Origin of elevational replacements in a clade of nearly flightless birds – most diversity in tropical mountains accumulates via secondary contact following allopatric speciation

Abstract

Tropical mountains are biodiversity hotspots. In particular, mountains in the Neotropics exhibit remarkable beta diversity reflecting species turnover along elevational gradients. Elevational replacements of species have been known since early surveys of the tropics, but data on how such replacements arise are scarce, limiting our understanding of mechanisms underlying patterns of diversity. We employed a phylogenetic framework to evaluate hypotheses accounting for the origin of elevational replacements in the genus Scytalopus (Rhinocryptidae), a speciose clade of passerine birds with limited dispersal abilities occurring broadly in the Neotropical montane region. We found that species of Scytalopus have relatively narrow elevational ranges, closely related species resemble each other in elevational distributions, and most species replacing each other along elevational gradients are distantly related to each other. Although we cannot reject the hypothesis that a few elevational replacements may reflect parapatric speciation along mountain slopes, we conclude that speciation in Scytalopus occurs predominantly in allopatry within elevational zones, with most elevational replacements resulting from secondary contact of formerly allopatric lineages. Our study suggests that accumulation of species diversity in montane environments reflects colonization processes as opposed to in situ divergence even in dispersal-limited animals.

Species richness patterns in Fijian bees are explained by constraints in physiological traits

“…• Phylogenetic niche conservatism: species maintain their ancestral traits (Figure 1B) (Forbes 1846, Mayr and O'Hara 1986, Patton and Smith 1992, Cadena and Céspedes 2020.…”

“…Determining the processes that shape species geographic ranges and corresponding patterns in species richness across climatic gradients is a key question in evolutionary ecology, especially in the context of climate change. Physiological constraints, evolutionary history, and competition are thought to be some of the key factors that shape species ranges; however, exactly how these factors interact to form range limits has remained a contentious topic for almost two centuries (Forbes 1846, Mayr 1963, Endler 1977, 1982a, Morin and Chuine 2006, Kellermann et al 2012, Chan et al 2019, Cadena and Céspedes 2020. Tropical montane island ecosystems are ideal natural laboratories to investigate why patterns in species richness occur across environmental gradients because they often have high levels of endemism and climates change rapidly over limited geographic space (Jankowski et al 2013).…”

“…Within tropical montane ecosystems, two key models of speciation have been proposed: gradient/parapatric and vicariant/allopatric speciation (Box 1; Figure 1) (Funk et al 2016). Both models assume that mountains are species-generating machines, but in different ways (Patton and Smith 1992, Wiens et al 2019, Cadena and Céspedes 2020, García-Rodríguez et al 2021, and with different implications for patterns of phylogenetic conservatism of physiological traits across species. The gradient hypothesis is a form of parapatric speciation, where selective pressures (such as climatic variation across an elevational gradient or intraspecific-competition) can drive divergence between populations, leading to speciation across the elevational gradient without physical geographic barriers (Endler 1977, 1982a, b, Rundle and Nosil 2005 (Figure 1A).…”

“…As such, evolutionary history (e.g. speciation processes) can help explain species' geographic distributions and patterns in species richness (Kellermann et al 2009, 2012, Cadena and Céspedes 2020. Within tropical montane ecosystems, two key models of speciation have been proposed: gradient/parapatric and vicariant/allopatric speciation (Box 1; Figure 1) (Funk et al 2016).…”

Speciation and gene flow across an elevational gradient in New Guinea kingfishers

“…Phylogenetic and phylogeographic comparative studies suggest this process has occurred in Ithioma butterflies (Elias et al, 2009) and Andean amphibians and reptiles (Arteaga et al, 2016). In Andean birds, a broad consensus holds that elevational replacements primarily form through divergence in allopatry followed by secondary contact and displacement (Cadena et al, 2012;DuBay and Witt, 2014;Caro et al, 2013;Cadena and Céspedes, 2019), despite equivocal results in some tests. Yet we are aware of only one study that has used population genomics to evaluate speciation across elevational gradients, which found strong evidence that adaptation to altitude drives speciation in Senecio ragwort plants in temperate Italy (Chapman et al, 2013;Osborne et al, 2013).…”