchwell & Komdeur, 2000), or ecological limitations on dispersal and gene flow that prevent or swamp adaptation (Kirkpatrick & Barton, 1997), may lead to the retention of current niche characteristics. These have been referred to as the ‘easier to move than to evolve’ principle (Donoghue, 2008). These are also the primary mechanistic factors by which PNC as a process would arise in natural populations, as adaptation to novel environmental conditions lags behind the pace of ecological change, resulting in a selective advantage for individuals maintaining the closest analogue of the current niche. This process could also be viewed from the perspective of extinction as the operative force (Waldron, 2010). Populations exceeding their habitable niche go extinct due to selection on individuals (Kirkpatrick & Barton, 1997). Thus, the greatest selective advantage is derived from inhabiting the niche with the largest area supporting the largest population size. Ecological pressures subdividing this niche inherently decrease area and population size, increasing the probability
Fig. 4. Conceptual illustration of phylogenetic distributions of niches and tests for phylogenetic niche conservatism (PNC). The niche breadth of the clade (all species combined) can be calculated as a subset of the total variation that is present in the environment. Then one can ask whether sister-species pairs are more similar than expected (A) or more divergent than expected (B) under a null model of drift, which would produce random divergence between species (C). The scenario illustrated here combines high levels of physical heterogeneity (the heights of the mountains) and environmental heterogeneity (the climate on the mountains, illustrated by the colour of the populations). The phylogeny in the lower left illustrates the relationships between the populations on the different mountains, illustrating the niche divergences.
(both environmental and geographic) that lead local populations to persist in their current niche, with the net effect of selection against adaptation to novel niches. This interpretation is fairly straightforward and consistent with the definitions of most previous authors (Harvey & Pagel, 1991; Wiens & Graham, 2005; Losos, 2008a). As mentioned above, the action of PNC to promote retention of the current niche by a population can lead to the radically different patterns of high similarity, random variation, and significant divergence in niche among species (Fig. 2). It is thus desirable to quantify the potential pattern-based outcomes of PNC, with regard to the environmental and geographic setting of speciation (Figs 2 and 3). To reiterate, we contend that it is crucial to define PNC in terms of both lineages, rather than species, and in terms of the current, rather than the ancestral niche. An important outcome of this definition, which has been overlooked by most recent literature, is that patterns produced by the process of PNC vary dramatically. A hypothesis of PNC-mediated speciation is thus one that implies a concrete mechanism for the evolutionary direction of ancestral–descendant populations through ecological-niche space. Any such hypothesis must be framed as an explicit test, and evaluated using analytical methods (e.g. Warren et al., 2008). Crucially, a hypothesized evolutionary mechanism for each specific instance of PNC (e.g. stabilizing selection) is needed.
Niche conservatism and speciation (2) PNC as a pattern Confusingly, most literature on speciation invoking PNC as a mechanism also refers to niches of species as being ‘conserved’ and those species (or their ecological characteristics) as exhibiting PNC (Peterson et al., 1999; Pyron & Burbrink, 2009b; Crisp & Cook, 2012). This is true for niches that fit simple null models of drift relative to the phylogeny as well as those that are more similar than expected given the phylogenetic relatedness of the species (Holt, 2009). As noted above, such similarity might occur as a result of at least four mechanisms. Alternatively, those species that are less similar than expected under given models of phylogenetic niche evolution are typically taken to represent niche divergence (Warren et al., 2008; Glor & Warren, 2011). Thus, the process and expected patterns of PNC become conflated in discussions of ecological speciation. Given the framework outlined above, we can delineate at least some of the expected outcomes of PNC as an evolutionary process in the context of speciation. Previous authors have focused on the potential patterns of speciation via PNC (Lindeman, 2000; Graham et al., 2004; Wiens, 2004; Costa et al., 2008; Simard et al., 2009; McCormack et al., 2010; Pyron & Burbrink, 2010; Svensson, 2012). However, a full consideration of the geographic and ecological contexts in which speciation may arise via PNC requires considering additional axes of environmental variation, which suggests several discrete patterns (Fig. 2). The primary axes along which allopatric separation may occur are physical heterogeneity (topography, etc.) and ecological heterogeneity (temperature, precipitation, etc.), either of which may promote speciation (Coyne & Orr, 2004). This yields at least four distinct avenues (Fig. 2) via which PNC as a process (instantaneous niche retention) might promote speciation (PNC as a pattern), resulting either in niche divergence or niche conservatism.
1255 dispersal (Wiens, 2004). This can yield ‘leaky’ contact zones between species or populations, as the forces separating them at their interface are relatively weak, depending on the strength of the ecological differences in their respective habitats. Evidence for this pattern of speciation, putatively resulting from PNC, seems to be relatively common (Gee, 2004; Hickerson & Meyer, 2008). In this case, species experiencing PNC as a local process in their current niche may undergo niche divergence as a result of PNC, a somewhat counterintuitive result. As described above, this would result when PNC is acting in the opposite direction to ecological selection while environments are changing, leading to adaptation for a niche that resembles, but is distinct from, the ancestral niche. This would lead to divergent niches among related species with strong phylogenetic signal in the clade as a whole (Fig. 4). (c) High physical heterogeneity and low environmental heterogeneity This scenario (Fig. 2D) has been referred to as ‘hard allopatry’ by some authors (Hickerson & Meyer, 2008; Pyron & Burbrink, 2010). If physical barriers to gene flow (e.g. a river through an open plain, or an isthmus separating two oceans) disrupt an ecologically homogeneous habitat, then ecological selection may be relatively unimportant for driving speciation, as geographic barriers limit gene flow. Thus PNC as a process may not be important for separating populations, even if those populations are all still adapted to similar habitats and exhibit ‘PNC’ as a pattern as a result of stabilizing selection to maintain their existing ecological niches in different geographic areas (Wiens, 2004), separated by a physical barrier to dispersal. Such a process is thought to be a major driver of phylogeographic diversity in many regions, particularly around rivers (Pellegrino et al., 2005; Soltis et al., 2006; Jackson & Austin, 2010; Pyron & Burbrink, 2010).
(a) Low physical and environmental heterogeneity
(d) High physical and environmental heterogeneity
In this case (Fig. 2C), PNC will not act strongly to promote population divergence, and speciation must occur either by intrinsic mechanisms (e.g. autopolyploidy) or non-ecological extrinsic mechanisms, such as isolation-by-distance, host-shifting (for symbiotic species), or sexual selection (Coyne & Orr, 2004). In this context, we would expect ecological differences between species to be proportional to their phylogenetic distance under null models of drift (Losos, 2008a; Pearman et al., 2008), as PNC has not driven either conservatism or divergence in niche relative to congeners.
When both geographic and ecological variation is high (Fig. 2B), the two processes described above (hard and soft allopatry) may interact synergistically to promote rapid divergence between populations (‘species pumps’), such as in tropical montane regions (Janzen, 1967; Brehm, Colwell & Kluge, 2007; Smith et al., 2007; Bryant et al., 2008; Kozak & Wiens, 2010b). In such areas, any geographic distance also results in environmental distance, promoting niche divergence, while the combined topographic variation and ecological distance reduces dispersal and, therefore, gene flow between adjacent populations (Gascon et al., 2000; Gehring et al., 2012). Lineages may thus exhibit either niche divergence or conservatism as a result of PNC driving continued adaptation to local niches. These effects may be intensified by climatic shifts (Haffer, 1969), particularly in temperate regions (Hewitt, 1996; Dynesius & Jansson, 2000). Why are these populations not simply moving up and down the mountains (Fig. 4), and maintaining stasis within a climatic band, as might be expected under strong PNC?
(b) Low physical heterogeneity and high environmental heterogeneity This scenario (Fig. 2A) has been referred to as ‘soft allopatry’ or ‘soft vicariance’ by some authors (Hickerson & Meyer, 2008; Pyron & Burbrink, 2010). In this case, PNC drives parapatry along ecological gradients or across ecological disruptions in the relative absence of geographic barriers to
R. A. Pyron and others greater niche differences following speciation (Graham et al., 2004; Carnicer et al., 2012). This leads to the problem touched on earlier, that even when PNC as a process acts to promote ecological speciation, the resulting species may exhibit a pattern of niche divergence (Fig. 2). The degree to which this is true is likely to be inversely proportional to the niche breadths of the original lineages (Figs 1 and 3). We can now consider the various process- and pattern-based outcomes of PNC given genetic variation, environmental heterogeneity, and ecological selection (Fig. 3). In the case of a population with a wide niche-breadth that is divided by a barrier to dispersal (Fig. 3A), the resulting lineages may merely occupy the same realized niche in allopatric populations (i.e. see Section II.2c). If populations are simply on opposite sides of a barrier (such as a river) dividing an ecologically homogeneous area, then the resulting species may bear the pattern of PNC (high niche similarity) due to stabilizing selection being directly responsible for that similarity. By contrast, if populations occupy different geographic regions with similar niches separated by ecologically heterogeneous habitats (Sections II.2b, c), this has been considered evidence for PNC driving allopatric speciation via ecologically mediated selection (Peterson et al., 1999; Wiens, 2004). In regions undergoing rapid ecological change, the ancestral niche may persist in one part of the range while conditions on the other side of the barrier may shift rapidly (Fig. 3B). Thus, even though the descendent populations track the instantaneous niche, this niche may shift rapidly, resulting in the process of PNC again yielding a pattern of niche divergence (Section II.2d). In highly heterogeneous environments such as tropical mountain ranges (Fig. 2), both populations may experience such pressures, resulting in none of the descendant species resembling the ancestral niche (Fig. 3C). Similar patterns may occur for a population occurring along an ecological gradient with less dispersal limitation (Fig. 3D), such as a filter barrier (Pyron & Burbrink, 2010; Cicero & Koo, 2012; Zink, 2014), where disruptive ecological selection may result in allopatric speciation, although the resulting species have different niches than the ancestor. By contrast, if niche width of the ancestor is narrow, selection resulting from different ecological conditions on one side of a barrier may result in the extirpation of that population (Fig. 3E), while selection across an ecological gradient may simply result in a single population that does not move (Fig. 3F). As noted earlier, this bears some resemblance to the population-genetic model for canalization, wherein phenotypic variation is suppressed by either environmental or genetic factors (Wagner et al., 1997). Environmental canalization can be thought of either as the inverse of phenotypic plasticity, where characters are insensitive to environmental variation, or as the reduction in environmental variance by a quantitative trait. Genetic canalization results from the insensitivity of a character to epistatic mutations. Under stabilizing selection, both result in reduced phenotypic variance around the optimum phenotype.
Niche conservatism and speciation Under this model, genetic variation may be higher than the niche breadth of the organism, but this is not expressed phenotypically. In the context of ecologically relevant traits, this provides a population-level mechanism for PNC integrating several of the factors mentioned previously. Genetic canalization in particular may allow for the accumulation of cryptic genetic variation (Wagner et al., 1997), allowing for divergence when niches shift. However, there is little apparent relationship between rates of climatic-niche evolution and climatic-niche breadth (Fisher-Reid et al., 2012), suggesting that the relationship between observed variation (as measured by niche breadth) and ability to adapt (as measured by rate) is not linear. Determining the association between observed variation and underlying genetic variation in ecologically relevant traits (i.e. degree of canalization) will likely be important for pinpointing the environmental and genetic causes of PNC. (4) Tests for PNC The final major consideration is how we can test for PNC (Losos, 2008a,b; Wiens, 2008; Wiens et al., 2010). This is again a matter of scale and perspective regarding the hypotheses and processes we are interested in; the aquatic niche of fish is clearly conserved, but this pattern tells us little about speciation of cichlids in African rift lakes. The question of how we can test for PNC has been debated extensively (Wiens et al., 2010), and we review the available options and describe some alternative possibilities. The original tests involved model fitting for continuous traits on a phylogeny, which can be applicable across scales. Key models include Brownian motion (BM; genetic drift), Ornstein–Uhlenbeck (OU; stabilizing selection), and white noise (WN; no phylogenetic signal). Fit of BM or OU over WN is evidence that evolutionary factors are involved in determining the distribution of trait values among species, although care is needed when interpreting this as a result of evolutionary processes (Revell et al., 2008; Harmon et al., 2010). The ‘k’ statistic (Blomberg, Garland & Ives, 2003) compares the observed distribution of species to that expected under a simulated BM model using the phylogeny, allowing one to determine if the empirical similarity is greater or less than expected. Some authors have suggested that similarity beyond that expected via BM is needed for evidence of PNC (Losos, 2008a), although as noted here and elsewhere even a BM drift model can produce some of the expected patterns from ‘PNC’ (Wiens et al., 2010). Strong fit of an OU model provides evidence that some factors constrain niche divergence around an evolutionary optimum, but additional evidence would be needed to interpret this as evidence for a driver of speciation (Beaulieu et al., 2012). However, such strong evidence for stabilizing selection could be taken as evidence that PNC is acting as a process, given that divergence from the optimum is constrained (or canalized). Furthermore, the actual elements of the niche examined could differ widely considering the traits examined. For instance, we could imagine that some
R. A. Pyron and others will tend to revert to the mean, giving the signature of a selective force towards a central tendency. Thus, model support alone cannot be considered support for PNC in the absence of corroborating evidence of physiological tolerances or quantitative genetic variation.
III. FUTURE DIRECTIONS There are numerous areas for improvement in our understanding of PNC as a process and of the pattern of conserved niches, particularly as it relates to ecological speciation. We have assumed that the realized niche breadth of species corresponds to the amount of standing genetic variation in the population (Fig. 3), but this is unlikely to be true in practice. Understanding the genetic and physiological limits of species (and associated canalization) compared to their realized niche will give us greater insight into both the potential role of PNC in driving ecological speciation, as well as the clade-level niche and PNC at deeper phylogenetic scales (Holt, 2009). Integration of global information system (GIS)-based data (Kozak et al., 2008) may allow us to test for more fine-scale processes in recent times (Kidd & Ritchie, 2006; Tingley et al., 2009). Furthermore, increasing integration of palaeontological data (such as palynological or palaeolimnetic series) will help examine niche evolution and conservatism over time (Maguire & Stigall, 2008; Holland & Zaffos, 2011). There are several questions that could be answered in future studies relating to PNC that have not been thoroughly addressed previously. The first set has to do with the accumulation of niche divergence over time. What is the shape of the relationship between time-since-speciation and niche divergence between sister-species pairs? A flat relationship might indicate that PNC is not a consistent driver of speciation in a given group. By contrast, we might expect a negative linear relationship under a null model of BM niche divergence. A slope significantly less than 1 might indicate that tracking niche similarity was driving speciation, the traditional interpretation of PNC-mediated ecological speciation (Wiens, 2004). A slope much greater than 1, or an exponentially decreasing amount of similarity, might indicate heavy levels of niche divergence in heterogeneous habitats driving speciation (Graham et al., 2004; Pyron & Burbrink, 2009b). Latitudinal variation in such mechanisms has been shown to be important in some groups, such as salamanders (Kozak & Wiens, 2007), but not others, such as frogs (Hua & Wiens, 2010). More explicit characterization is needed for testing hypotheses of PNC as it relates to ecological speciation, rather than simply attributing allopatric divergence to PNC sensu lato. We have outlined several of the major tests above, including model fitting, ecological-niche modelling, geographic simulations, and novel methods based on sister-species divergence and community assembly. For future studies, outlining the mechanisms by which PNC could have initiated speciation and the expected outcomes under
Niche conservatism and speciation different historical scenarios, are needed to test hypotheses fully. In general, we suggest that ecological speciation via PNC is likely to be a common null model (Wiens, 2004), based on the generally observed tendency for species to track their niche geographically (Tingley et al., 2009), an observation that has led to ecological species concepts (Van Valen, 1976). However, this may produce multiple distinct patterns (Figs 2 and 4) that could be termed PNC, and which need to be enumerated beforehand. Ultimately, we believe a clearer understanding of the origin and outcome of PNC can help simplify and extend our conception of speciation via ecological selection, and foster a more complete synthesis of ecology and evolutionary biology, as well as phylogeography and historical biogeography. Thus, for a study attempting to elucidate the ecological mechanisms of speciation in a group, several pieces are needed when considering the potential occurrence and influence of PNC. As we have shown, PNC can act more broadly than has been considered by most recent authors (e.g. causing niche divergence). At minimum, researchers should consider several points. First, are species more similar or more divergent than expected under a null model of diversification (Losos, 2008a), particularly when compared to the group as a whole (Holt, 2009)? This establishes conservatism, constraint, or divergence (Fig. 1) as a pattern with respect to the available niche volume (Warren et al., 2008). Second, what is the degree of physical and ecological heterogeneity in the focal region (Fig. 2), from which the process of PNC can be inferred (Pyron & Burbrink, 2010)? Third, what is the phylogeographic history of the constituent lineages with respect to ecological niches and geographic location (Lemmon & Lemmon, 2008; Knowles & Alvarado-Serrano, 2010)? These elements combined give a cohesive historical framework for the interplay between the evolutionary processes of niche evolution and ecological speciation, interpreted relative to observable patterns of climatic-niche variability.
IV. CONCLUSIONS (1) The endogenous factors of genetic, physiological and ecological constraints limit the ability of individuals, and thus populations, to adapt rapidly to changing environmental conditions. The selective forces resulting from ecological variation experienced by populations thus result in the tendency of populations to maintain their current niche through time. We argue that this is the true underlying process of PNC (Fig. 1). Within a total ecosystem niche volume (i.e. the environmental background), this can result in several potential patterns (Figs 1–4). (2) First is niche divergence: two populations maintaining their current niche (driven by PNC) may nonetheless end up in drastically different niches if the ecological trajectories of their local environments differ to a sufficient degree (Figs 2 and 4). For instance, a population in a rapidly aridifying desert will rapidly diverge in niche from a sister species in a
1259 temperate environment that is becoming more mesic if both experience PNC in their respective niches. (3) Second, the fundamental niche of a species or group of related species may be relatively fixed, but taxa may differ in their realizations of that fundamental niche, a process we term niche constraint (Holt, 2009). This may give the appearance of niche divergence, when niches are in reality more similar than might be expected given the phylogeny, due to PNC. (4) Third is that of high overlap in realized niche between species that are evolutionarily divergent enough for large ecological differences to be expected (Fig. 1). If certain habitats are particularly suitable for populations, then selection for that niche may persist for relatively long periods of evolutionary time, leading to greater-than-expected similarity. (5) The latter pattern is the only one typically referred to as ‘PNC’ (Wiens, 2004, 2008; Losos, 2008a,b). However, all three patterns (niche divergence, niche constraint, and niche conservatism) may result from the same underlying process of niche conservatism, which results from the endogenous factors we have enumerated (Fig. 1). The process of PNC may promote ecological speciation along axes of both physical and environmental heterogeneity, yielding any of the three patterns listed above. We suggest, as do other recent authors (Wiens & Graham, 2005), that testing for the presence of this ‘pattern of PNC’ is not the key question of interest for ecology and evolutionary biology, as the tendency of related species to be similar is a foregone conclusion. (6) Instead, we suggest that observed patterns in ecological speciation be considered in terms of an underlying mechanism of PNC generating multiple potential patterns, including that of ‘niche conservatism.’ However, PNC as a process may also yield niche divergence, as well as intermediate patterns (Figs 1 and 2). Distinguishing between these based on phylogenetic signal may be difficult (Revell et al., 2008), but the ecophysical roots of allopatric speciation can likely be classified in most cases into one of the four general categories we have outlined, driven by the single underlying process of PNC, and exhibiting patterns along a continuum from conservatism to divergence in ecological niche, both fundamental and realized. By distinguishing between PNC as a process and a pattern, we can more easily understand ecological speciation.
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(Received 16 December 2013; revised 16 September 2014; accepted 15 October 2014; published online 26 November 2014)