How close relatives maintain species boundaries in sympatry remains a critical question in biodiversity research. Here we introduce Lobelia sect. Lobelia (Campanulaceae) as a useful clade for investigating such questions. Polyphyly within this clade was strongly suspected because many of the 26 species are cross-compatible and show remarkable overlap in distribution, morphology, ecology, and life history. Indeed, the species Lobelia × rogersii has a purported hybrid origin from Lobelia puberula and Lobelia brevifolia, and the well-known cultivar Lobelia × speciosa results from mating between Lobelia siphilitica and Lobelia cardinalis. We carried out a comprehensive evolutionary investigation of Lobelia sect. Lobelia, including phylogenetic inference, divergence time estimates, and population structure analyses using 729 accessions from 193 natural population sites representing 1-13 individuals per population per species. In contrast to expectations, nearly all species were recovered as reciprocally monophyletic with strong topological support and low levels of interspecific gene flow. An exception to this general pattern is observed in the Florida panhandle, where Lobelia glandulosa and Lobelia apalachicolensis co-occur and appear to be actively hybridizing. We conclude that North American Lobelia species are genetically cohesive, despite significant geographic overlap, frequent co-occurrence, morphological similarity, and broad interfertility in artificial crosses.

There is considerable evidence for local adaptation in nature, yet important questions remain regarding its genetic basis. How many loci are involved? What are their effect sizes? What is the relative importance of conditional neutrality versus genetic trade-offs? Here we address these questions in the self-pollinating, annual plant Arabidopsis thaliana. We used 400 recombinant inbred lines (RILs) derived from two locally adapted populations in Italy and Sweden, grew the RILs and parents at the parental locations, and mapped quantitative trait loci (QTL) for mean fitness (fruits/seedling planted). We previously published results from the first 3 years of the study, and here add five additional years, providing a unique opportunity to assess how temporal variation in selection might affect QTL detection and classification. We found 10 adaptive and one maladaptive QTL in Italy, and six adaptive and four maladaptive QTL in Sweden. The discovery of maladaptive QTL at both sites suggests that even locally adapted populations are not always at their genotypic optimum. Mean effect sizes for adaptive QTL, 0.97 and 0.55 fruits in Italy and Sweden, respectively, were large relative to the mean fitness of the RILs (approximately 8 fruits/seedling planted at both sites). Both genetic trade-offs (four cases) and conditional neutrality (seven cases) contribute to local adaptation in this system. The 8-year dataset provided greater power to detect QTL and to estimate their locations compared to our previous 3-year study, identifying one new genetic trade-off and resolving one genetic trade-off into two conditionally adaptive QTL.

The spiral gingers (Costus L.) are a pantropical genus of herbaceous perennial monocots; the Neotropical clade of Costus radiated rapidly in the past few million years into over 60 species. The Neotropical spiral gingers have a rich history of evolutionary and ecological research that can motivate and inform modern genetic investigations. Here, we present the first 2 chromosome-level genome assemblies in the genus, for C. pulverulentus and C. lasius, and briefly compare their synteny. We assembled the C. pulverulentus genome from a combination of short-read data, Chicago and Dovetail Hi-C chromatin-proximity sequencing, and alignment with a linkage map. We annotated the genome by mapping a C. pulverulentus transcriptome and querying mapped transcripts against a protein database. We assembled the C. lasius genome with Pacific Biosciences HiFi long reads and alignment to the C. pulverulentus genome. These 2 assemblies are the first published genomes for non-cultivated tropical plants. These genomes solidify the spiral gingers as a model system and will facilitate research on the poorly understood genetic basis of tropical plant diversification.

Ecological heterogeneity can lead to local adaptation when populations exhibit fitness trade-offs among habitats. However, the degree to which local adaptation is affected by the spatial and temporal scale of environmental variation is poorly understood. A multiyear reciprocal transplant experiment was performed with populations of the annual plant Leptosiphon parviflorus living on adjacent serpentine and nonserpentine soil. Local adaptation over this small geographic scale was observed, but there were differences in the temporal variability of selection across habitats. On serpentine soil, the local population had a consistently large survival advantage, presumably as a result of the temporal stability in selection imposed by soil cation content. In contrast, a fecundity advantage was observed for the sandstone population on its native soil type but only in the two study years with the highest rainfall. A manipulative greenhouse experiment demonstrated that the fitness advantage of the sandstone population in its native soil type depends critically on water availability. The temporal variability in local adaptation driven by variation in precipitation suggests that continued drought conditions have the potential to erode local adaptation in these populations. These results show how different selective factors can influence spatial and temporal patterns of variation in fitness trade-offs.

Habitat degradation combined with climate change increases the threat of extinction for stream fishes. In response to these threats, efforts to reestablish species within formerly occupied streams or translocation to suitable areas may be effective conservation strategies. In the absence of historic species presence data, identifying locations where suitable habitat exists across many fluvial habitats may limit the effectiveness of reestablishments. We present an approach that ranks habitat for stream fish reestablishment over large areas using best available information. Using the locally extirpated Arctic grayling (Thymallus arcticus) in Michigan, USA as an example, we integrate information on species preferences and relationships between species with similar habitat requirements and landscape predictors of habitat to rank stream suitability. We find that unfragmented streams throughout the historical range of Arctic grayling and areas previously unoccupied by the species are potential locations for conservation action. However, we note that projected increases in summer water temperatures may reduce the amount of thermally suitable habitat in some top-ranked locations by up to 30%. Given its inherent flexibility in data requirements, our landscape-level approach may be a valuable tool that supports planning for species reestablishment.

The importance of genetic drift in shaping patterns of adaptive genetic variation in nature is poorly known. Genetic drift should drive partially recessive deleterious mutations to high frequency, and inter-population crosses may therefore exhibit heterosis (increased fitness relative to intra-population crosses). Low genetic diversity and greater genetic distance between populations should increase the magnitude of heterosis. Moreover, drift and selection should remove strongly deleterious recessive alleles from individual populations, resulting in reduced inbreeding depression. To estimate heterosis, we crossed 90 independent line pairs of Arabidopsis thaliana from 15 pairs of natural populations sampled across Fennoscandia and crossed an additional 41 line pairs from a subset of four of these populations to estimate inbreeding depression. We measured lifetime fitness of crosses relative to parents in a large outdoor common garden (8,448 plants in total) in central Sweden. To examine the effects of genetic diversity and genetic distance on heterosis, we genotyped parental lines for 869 SNPs. Overall, genetic variation within populations was low (median expected heterozygosity = 0.02), and genetic differentiation was high (median FST  = 0.82). Crosses between 10 of 15 population pairs exhibited significant heterosis, with magnitudes of heterosis as high as 117%. We found no significant inbreeding depression, suggesting that the observed heterosis is due to fixation of mildly deleterious alleles within populations. Widespread and substantial heterosis indicates an important role for drift in shaping genetic variation, but there was no significant relationship between fitness of crosses relative to parents and genetic diversity or genetic distance between populations.

Seasonal drought has been shown to greatly influence the distributions and species composition of plants in tropical rain forests. By conducting a series of field, greenhouse, and growth chamber experiments, we examined how Costus villosissimus, a forest edge species, has adapted to drought and differentiated from C. allenii, its closely related species in the understory. We hypothesize that delayed seed germination and high drought tolerance may lead to habitat differentiation and thus reproductive isolation between closely related plant species in the tropics. in Spanish is available with online material.

The biotic interactions hypothesis posits that biotic interactions are more important drivers of adaptation closer to the equator, evidenced by "stronger" contemporary interactions (e.g., greater interaction rates) and/or patterns of trait evolution consistent with a history of stronger interactions. Support for the hypothesis is mixed, but few studies span tropical and temperate regions while experimentally controlling for evolutionary history. Here, we integrate field observations and common garden experiments to quantify the relative importance of pollination and herbivory in a pair of tropical-temperate congeneric perennial herbs. Phytolacca rivinoides and P. americana are pioneer species native to the Neotropics and the eastern United States, respectively. We compared plant-pollinator and plant-herbivore interactions between three tropical populations of P. rivinoides from Costa Rica and three temperate populations of P. americana from its northern range edge in Michigan and Ohio. For some metrics of interaction importance, we also included three subtropical populations of P. americana from its southern range edge in Florida. This approach confounds species and region but allows us, uniquely, to measure complementary proxies of interaction importance across a tropical-temperate range in one system. To test the prediction that lower-latitude plants are more reliant on insect pollinators, we quantified floral display and reward, insect visitation rates, and self-pollination ability (autogamy). To test the prediction that lower-latitude plants experience more herbivore pressure, we quantified herbivory rates, herbivore abundance, and leaf palatability. We found evidence supporting the biotic interactions hypothesis for most comparisons between P. rivinoides and north-temperate P. americana (floral display, insect visitation, autogamy, herbivory, herbivore abundance, and young-leaf palatability). Results for subtropical P. americana populations, however, were typically not intermediate between P. rivinoides and north-temperate P. americana, as would be predicted by a linear latitudinal gradient in interaction importance. Subtropical young-leaf palatability was intermediate, but subtropical mature leaves were the least palatable, and pollination-related traits did not differ between temperate and subtropical regions. These nonlinear patterns of interaction importance suggest future work to link interaction importance to climatic or biotic thresholds. In sum, we found that the biotic interactions hypothesis was more consistently supported at the larger spatial scale of our study.

PREMISE:Despite myriad examples of local adaptation, the phenotypes and genetic variants underlying such adaptive differentiation are seldom known. Recent work on freezing tolerance and local adaptation in ecotypes of Arabidopsis thaliana from Italy and Sweden provides an essential foundation for uncovering the genotype-phenotype-fitness map for an adaptive response to a key environmental stress.METHODS:We examined the consequences of a naturally occurring loss-of-function (LOF) mutation in an Italian allele of the gene that encodes the transcription factor CBF2, which underlies a major freezing-tolerance locus. We used four lines with a Swedish genetic background, each containing a LOF CBF2 allele. Two lines had introgression segments containing the Italian CBF2 allele, and two contained deletions created using CRISPR-Cas9. We used a growth chamber experiment to quantify freezing tolerance and gene expression before and after cold acclimation.RESULTS:Freezing tolerance was lower in the Italian (11%) compared to the Swedish (72%) ecotype, and all four experimental CBF2 LOF lines had reduced freezing tolerance compared to the Swedish ecotype. Differential expression analyses identified 10 genes for which all CBF2 LOF lines, and the IT ecotype had similar patterns of reduced cold responsive expression compared to the SW ecotype.CONCLUSIONS:We identified 10 genes that are at least partially regulated by CBF2 that may contribute to the differences in cold-acclimated freezing tolerance between the Italian and Swedish ecotypes. These results provide novel insight into the molecular and physiological mechanisms connecting a naturally occurring sequence polymorphism to an adaptive response to freezing conditions.

More than five decades ago, Ehrlich and Raven proposed a revolutionary idea-that the evolution of novel plant defense could spur adaptive radiation in plants. Despite motivating much work on plant-herbivore coevolution and defense theory, Ehrlich and Raven never proposed a mechanism for their "escape and radiate" model. Recent intriguing mechanisms proposed by Marquis et al. include sympatric divergence, pleiotropic effects of plant defense traits on reproductive isolation, and strong postzygotic isolation, but these may not be general features of herbivore-mediated speciation. An alternate view is that herbivores may impose strong divergent selection on defenses in allopatric plant populations, with plant-herbivore coevolution driving local adaptation resulting in plant speciation. Building on these ideas, we propose three scenarios that consider the role of herbivores in plant speciation. These include (1) vicariance, subsequent coevolution within populations and adaptive divergence between geographically isolated populations, (2) colonization of a new habitat lacking effective herbivores followed by loss of defense and then re-evolution and coevolution of defense in response to novel herbivores, and (3) evolution of a new defense followed by range expansion, vicariance, and coevolution. We discuss the general role of coevolution in plant speciation and consider outstanding issues related to understanding: (1) the mechanisms behind cospeciation of plants and insects, (2) geographic variation in defense phenotypes, (3) how defensive traits and geography map on plant phylogenies, and (4) the role of herbivores in driving character displacement in defense phenotypes of related species in sympatry.

Local adaptation is common, but the traits and genes involved are often unknown. Physiological responses to cold probably contribute to local adaptation in wide-ranging species, but the genetic basis underlying natural variation in these traits has rarely been studied. Using a recombinant inbred (495 lines) mapping population from locally adapted populations of Arabidopsis thaliana from Sweden and Italy, we grew plants at low temperature and mapped quantitative trait loci (QTLs) for traits related to photosynthesis: maximal quantum efficiency (Fv/Fm), rapidly reversible photoprotection (NPQfast), and photoinhibition of PSII (NPQslow) using high-throughput, whole-plant measures of chlorophyll fluorescence. In response to cold, the Swedish line had greater values for all traits, and for every trait, large effect QTLs contributed to parental differences. We found one major QTL affecting all traits, as well as unique major QTLs for each trait. Six trait QTLs overlapped with previously published locally adaptive QTLs based on fitness measured in the native environments over 3 years. Our results demonstrate that photosynthetic responses to cold can vary dramatically within a species, and may predominantly be caused by a few QTLs of large effect. Some photosynthesis traits and QTLs probably contribute to local adaptation in this system.

The longstanding biotic interactions hypothesis predicts that herbivore pressure declines with latitude, but the evidence is mixed. To address gaps in previous studies, we measured herbivory and defence in the same system, quantified defence with bioassays, and considered effects of leaf age. We quantified herbivory and defence of young and mature leaves along a continental gradient in eastern North America in the native herb Phytolacca americana L. Herbivory in the field declined with latitude and was strongly correlated with lepidopteran abundance. Laboratory bioassays revealed that leaf palatability was positively correlated with latitude of origin. Young leaves were more damaged than mature leaves at lower latitudes in the field, but less palatable in bioassays. Both defence and palatability displayed non-linear latitudinal patterns, suggesting potential mechanisms based on biological or climatic thresholds. In sum, observational and experimental studies find patterns consistent with high herbivore pressure and stronger plant defences at lower latitudes.

Flowering time is an important life history trait in plants that often affects fitness. The optimal time to flower may be influenced by trade-offs between flowering time and growth-related traits and is thus likely to differ among habitats. Because flowering-time differences between populations can also reduce gene flow, understanding the factors that contribute to variation in flowering time among closely adjacent populations that experience gene flow is of particular interest. Plant adaptation to different edaphic environments provides some of the best examples of adaptive divergence at small spatial scales, and often coincides with flowering-time shifts. The current study addresses the causes of flowering-time differences in two populations of Leptosiphon parviflorus that are locally adapted to adjacent serpentine and sandstone soils despite moderate levels of gene flow and close geographic proximity. Field reciprocal-transplant studies and watering manipulations in the greenhouse demonstrate the contribution of both the genotype and the environment to observed flowering-time differences. The plasticity of flowering time in response to soil type appears to be driven by differences in soil moisture. In addition, selection on flowering time was measured in both soil types across 4 years of study using a set of F5 advanced-generation hybrids and found to differ between the habitats. Therefore, both selection and plasticity contribute to flowering-time differences between these populations and thus have likely played an important role in the initiation and/or maintenance of adaptive divergence in this system.

The nearly universal pattern that species richness increases from the poles to the equator (the latitudinal diversity gradient [LDG]) has been of intense interest since its discovery by early natural-history explorers. Among the many hypotheses proposed to explain the LDG, latitudinal variation in (1) productivity, (2) time and area available for diversification, and (3) speciation and/or extinction rates have recently received the most attention. Because tropical regions are older and were formerly more widespread, these factors are often intertwined, hampering efforts to distinguish their relative contributions to the LDG. Here we examine the global distribution of endemic lake fishes to determine how lake age, area, and latitude each affect the probability of speciation and the extent of diversification occurring within a lake. We analyzed the distribution of endemic fishes worldwide (1,933 species and subspecies from 47 families in 2,746 lakes) and find that the probability of a lake containing an endemic species and the total number of endemics per lake increase with lake age and area and decrease with latitude. Moreover, the geographic locations of endemics in 34 of 41 families are found at lower latitudes than those of nonendemics. We propose that the greater diversification of fish at low latitudes may be driven in part by ecological opportunities promoted by tropical climates and by the coevolution of species interactions.

Previous articleNext article FreeHistorical Comment"Latitudinal Gradients in Species Diversity": Reflections on Pianka's 1966 Article and a Look ForwardDouglas W. Schemske and Gary G. MittelbachDouglas W. Schemske1. W. K. Kellogg Biological Station, Michigan State University, Hickory Corners, Michigan 490602. Department of Plant Biology, Michigan State University, East Lansing, Michigan 48824*Corresponding author; e-mail: [email protected]. Search for more articles by this author and Gary G. Mittelbach1. W. K. Kellogg Biological Station, Michigan State University, Hickory Corners, Michigan 490603. Department of Integrative Biology, Michigan State University, East Lansing, Michigan 48824 Search for more articles by this author PDFPDF PLUSFull Text Add to favoritesDownload CitationTrack CitationsPermissionsReprints Share onFacebookTwitterLinked InRedditEmailQR Code SectionsMoreI was a mere graduate student, wet behind the ears, only 25 years old, when I wrote it. I don't usually re-read my own papers—but now, five decades later, I am pleased to find it cerebral and fairly well written.Eric Pianka, 2016Just as the 1960s were marked by revolutionary changes in music, culture, and politics, so too were they foundational for the emergence of modern ecology, with classical observational approaches giving way to mathematical theory and experiments. The University of Washington was a crucible for this movement in the 1960s, with young faculty such as Bob Paine, Gordon Orians, and Alan Kohn working in close collaboration with a number of exceptionally creative graduate students, including Henry Horn, Eric Pianka, Christopher Smith, Jared Verner, and Mary Willson. Collectively, this group pioneered a new way of thinking about ecological problems and was instrumental in defining the discipline of evolutionary ecology, a union of ecological and evolutionary perspectives that mirrors the goals of the American Society of Naturalists: "to advance and to diffuse knowledge of organic evolution and other broad biological principles."Of the many notable contributions by the Washington group, Eric Pianka's 1966 article "Latitudinal Gradients in Species Diversity: A Review of Concepts" is emblematic of the creative energy at work during this era to address big problems in ecology and evolution. The paper was the first chapter of Eric's PhD dissertation (1965), "Species Diversity and Ecology of Flatland Desert Western North America," which (like many graduate students at the time) he painstakingly typed himself (fig. 1).Figure 1. Eric Pianka typing his dissertation. Photo courtesy of E. Pianka.View Large ImageDownload PowerPointEric's influential paper (as of February 2017: 1,536 citations, Google Scholar; 967 citations, Web of Science) provided the first synthesis of the major hypotheses for the latitudinal diversity gradient (LDG) and stimulated the research trajectories of legions of naturalists. It has been reprinted in three different volumes of classic papers, including tropical ecology (Jordan 1981), tropical forest biology (Chazdon and Whitmore 2002), and biogeography (Lomolino et al. 2004), and thus it continues to serve as a foundational reference for the study of geographic patterns of biodiversity.In reviewing previous studies, Pianka compiled six hypotheses that addressed possible causes of the LDG (Table 1). For each, he provided a rationale for the biological processes involved. He did not evaluate the validity of the hypotheses or identify those he considered most plausible, but where possible, he did identify areas of future research. One important theme he highlighted throughout the paper is the challenge of distinguishing different mechanisms that might generate the same pattern: "Obviously, there is room for considerable overlap between these different hypotheses, and several may be acting in concert or in series in any particular situation" (Pianka 1966, p. 42). Below we briefly summarize Pianka's six hypotheses.Table 1. Pianka's (1966) six hypotheses and Fine's (2015) five hypotheses for the latitudinal diversity gradientHypothesisOur interpretation of the main focusPianka 1966: 1. The time theoryEcology and evolution 2. The theory of spatial heterogeneityEcology 3. The competition hypothesisEcology 4. The predation hypothesisEcology 5. The theory of climatic stabilityEcology and evolution 6. The productivity hypothesisEcologyFine 2015: 1. Time-integrated area, energy, and tropical niche conservatismEvolution 2. Climate stabilityEvolution 3. Temperature and evolutionary speedEvolution 4. Biotic interactions and speciation rateEvolution 5. Biotic interactions and finer nichesEcologyView Table ImageHypothesis 1. The time theory proposes that the species richness of communities increases with time due to ecological (immigration) and evolutionary (speciation) processes. Multiple glaciations in northern latitudes reduced the time available for diversification, while tropical regions remained relatively undisturbed, leading to the LDG.Hypothesis 2. The theory of spatial heterogeneity suggests that the LDG results from the greater heterogeneity and/or complexity of physical and biotic factors (e.g., foliage height diversity) in the tropics. This is essentially an argument that the number of available habitats controls the number of species in a community.Hypothesis 3. The competition hypothesis proposes that natural selection in the temperate zone is governed more by abiotic than by biotic factors, and as a result, competition is stronger in the tropics, niches are narrower, and more species can be supported.Hypothesis 4. The predation hypothesis is an alternative to the competition hypothesis, suggesting that competition is actually lower in the tropics due to a reduction in population sizes caused by higher predation in tropical environments. Lower competition reduces the likelihood of competitive exclusion and increases species richness.Hypothesis 5. The theory of climatic stability predicts that the stable climate in tropical regions leads to greater specialization, narrower niches, and higher species richness.Hypothesis 6. The productivity hypothesis suggests that the greater productivity of tropical regions increases species richness, perhaps by allowing narrower niches, tighter species packing, and greater niche overlap.Pianka's Hypotheses RevisitedPianka's 1966 article synthesized the wide range of ideas regarding the causes of the LDG in hopes of stimulating new research. Fifty years after its publication, there is still no consensus as to the primary mechanisms that contribute to the origin and maintenance of the LDG. Yet the remarkable diversity of life found in the humid tropics is as fascinating to biologists today as it was to Pianka in 1966 or to Wallace and Darwin more than 150 years ago. Here we briefly review how each of the six hypotheses summarized by Pianka has fared over the past 50 years.The time hypothesis, which dates back to A. R. Wallace, is perhaps the oldest and most widely accepted of the six hypotheses. Since Pianka, development of detailed databases on the fossil record and knowledge of paleoclimates has provided a clearer of picture of the development of biodiversity through deep time, and this has provided new opportunities for examining the time hypothesis (Jablonski et al. 2017). For example, a time-integrated biogeographic analysis of diversification in forest trees has suggested that tropical environments are older and thus have had more time for diversification (reviewed in Fine 2015), consistent with the time hypothesis. However, empirical tests of this hypothesis are challenging because it is difficult to separate the effects of age and climate (tropical vs. temperate). If tropical environments are older, historically larger, and more diverse, is the increased diversity due to age, area, climate, or all of the above? A recent approach to this problem examined the joint effects of time, area, and latitude on speciation of endemic fish and found that age, area, and latitude have significant and equivalent effects (Hanly et al. 2017). As one striking example that time per se is not the only factor contributing to fish diversification, consider that Lake Baikal, located at 55.63°N, is the largest by volume and the oldest (>27 million years) lake in the world, yet it has just 52 species of fish, 37 of which are endemic. In comparison, Lake Victoria, located at 1.30°S, is just 18,000 years old, yet it has 566 species, of which 450 are endemic.The theory of spatial heterogeneity has received little attention since Pianka's review, which probably reflects the general opinion that tropical regions do not display greater heterogeneity of the physical environment. However, there is evidence that tropical forests have greater heterogeneity in tree height, which may contribute to the increased diversity of short-statured species in tropical forests (King et al. 2006). The role of biotic interactions in the LDG has expanded beyond the competition and predation hypotheses that were the focus of Pianka's review to include mutualisms such as pollination, which may promote reproductive isolation of plants and their pollinators. This has led to renewed interest in Dobzhansky's proposal that the greater importance of biotic interactions in the tropics promotes diversification (Dobzhansky 1950). A recent review of latitudinal patterns in the strength of biotic interactions largely confirms Dobzhansky's hypothesis, although more data are needed (Schemske et al. 2009). Although Dobzhansky did not provide a mechanism linking biotic interactions to the LDG, a recent extension of his ideas suggests that strong biotic interactions coupled with coevolution lead to faster rates of adaptation and speciation in the tropics (Schemske 2009).The theory of climatic stability remains relatively unexplored, although interest has been renewed in Janzen's idea that mountains are a greater barrier to gene flow in the tropics than in temperate zones and that this may contribute to diversification. This hypothesis that "mountain passes are higher in tropics" is based on the idea that populations physiologically adapted to cold, high-elevation habitats in the aseasonal tropics are less likely to migrate across warmer valleys than are their temperate counterparts that are adapted to far greater seasonal variation in temperature (Janzen 1967). Finally, although the productivity hypothesis has been reviewed extensively, both with respect to geographic patterns in community diversity and to the LDG specifically, a direct mechanism linking productivity to species richness remains elusive.Current Hypotheses for the LDG: An Emphasis on EvolutionPianka's six hypotheses for the latitudinal diversity gradient (Table 1) focus predominantly on mechanisms of species coexistence: "The question of basic ecological interest is … what are the factors that allow ecological coexistence of more species at low latitudes?" (Pianka 1966, p. 34). Interestingly, in the more than 50 years since Pianka's review, explanations for the LDG now place greater emphasis on evolutionary mechanisms (Mittelbach et al. 2007). For example, Fine (2015) reviews five hypotheses for the LDG (Table 1), including latitudinal differences in rates of diversification (speciation and/or extinction) and their drivers and differences in the time and area available for diversification in tropical and temperate biomes (in combination with tropical niche conservatism and available energy). Only one of Fine's hypotheses is purely ecological.Understandably, Pianka's 1966 review focused mainly on ecology. Interspecific competition and species coexistence via niche partitioning formed the foundation of community ecology in the 1960s. These processes were thought to determine the number of species found in most local communities and, by extension, patterns of species diversity at broader spatial scales. Pianka also may have given evolution less emphasis because evolutionary hypotheses were thought untestable at the time. For example, he suggested (Pianka 1966, p. 35) that "the evolutionary time theory is not readily amenable to conclusive tests, and will probably remain more or less unevaluated for some time." Since Pianka's review, technological advances on a variety of fronts have made it possible to test evolutionary hypotheses. For example, molecular phylogenies allow the estimation of clade-specific diversification rates in relation to climate, latitude, and other factors. Further, satellite imagery and GIS mapping now provide unprecedented means to characterize global patterns of landform, climate, productivity, and species richness. These and other technological achievements have greatly advanced both paleo- and neontological studies of the LDG (Jablonski et al. 2017.The Question of Coexistence Still MattersThe current emphasis on historical and evolutionary hypotheses for the LDG is a welcome and needed addition to the ecological hypotheses that figured so prominently in Pianka's early review. However, we should not lose sight of the question of species coexistence. Even if evolution produces more species in the tropics compared to the temperate zone (e.g., due to higher speciation rates and/or a longer evolutionary history), unless interspecific competition is always weak and communities are unsaturated, we still need to explain how more species can coexist at low latitudes than high.One explanation for why more species are able to coexist in the tropics than in the temperate zone is that primary productivity is higher in the tropics, allowing more individuals to be supported per unit area and, therefore, more species (hypothesis 6 in Pianka's list; Table 1). However, for well-documented taxa such as birds and trees, the increase in numbers of individuals per unit area from high latitudes to low latitudes is small compared to the dramatic increase in species richness (Currie et al. 2004; Brown 2014), suggesting that the productivity hypothesis by itself is insufficient. Another long-held hypothesis for enhanced species coexistence in the tropics (included at multiple points by Pianka) is that tropical species have narrower niches, which allows tighter species packing in tropical communities. Indeed, many examples of tropical species with very specialized niches exist, yet results from individual studies and from meta-analyses are mixed. Major obstacles to testing the narrower-niches hypothesis include the poor quality of the data (few studies have collected data specifically to test the hypothesis), the limited geographic scope of studies, and the wide variation in sampling methods. Much more work is needed to better test the narrower-niches hypotheses. Below, we suggest two additional mechanisms (not discussed in Pianka's review) that may promote greater niche diversity and allow more species to coexist in the tropics relative to the temperate zone.Species as NichesBiotic interactions coupled with speciation may greatly expand the available niche space. As suggested by Vermeij (2005), "Every species is potentially a resource on which some other species can in principle specialize or to which another species must adapt." More simply, species can be niches for other species. Consider the army ant Eciton burchellii, which forages in large swarms and collects thousands of arthropod prey for the colony in a single day. More than 300 species of insects, birds, and other organisms depend on E. burchellii for all or part of their livelihood (e.g., eating prey the ants collect or flush, consuming the waste they discard, living in ant nests, riding on ant bodies, and more). Rettenmeyer et al. (2011) speculate that the more than 300 known associate species of E. burchellii are "likely only the tip of the iceberg," with thousands of specimens yet to be described and their associations characterized. Army ants are a spectacular example of how even a single species may create niches for other species. There are many such examples in nature, and they seem far more common at low latitudes than high, but no studies have attempted to quantify this difference.Novel NichesPrice (2008) suggested that "at least 50% of the increase in (bird) species numbers in the tropics is due to the presence of unusual niches." This is likely a "best guess," but there is little doubt that tropical species occupy a variety of novel niches rarely found in the temperate zone. Further, many of these niches are functionally associated with warm temperatures and stable environments and thus may not be possible in a seasonally cold climate. For example, many tropical fish species are herbivorous or frugivorous, but these feeding modes are rare at high latitudes. Other, more unusual, feeding modes such as scale and fin eating have evolved many times in tropical fishes but are essentially absent in temperate fishes. A strong case can be made that herbivory is metabolically feasible only in warm environments for fishes and probably most poikilotherms. However, why other novel niches (e.g., scale eating, parasite cleaning, electrical communication, and predation in fishes; flower piercing, ant following, and herbivory in birds; ant eating, blood feeding in mammals) are common in the tropics but are much rarer in the extratropics deserves further study.The Next 50 Years?Pianka's article was a milestone in organizing the myriad hypotheses for the LDG into a manageable framework for future study. What will the next 50 years bring? First, we are hopeful that current and future naturalists will continue to investigate the ecological and evolutionary mechanisms that contribute to the LDG. Second, and perhaps most important, is to ask how much tropical diversity will remain 50 years from now and to explore what can be done to preserve it. At the very time when we are developing the scientific tools to deeply probe and understand the causes of the LDG, that diversity is rapidly disappearing. What will be left for future naturalists to study? This question was largely absent 50 years ago but now demands our attention.AcknowledgmentsWe thank J. Bronstein for encouragement to write the article, E. Pianka and J. Bronstein for thoughtful comments, and the National Science Foundation for support through grant DEB-1456615. This is contribution 1982 from the Kellogg Biological Station.Literature CitedBrown, J. H. 2014. Why are there so many species in the tropics? 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To identify the ecological and genetic mechanisms of local adaptation requires estimating selection on traits, identifying their genetic basis, and evaluating whether divergence in adaptive traits is due to conditional neutrality or genetic trade-offs. To this end, we conducted field experiments for three years using recombinant inbred lines (RILs) derived from two ecotypes of Arabidopsis thaliana (Italy, Sweden), and at each parental site examined selection on flowering time and mapped quantitative trait loci (QTL). There was strong selection for early flowering in Italy, but weak selection in Sweden. Eleven distinct flowering time QTL were detected, and for each the Italian genotype caused earlier flowering. Twenty-seven candidate genes were identified, two of which (FLC and VIN3) appear under major flowering time QTL in Italy. Seven of eight QTL in Italy with narrow credible intervals colocalized with previously reported fitness QTL, in comparison to three of four in Sweden. The results demonstrate that the magnitude of selection on flowering time differs strikingly between our study populations, that the genetic basis of flowering time variation is multigenic with some QTL of large effect, and suggest that divergence in flowering time between ecotypes is due mainly to conditional neutrality.

Retention of nonfunctional traits over evolutionary time is puzzling, because the cost of trait production should drive loss. Indeed, several studies have found nonfunctional traits are rapidly eliminated by selection. However, theory suggests that complex genetic interactions and a lack of genetic variance can constrain evolution, including trait loss. In the mustard family Brassicaceae the conserved floral condition includes four long and two short stamens, but we show that short stamens in the highly self-pollinating mustard Arabidopsis thaliana do not significantly increase selfed seed set, suggesting that the trait has lost most or all of its function after the transition to selfing. We find that short stamen loss is common in native populations. Loss is incomplete and decreases with increasing latitude, a cline unexplained by correlations with flowering time or ovule count (which also vary with latitude). Using recombinant inbred lines derived from a cross between plants at the latitudinal extremes of the native range, we found three QTLs affecting short stamen number, with epistasis among them constraining stamen loss. Constraints on stamen loss from both epistasis and low genetic variance may be augmented by high selfing rates, suggesting that these kinds of constraints may be common in inbred species.

The distribution of effect sizes of adaptive substitutions has been central to evolutionary biology since the modern synthesis. Early theory proposed that because large-effect mutations have negative pleiotropic consequences, only small-effect mutations contribute to adaptation. More recent theory suggested instead that large-effect mutations could be favoured when populations are far from their adaptive peak. Here we suggest that the distributions of effect sizes are expected to differ among study systems, reflecting the wide variation in evolutionary forces and ecological conditions experienced in nature. These include selection, mutation, genetic drift, gene flow, and other factors such as the degree of pleiotropy, the distance to the phenotypic optimum, whether the optimum is stable or moving, and whether new mutation or standing genetic variation provides the source of adaptive alleles. Our goal is to review how these factors might affect the distribution of effect sizes and to identify new research directions. Until more theory and empirical work is available, we feel that it is premature to make broad generalizations about the effect size distribution of adaptive substitutions important in nature.

Reciprocal transplant experiments have often provided evidence of local adaptation in temperate plants, but few such studies have been conducted in the tropics. To enhance our knowledge of local adaptation in tropical plants, we studied natural populations of two recently diverged Neotropical plant species, Costus allenii and C. villosissimus, in central Panama. We found that these species display a parapatric distribution that reflects local environmental differences on a fine geographic scale: C. allenii is found along ravines in the understory of primary forest, while C. villosissimus is found along forest edges. Light availability was lower in C. allenii habitats, while precipitation and soil moisture were lower in C. villosissimus habitats. We carried out reciprocal transplant experiments with seeds and clones of mature plants to test the hypothesis that the parapatric distribution of these species is due to divergent adaptation to their local habitats. We found strong evidence of local adaptation, i.e., when grown in their home sites, each species outperformed the species from an away site. Our finding that C. allenii and C. villosissimus are mainly isolated by their microhabitats provides a first step toward understanding the mechanisms of adaptation and speciation in the tropics.

Ecologists often view community assembly as a process involving the dispersal of species from a static regional species pool followed by environmental filtering to establish the local community. This conceptual framework ignores the dynamic nature of species pools and fails to recognize that communities are assembled by processes operating over a vast range of temporal and spatial scales. Species pool richness and composition are influenced by metacommunity dynamics over short timescales and by speciation, extinction, and dispersal over long timescales. We suggest that a stronger focus on the geography of speciation, the formation of secondary sympatry, and the feedback between local and regional processes is needed to fully understand community assembly and the importance of dynamic species pools.