Introduction

            The environmental conditions under which species thrive (i.e. their fundamental ecological niches defined in abiotic environmental dimensions) evolve over time. The frequency and speed at which niches evolve in speciating lineages remains a key question in evolutionary biology (e.g., Peterson et al. 1999; Graham et al. 2004; Knouft et al. 2006; Losos 2008; Pearman et al. 2008; Evans et al. 2009; Vieites et al. 2009; Nyári and Reddy 2013; Owens et al. 2017; García-Navas and Rodríguez-Rey 2018). Methods for estimating fundamental ecological niches (Peterson et al. 2011; Hijmans and Elith 2015) and inferring macroevolutionary patterns from phylogenies (Swofford and Maddison 1987; Lanyon 1993; Freckleton et al. 2002; Pagel et al. 2004; O'Meara 2012; Revell 2012) have both advanced greatly in recent decades. These developments have facilitated a paradigm shift toward investigating biogeographic history in the context of reconstructed ancestral ecological niche characteristics (e.g., Rice et al. 2003; Graham et al. 2004; Knouft et al. 2006; Pearman et al. 2008; Anciães and Peterson 2009; Evans et al. 2009; Vieites et al. 2009; Smith and Donoghue 2010; Nyári and Reddy 2013; Ribeiro et al. 2016; Owens et al. 2017). Still, modeling complex traits and their evolution remains a major challenge, and indeed reconstructing the evolution of abiotic ecological niches is particularly difficult.
            Researchers have used several approaches to characterize ecological niches when attempting to reconstruct their evolutionary history. Studies have used means and standard errors of suitable abiotic niche characteristics (Rice et al. 2003; Anciães and Peterson 2009), minimum and maximum suitable abiotic niche values (Graham et al. 2004; Yesson and Culham 2006), central tendencies of suitable niche values (i.e. mean or median; Ackerly et al. 2006; Kozak and Wiens 2010; Cooper et al. 2011), and distributions of suitable niche values (Evans et al. 2009; Smith and Donoghue 2010). These data were derived either directly from the occurrence data (e.g. Ackerly et al. 2006; Kozak and Wiens 2010; Cooper et al. 2011) or from ecological niche model outputs (e.g. Rice et al. 2003; Smith and Donoghue 2010; Nyári and Reddy 2013). These approaches fit exisiting ancestral state reconstruction methodology relatively well, but at the cost of simplifying complex niches to summary statistics for each species.
Fundamental ecological niches, furthermore, are rarely characterized completely and unambiguously when they are estimated for real-world species on real landscapes, owing to biases and limitations in environmental conditions available across accessible areas of geographic space (Fig. 1; Saupe et al. 2012; Veloz et al. 2012; Owens et al. 2013; Guisan et al. 2014; Warren et al. 2014; Saupe et al. 2017). The fundamental ecological niche of a species is defined as the set of conditions under which it is able to maintain populations without immigrational input (Soberón 2007), and is the result of phenotypic traits subject to natural selection (Peterson 2011). However, the full suite of environmental conditions within a species’ fundamental niche is not necessarily represented on Earth, or across areas that are accessible to a species. This subset of the fundamental ecological niche that is present in geographic space at the time period of interest and is accessible to the species is referred to as the existing niche (Barve et al. 2011). A species’ realized ecological niche (i.e. environments where the species is observed) is determined by the further reduction of the existing niche by biotic factors such as competition, parasitism, etc. (Soberón 2007).