Adaptive radiation and ecological opportunity - insight from cichlid fishes
I'm interested in broad patterns and themes of adaptive radiation (i.e., how adaptive radiation proceeds, how predictable are adaptive radiations, and what mechanisms serve as the catalyst of adaptive radiation). We've learned tons from model groups such as Anolis lizards, Darwin's finches, sticklebacks, and cichlids, among many others. Cichlids have earned their status as a model group for studying adaptive radiation by exhibiting bewildering fast rates of species and morphological diversification. There is a rich literature pertaining to the adaptive radiation of cichlid fishes, but I am convinced there is much more left to learn and even hazard a guess that we don't know that much about how (and why) cichlids have been so successful and exhibit the ecological diverity that we observe among extant cichlids.
The particulars of adaptive radiation are perhaps contentious, but I usually think about adaptive radiation as the rise of ecological diversity and its corresponding adaptations in response to ecological opportunity. To be honest, that is somewhat of an amalgamation of definitions derived from Simpson, Schluter, Losos, and Givnish (google any of their names with "adaptive radiation" to get an idea of the nuanced ins and outs of the concept). All of those guys have played a large role in my own interest in adaptive radiation, particularly the role that ecological opportunity, specifically the various sources of ecological opportunity, have played during the process of adaptive radiation. In terms of cichlids, there are two major hypothesized catalysts of adaptive radiation: (i) novel innovation (i.e., pharyngognathy - a series of modifications to the pharyngeal arches that result in a functional second set of jaws) and (ii) colonization events (i.e., of lakes). Both of these presumably provide access to novel resources and subsequently alter the trajectory of lineages across the adaptive landscape (i.e., spur adaptive radiation).
Cichlids are a good group to evaluate the role of colonization-associated ecological opportunity because they have colonized many environments that vary tremendously in scale and type, ranging from continents (i.e., of South America and subsequently Middle America) via trans-Atlantic dispersal, to ecosystem-scale colonization events that range from large-scale (i.e., Lakes Tanganyika, Malawi, and Victoria) to small-scale (i.e., Lakes Barombi, Bermin, Natron, and Nicaragua), as well as river systems (i.e., of the La Plata, Congo, and Malagarasi basins). We know a fair amount about the colonization of the East African Great Lakes, but little about the colonization of the Americas, and essentially nothing about the colonization of river basins. For example, we know that cichlids have diversified rapidly in the East African Great lakes, but what about following the colonization of the Americas or following the colonization of rivers? Presumably all of these colonization events provide ecological opportunity, but how have cichlids responded?.
In a recent paper (Burress and Tan 2017; PDF), my collaborator and I demonstrate that colonization events, regardless if they correspond to continental, lake, or river colonization, result in bursts in species diversification (generally considered an inherent quality of adaptive radiation). Furthermore, across the cichlid phylogeny, nodes that correspond with colonization events are far more likely (approximately 43-times as likely based on marginal odds ratios) to exhibit bursts in diversification rates than other, non-colonization-associated, nodes. In addition, we demonstrate that some river-dwelling cichlids, particularly pike cichlids (Crenicichla) that have colonized the La Plata basin, and subsequently diversified in parallel in the Uruguay and Paraná Rivers, exhibit diversification rates comparable with those of some African lake radiations. Long story short, we hypothesize that physiological plasticity and adaptability permitted cichlids to capitalize upon opportunities to colonize novel environments when they arise (i.e., salinity, alkalinity, and temperature tolerance) and therefore, likely played a central role in the adaptive radiation of cichlid fishes. We hope that this provides a more mechanistic interpretation of the long-standing hypothesis that colonizing novel environments plays a key role in spurring adaptive radiation. Testing this hypothesis directly should be feasible, even if no simple feat, by evaluating (i) salinity-tolerance (as well as alkalinity and temperature tolerance) across cichlids and (ii) exploring the genetic basis for these physiological characteristics (i.e., perhaps cichlids maintain signals in their genome that are shared by marine species that may have facilitated their capacity to successfully colonize the Americas via trans-Atlantic dispersal from Africa).
Despite our hypothesis that colonization-associated ecological opportunity explains much of the diversification rate heterogeneity across the cichlid phylogeny, I have also sunk a great deal of time into studying the implications of a key phenotypic innovation upon the adaptive radiation of cichlids - pharyngognathy - a series of modifications to the pharyngeal arches that permits the pharyngeal jaws to literally function as a second set of jaws. Pharyngognathy is one of the prominent hypothesized key innovations (originally by Liem 1973) that improved functional capacity by releasing the oral jaws from demands associated with prey processing and thereby permitting their diversification in terms of prey capture. We know surprisingly little about this hypothesis. Lots of work by Wainwright and subsequently Hulsey (and even myself to a much lesser extent) has revealed that pharyngognathy facilitated the exploration of some ecological roles (namely durophagy - the exploitation of hard-shelled prey); although, I have speculated that this may also extend to difficult-to-digest prey such as algae by permit the mechanistic rupture of cells and thereby facilitate digestion (Burress 2016; although, admittedly this has not been experimentally shown in cichlids). In addition, several studies have clearly shown that pharyngeal jaws can constraint evolution in some conditions (McGee et al. 2015; Burress et al. 2015), particularly among piscivores where there is little demand for prey processing by the pharyngeal jaws and pharyngeal jaw-associated gape limitations constrain the consumption of whole prey (i.e., with minimal or no processing prior to ingestion).
So, there are aspects of pharyngeal jaw shape and dentition that have clear ecological implications and supposedly subsequent adaptive implications as well as clear implications of colonization-associated opportunity upon the evolutionary histories of cichlids. Therefore, I am left fascinated by the relative importance of innovation-and colonization-associated ecological opportunity during the adaptive radiation of cichlid fishes and a large portion of my research interests deal with testing these two hypotheses as well as teasing these two mechanisms apart (or more likely, revealing how they are intertwined).
Literature referenced in the above text
Burress and Tan. 2017. Ecological opportunity alters the timing and shape of adaptive radiation. Evolution. doi:10.1111/evo.13362. PDF.
Burress. 2016. Ecological diversification associated with the pharyngeal jaw diversity of Neotropical cichlid fishes. Journal of Animal Ecology 85: 302-313. PDF.
Burress et al. 2015. Rates of piscivory predict pharyngeal jaw morphology in a piscivorous lineage of cichlid fishes. Ecology of Freshwater fish 25: 590-598. PDF.
Liem. 1973. Evolutionary strategies and morphological innovations: cichlid pharyngeal jaws. Systematic Zoology 22: 425-441.
McGee et al. 2015. A pharyngeal jaw evolutionary innovation facilitated extinction in Lake Victoria cichlids. Science 350: 1077-1079.