Experimental evidence that predation promotes divergence in adaptive radiation

Abstract

Adaptive radiation is the evolution of ecological and phenotypic diversity within a rapidly multiplying lineage. Recent studies have identified general patterns in adaptive radiation and inferred that resource competition is a primary factor driving phenotypic divergence. The role and importance of other processes, such as predation, remains controversial. Here we use Timema stick insects to show that adaptive radiation can be driven by divergent selection from visual predators. Ecotypes using different host-plant species satisfy criteria for the early stages of adaptive radiation and differ in quantitative aspects of color, color pattern, body size, and body shape. A manipulative field experiment demonstrates that the direction and strength of divergent selection on these traits is strongly positively correlated with the direction and magnitude of their population divergence in nature but only when selection is estimated in the presence of predation. Our results indicate that both competition and predation may commonly serve as mechanisms of adaptive radiation.T he ''ecological theory of adaptive radiation'' states that (i) divergent natural selection drives the phenotypic divergence and speciation of lineages and (ii) divergent selection itself stems from ecological differences between environments or from ecological interactions (1-3). A prediction of the theory is that the direction and magnitude of divergent selection in the wild is positively correlated with the direction and magnitude of trait divergence among natural populations (4, 5). Our understanding of adaptive radiation has been greatly increased by studies describing general patterns or documenting the process of divergent selection (6-11). However, such studies do not test the critical prediction of a correlation between selection and trait divergence. With respect to the causes of divergent selection, support for even the best-studied mechanism of interspecific competition is mostly indirect (2). The role of other processes, such as predation, has long been discussed (12-14) but remains controversial (2,(15)(16)(17)(18). Part of the controversy stems from the fact that predation is notoriously difficult to study in the wild. Here we demonstrate that divergent selection and trait divergence are strongly correlated in natural populations of walking stick insects, and we elucidate predation as the source of divergent selection by using a manipulative field experiment.Timema walking sticks are plant-feeding insects distributed throughout southwestern North America (19). The genus as a whole satisfies three of the four criteria for adaptive radiation (2): recent ancestry, environment-phenotype correlations, and rapid bursts of speciation (19,20). Only experimental tests for trait utility at the among-species level remain to be conducted. By contrast, all four criteria are satisfied for host-associated ecotypes of Timema cristinae adapted to feeding on two different host-plant species. Ecotypes are defined by which host-plant species they are found on...

Fine-scale local adaptation in an invasive freshwater fish has evolved in contemporary time

“…Notwithstanding extensive sampling of potential fish predators, we documented only one case of predation: cannibalism of a tagged Waterford individual by an untagged yearling brown trout in Middle Rocky Brook. The lack of tag recoveries in predators is not surprising as predation is notoriously difficult to observe and quantify in the wild [61,62]. Based on water temperatures during the experiment, ingested tags would probably have spent only around 20 h in the digestive system of predators according to gastric evacuation models [63].…”

Adaptive Radiation, Ecological Opportunity, and Evolutionary Determinism

“…Field studies of natural selection (e.g., Grant and Grant 2006), sometimes experimental (e.g., Losos et al 2006), are testing the hypothesis that interspecific interactions drive adaptive diversification in adaptive radiations. Studies on nascent adaptive radiations, such as those of sticklebacks and walking sticks, are shedding light on how speciation and adaptive diversification occur in the early stages of adaptive radiation (e.g., Rundle et al 2003;Schluter 2003;Nosil and Crespi 2006), although of course there is no guarantee that these taxa will eventually become full-blown adaptive radiations. In this light, invasive species, in so many ways potentially disastrous ecologically, may serve a positive role by creating quasi experiments that could never be conducted intentionally, allowing evolutionary biologists to study the early stages of adaptive radiation when a species arrives in an environment in which it has no previous evolutionary history (e.g., Carroll et al 1998;Phillips and Shine 2004;Sax et al 2007;Vellend et al 2007).…”

It’s a bear market: evolutionary and ecological effects of predation on two wild sockeye salmon populations

“…Despite these interests, the selection coefficients and demographic and evolutionary effects associated with natural predation are rarely quantified. Even in well-established model systems, such as Trinidadian guppies (Poecilia reticulata; Reznick et al, 1996) and stick insects (Timema cristinae; Nosil and Crespi, 2006), there are few quantitative estimates of predation intensity and selectivity, which are critical for determining mechanistic links between predation and evolution in wild populations.…”

“…Predation can regulate sizes of natural populations (Hixon and Carr, 1997) and determine foodweb and ecosystem dynamics (Mills et al, 1993). Evolutionarily, predation can influence phenotypic divergence within species (Vamosi, 2005), affecting life history, morphology and other traits (Reznick and Endler, 1982;Nosil and Crespi, 2006). Predators may show a preference for killing individuals with specific phenotypes, a phenomenon termed 'selectivity.'…”