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The Neotropical Realm is unparalleled in its biotic complexity (Gentry, 1982a; Graham, 2010). The northern Neotropical region geologically corresponds to southwestern Laurasia. It is occupied today by Mexico and Central America, and is here loosely referred to as ‘Mesoamerica’. It includes, but is not limited to, the Mesoamerican Biodiversity Hotspot (Myers et al., 2000), and encompasses a vast diversity of habitats, functional adaptations, and species richness. Substantial efforts to document Mesoamerican biodiversity have been ongoing for centuries (e.g., Sessé & Mociño, 1887; Hernández, 1942; CONABIO, 2008). However, its sheer richness, overlaid on an extensive and physically complex territory, have rendered these fundamental efforts still incomplete. Very little is known about the main evolutionary processes underlying high species richness in different Mesoamerican biomes. Is it caused by high speciation, or by low extinction? Is it derived from temporally circumscribed speciation bursts, or from long-term species accumulation? What is the relative relevance of in situ diversification vs the prevalence of immigrants? The main aim of this study is to investigate the evolutionary processes that contribute to high species richness in a particular biome in Mesoamerica.
Arid biomes are prominent in Mesoamerica, providing emblematic landscapes and a substantial part of its biodiversity. While less diverse than wet forests, Mesoamerican arid biomes are richer than their South American counterparts (Gentry, 1982a; Lott et al., 1987; Trejo & Dirzo, 2002; Villaseñor, 2004; Pennington et al., 2009). Seasonally dry tropical forests (SDTFs) encompass a variety of plant associations, from relatively moist tall forests to dry, succulent-rich scrubs, that grow on fertile soils where rainfall is below 1800 mm yr−1, and receive < 100 mm for 5–6 months (Pennington et al., 2009). They are fire- and frost-intolerant, and, in South America, are distributed in small isolated patches separated by areas that are wetter, colder, extremely arid, or at very high elevation (Pennington et al., 2009). Contrary to other biomes (e.g. lowland rainforests; Gentry, 1982a) and biological lineages (e.g. hylid frogs, Wiens et al., 2006), the richest Neotropical SDTFs are distant from the Equator, leading some workers to discuss an ‘inverse latitudinal diversity gradient’ (Gentry, 1995; Trejo & Dirzo, 2002). Lott et al. (1987) documented 83–105 spp. km−2 at 19°N in Chamela, Mexico, close to the northern end of the distribution of this vegetation type, a substantially higher richness than in sampled South American dry forest sites with equivalent precipitation, and much higher than predicted by the postulated linear relationship between mean annual precipitation and Neotropical plant species richness (Gentry, 1982b; Lott et al., 1987; Trejo & Dirzo, 2002). In particular, Mexican SDTFs have high degrees of endemism and a pronounced species turnover displayed by very low among-site floristic similarity (Trejo & Dirzo, 2002).
After its emergence from the North American epicontinental sea in the early Tertiary, the area corresponding to central and northern Mexico was at least somewhat arid because of its latitudinal position in the descending arm of the Hadley global convection cell and the rain shadows cast by the western and eastern Mexican mountain ranges (Sierra Madre Occidental and Sierra Madre Oriental, respectively) which resulted from orogenic processes starting in the middle Cretaceous and mostly culminating by the late Eocene (Graham, 2010). Between the Miocene and Pliocene, aridity was further enhanced by a global cooling process that resulted in polar ice sheets and local volcanism that enhanced rain shadows (Graham, 2010, pp. 60–66). In response to Miocene aridification, preadapted lineages that existed in North America in the middle Eocene expanded southwards and, together with newly evolved species, represented the onset of the vegetation of the contemporary Sonoran and Chihuahuan Deserts (Graham, 2010, pp. 62–63). Climatic and fossil evidence, as well as molecular clock ages of SDTF-centered lineages (Lavin et al., 2003, 2004; Pennington et al., 2004; Pirie et al., 2009; Schrire et al., 2009), congruently indicate that Neotropical SDTFs may have arisen in North America during the middle Eocene (Pennington et al., 2009).
Considering the early Cenzoic origin, aridity and patchy distribution of South American SDTFs, Pennington et al. (2009) postulated that these attributes have shaped the evolution of resident woody plant lineages. Seasonal dryness represents a barrier to potential colonists not adapted to some degree of aridity. Their patchy distribution separated by areas that are difficult to surmount causes limited dispersal of propagules and pollen. These barriers to migration, combined with their prolonged existence, have resulted in a distinctive species composition for each patch. As a consequence, South American SDTFs are compositionally stable, dispersal-limited systems with high among-patch beta diversity, which is positively correlated with geographical distance (Linares-Palomino et al., 2011). SDTF-centered lineages consequently include ancient species and exhibit strong geographic structure in their phylogenies (Pennington et al., 2009). Moreover, South American SDTF lineages exhibit strong niche conservatism where closely related species share the same type of habitat, and habitat shifts are strongly directional: predominantly from SDTFs to other types of vegetation (Pennington et al., 2009). Mesoamerican SDTFs, while sharing with their South American counterparts aridity and origin since at least the Miocene, occupy a variety of physical conditions (e.g. edaphic, altitudinal), a wide latitudinal range, and have a continuous distribution over extensive areas (e.g. the Mexican Pacific slope and the Balsas river basin; Fig. 1), which is unusual for other Neotropical SDTFs (Trejo & Dirzo, 2002).
Figure 1. Main areas of Bursera distribution in seasonally dry tropical forests (SDTFs). The colored areas represent nine main disjunct areas of distribution of Bursera in SDTFs, delimited mainly by considering species distributions (see main text). SDTFs are continuously distributed among several of these areas. Nevertheless, the distribution of Bursera species is discontinuous as a result of altitudinal barriers. Map based on Brown et al. (2007).
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To document some of the diversification processes underlying the high species richness of Mesoamerican SDTFs, and to evaluate if the old species age, high geographical structure and directionality of habitat shifts observed for South American SDTF-centered lineages prevail in their Mesoamerican counterparts, we use the genus Bursera (Burseraceae, Sapindales) as a study system. Bursera is arguably the most distinctive physiognomic component of Mesoamerican SDTFs. It includes c. 107 species which, except for B. tonkinensis from northern Vietnam, are approximately distributed between northern Mexico and northern South America (Fig. 1). Bursera’s diversification has been linked with the southward expansion of SDTFs in North America in response to the uplift of the Sierra Madre Occidental and the Mexican Transvolcanic Belt (Becerra, 2005; Cevallos-Ferriz & González-Torres, 2005; Dick & Wright, 2005). The strong association of Bursera with SDTFs has prompted attempts to reconstruct the history of this vegetation type in Mexico on the basis of the diversification of Bursera (Becerra, 2005; but see, e.g. Dick & Wright, 2005).
Here we estimate phylogenetic relationships for a nearly complete representation of Bursera species based on one plastid and four nuclear molecular markers. To document the diversification process leading to Bursera species diversity, we estimate the timing of lineage splitting and test for significant diversification heterogeneity among phylogenetic branches and through time. To evaluate specific predictions about SDTF-centered lineages (Pennington et al., 2009), we obtain the average age of Bursera species; conduct tests of geographical structure; and reconstruct habitat shifts. We expect the following: a pre-Pliocene age for most Bursera species, particularly for those centered in SDTFs; the presence of geographical structure in its phylogeny; and directional habitat shifts predominantly from SDTFs to other vegetation types.
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Fig. S1 Maximum likelihood phylogenetic tree for Bursera species.
Table S1 List of species and GenBank accessions for Bursera phylogenetic analysis
Table S2 List of species and GenBank accessions for eudicot phylogenetic analysis
Table S3 Fossil calibration and constraints for eudicot dating analysis
Table S4 Fossil calibration and constraints for Bursera dating analysis
Table S5 List of Bursera species inserted in phylogenetic diversification analysis
Table S6 Fit of birth-death likelihood (BDL) diversification models to Bursera
Methods S1 Detailed description of methods.
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