Edward D. Burress
Received: 24 January 2014 / Accepted: 21 June 2014 Springer International Publishing Switzerland 2014
Cichlid fishes are hypothesized to encompass several independent adaptive radiations that display increased diversification rates and impressive ecological heterogeneity. Here, I review major ecological patterns associated with the evolutionary history of cichlids, with particular focus on comparison of Afrotropical and Neotropical lineages. Specifically, I present major patterns of ecological diversification, potential mechanisms that may promote ecological diversification, and possible consequences of ecological diversification. Evolutionary convergence and specialization of ecological (e.g., diet), behavioral (e.g., benthic sifting), and morphological traits (e.g., oral dentition) characterize adaptive patterns that transcend continents. Craniofacial mechanics, the pharyngeal jaw apparatus, phenotypic plasticity, and hybridization may have facilitated diversification of cichlid fishes by generating functional, morphological, and/or genetic diversity. The benthic–pelagic axis has been an important source of divergence during adaptive radiation. Additionally, there are several discrepancies between Afrotropical and Neotropical lineages, such as the relative frequency of herbivorous species, the importance of hybridization in generating diversity, the relative frequency of dentition types, and relationships between dental organization and ecological function. Emphasis on contrasts between Neotropical and Afrotropical lineages improves characterization of patterns at a broader level of organization and indicates that the genetic basis, functional capacity, and ecological opportunity for many traits may be conserved across lineages.
The future of cichlids as model organisms for evolutionary and ecological research is bright, and several areas are of particular interest and in need of extensive investigation, for example, expanding our understanding of the interplay between genetic and ecological processes and how they relate to the processes of speciation and adaptive radiation. Specifically, utilization of high-throughput molecular techniques to investigate genome-wide patterns and how they relate to ecological processes is of special interest (reviewed in Fan et al., 2012), for example, what changes in genomic architecture precede (i.e., may initiate) or are ongoing during adaptive radiation? Investigation into the number, size, and distribution of genomic regions associated with morphological and ecological diversification is in its infancy, yet is the first step in understanding the genetic basis of adaptive radiation (Irschick et al., 2013). Furthermore, molecular factors that promote and constrain evolution, identification of candidate genes that underlie phenotypic variation and adaptive shifts, and the fate of heterospecific alleles are all fruitful topics that remain poorly understood. Such research would vastly expand our understanding of the genes to phenotype to function linkage and thus clarify questions pertaining to the processes of adaptive radiation and speciation. Numerous recent papers have embarked on this journey (Keller et al., 2012; Recknagel et al., 2013; Franchini et al., 2013). Additionally, high-throughput techniques will help resolve poorly understood evolutionary histories (e.g., Wagner et al., 2013; Ilves & Lo´pez-Ferna´ndez, 2014), which is a common problem among cichlids due to shallow divergence times and complications due to hybridization early in the speciation process (i.e.,P?´alek et al., 2012).
Cichlids are an ideal model for studying adaptive radiations. Afrotropical and Neotropical lineages are similar in many respects such as (1) common form–function relationships with regard to body shape, oral dentition, and pharyngeal jaws, (2) ubiquity of omnivory and benthic sifting, (3) a central role for craniofacial mechanics, the pharyngeal apparatus, and phenotypic plasticity as mechanisms for generating ecological diversity, and (4) comparable examples of ecological speciation and adaptive radiation, particularly the importance of the benthic–pelagic axis. However, these systems also differ in key aspects of processes and patterns of evolution such as (1) the relative frequency of herbivorous species, (2) the primary foraging substrate associated with algivorous species, (3) the relative importance of hybridization as a source of diversification, and (4) relationships between oral tooth integration, shape, and ecology. Both these lineages provide key components to understanding the interplay between ecology and evolution among cichlids, yet studies on the lineages have largely not been integrated (but see Hulsey et al., 2010). Thus, there would be great benefit from improved integration of both systems in the form of comparative analyses, particularly as we advance into linking the genome to form and function.
I am grateful to the many researchers cited herein whose work with cichlid fishes was instrumental in constructing this review and fostered my interest in cichlid fishes… This work was partially supported by the Jim Smith Endowment Fund and the Guy Jordan Endowment Fund. This paper is Contribution No. 703 of the Auburn University Museum of Natural History.