Case studies and mathematical models of ecological speciation. 1. Cichlids in a Crater Lake

Universität Konstanz, Constance, Baden-Württemberg, Germany
Molecular Ecology (Impact Factor: 6.49). 08/2007; 16(14):2893-909. DOI: 10.1111/j.1365-294X.2007.03305.x
Source: PubMed


A recent study of a pair of sympatric species of cichlids in Lake Apoyo in Nicaragua is viewed as providing probably one of the most convincing examples of sympatric speciation to date. Here, we describe and study a stochastic, individual-based, explicit genetic model tailored for this cichlid system. Our results show that relatively rapid (<20,000 generations) colonization of a new ecological niche and (sympatric or parapatric) speciation via local adaptation and divergence in habitat and mating preferences are theoretically plausible if: (i) the number of loci underlying the traits controlling local adaptation, and habitat and mating preferences is small; (ii) the strength of selection for local adaptation is intermediate; (iii) the carrying capacity of the population is intermediate; and (iv) the effects of the loci influencing nonrandom mating are strong. We discuss patterns and timescales of ecological speciation identified by our model, and we highlight important parameters and features that need to be studied empirically to provide information that can be used to improve the biological realism and power of mathematical models of ecological speciation.

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    • "Second, this genetically based (i.e., heritable) divergence generates some degree of reproductive isolation between the populations, for instance through performance tradeoffs between the habitats (Hendry 2004; Nosil et al. 2005; Thibert-Plante and Hendry 2009), or divergence in reproductive behavior (Coyne and Orr 2004; Ritchie 2007; Maan and Seehausen 2011; Thibert-Plante and Gavrilets 2013). Although it is debatable how fast reproductive isolation through this pathway can emerge (Hendry et al. 2007; Gavrilets et al. 2007; Nosil 2012), selection over multiple generations is certainly needed to achieve the underlying genetic divergence—even when selection is strong and genetic variation is abundant. However, a faster pathway to speciation can occur when the exposure to ecologically different habitats directly causes divergence between populations through phenotypic "
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    ABSTRACT: Speciation can be promoted by phenotypic plasticity if plasticity causes populations in ecologically different habitats to diverge in traits mediating reproductive isolation. Although this pathway can establish reproductive barriers immediately and without genetic divergence, it remains poorly investigated. In threespine stickleback fish, divergence in body size between populations represents a potent source of reproductive isolation because body size often influences reproductive behavior. However, the relative contribution of phenotypic plasticity and genetically based divergence to stickleback body size evolution has not been explored. We here do so by using populations residing contiguously in Lake Constance (Central Europe) and its tributaries, a system where lake fish exhibit strikingly larger size and greater age at maturity than stream fish. Laboratory experiments reveal the absence of substantial genetic divergence in intrinsic growth rates and maturation size thresholds between lake and stream fish. A field transplant experiment further demonstrates that lake fish display the life history typical of stream fish when exposed to stream habitats for one year, confirming that life history divergence in this system is mainly plastic. This plasticity appears to be driven by restricted food availability in the lake relative to the stream habitat. We thus propose that in this stickleback system, the exploitation of different trophic niches immediately promotes reproductive isolation via resource-based plasticity in life history.
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    • "Understanding local adaptation and speciation therefore requires inferences about the biogeography and past demography of populations, factors that may have changed substantially over the course of speciation (Hewitt 2011; Abbott et al. 2013). For example, speciation might be promoted by alternating cycles of separation by geographical barriers and secondary contact (Bierne et al. 2011), or local adaptation might be achieved more readily with some spatial arrangements of habitats than with others (Gavrilets et al. 2007). "
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    ABSTRACT: Parallel evolution of similar phenotypes provides strong evidence for the operation of natural selection. Where these phenotypes contribute to reproductive isolation, they further support a role for divergent, habitat-associated selection in speciation. However, the observation of pairs of divergent ecotypes currently occupying contrasting habitats in distinct geographical regions is not sufficient to infer parallel origins. Here we show striking parallel phenotypic divergence between populations of the rocky-shore gastropod, Littorina saxatilis, occupying contrasting habitats exposed to either wave action or crab predation. This divergence is associated with barriers to gene exchange but, nevertheless, genetic variation is more strongly structured by geography than by ecotype. Using approximate Bayesian analysis of sequence data and AFLP markers, we show that the ecotypes are likely to have arisen in the face of continuous gene flow and that the demographic separation of ecotypes has occurred in parallel at both regional and local scales. Parameter estimates suggest a long delay between colonisation of a locality and ecotype formation, perhaps because the postglacial spread of crab populations was slower than the spread of snails. Adaptive differentiation may not be fully genetically independent despite being demographically parallel. These results provide new insight into a major model of ecologically-driven speciation. This article is protected by copyright. All rights reserved.
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    • "Instead, this finding may be, in part, a result of our tendency to name species only with clear and definable morphological characters (Rocha & Bowen, 2008; see 'Intraspecific Comparisons', below). Moreover, several lines of evidence indicate that isolation neither has to be absolute nor of long duration for speciation to occur (Gavrilets et al., 2007; Rocha & Bowen, 2008). Selection driven by ecological factors could accelerate divergence despite gene flow or continue generating divergence that began in isolation. "
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