Del control de la acusación y su finalidad

Trait evolution mediated by selection to minimize resource competition – what Darwin named divergence of character – has a long history in evolutionary biology (Darwin 1859). Field and experimental evidence, accumulated since Darwin first proposed the idea, make clear that resource competition is an important force for

generating biological diversity (for reviews, Schluter 2000; Dayan and Simberloff 2005; Pfennig and Pfennig 2009). However, our current level of understanding raises many, new, interesting questions concerning both the causes and consequences of character displacement (Pfennig and Pfennig 2009). The research I have conducted in my

dissertation addresses several questions, while suggesting future lines of inquiry that may further our understanding of ecological character displacement.

In Chapter II, I document disruptive selection acting on trophic morphology in natural populations of spadefoot toad tadpoles (S. multiplicata). While disruptive selection has long been thought to drive divergence within populations (i.e. “intraspecific character displacement”), it was also believed to be rare (Bolnick 2004). Furthermore, I

demonstrated that this disruptive selection was likely caused by ecological specialization and resource competition. Because ecological specialization and competition are common in nature (Schluter 2000), my research suggests that disruptive selection may be a more

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common force contributing to evolutionary divergence than is currently recognized (Reviewed in Rueffler et al. 2006).

The possibility that disruptive selection is more common than generally thought opens several new avenues of research. First, it raises the question: what are the consequences of widespread disruptive selection in natural populations (Rueffler et al. 2006)? In Chapter III, I examine, one possible consequence of disruptive selection, the expression and evolution of resource polymorphism. I further discuss that disruptive selection is unlikely to lead to stable evolutionary divergence without the accompanying evolution of reproductive isolation between diverging lineages. Additionally, the

evolution of reproductive isolation may be more likely in cases where diverging lineages occupy separate niches (Ripa 2009). For example, when disruptive selection is driven by ecological specialization for bimodally distributed resources. In Chapter III, I show that disruptive selection alone is likely not sufficient for the evolution of resource

polymorphism (i.e. intraspecific character displacement). Indeed, my results suggest that intraspecific divergence may require ecological opportunity, in the form of diverse available resources, and intraspecific competition. Therefore, my research supports the widespread observation that alternative resource-use morphs occur most commonly in habitats where interspecific competition is relaxed, such as depauperate glacial and volcanic lakes, which leads to increased ecological opportunity (Smith and Skúlason 1996).

The fact that an experimental decrease in the range of resources available did not cause greater bimodality in trophic morphology, whereas an increase in the density of conspecifics did, raises an interesting question for the relative importance of exploitative

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versus interference competition, in the evolution of intraspecific character displacement. Generally, the relative contributions of exploitative versus interference competition to the evolution of resource polymorphism are largely unknown, and remain a potentially productive area of research.

While the ultimate causes of character displacement are well known, surprisingly little is known about the proximate mechanisms underlying inter and intraspecific character displacement. In Chapter IV, I show that maternal investment in offspring egg size influences the induction of resource polymorphism in S. multiplicata tadpoles. In addition, my research provides additional support that a condition dependent maternal effect mediates interspecific character displacement in spadefoot toads (Pfennig and Martin 2009). Given that the evolution of character displacement is often associated with the expansion of populations into novel environments (Rice and Pfennig 2008), and with encountering novel competitors, the role of environmental effects in the earliest stages of character displacement might be much more widespread and important than is currently believed (Pfennig and Martin 2009, 2010). Future research then, should attempt to better understand the proximate basis of character displacement in other systems.

Which particular traits experience selection during character displacement can

influence whether and how character displacement occurs. However, few studies identify the targets of selection during character displacement. In Chapter V, I address this

problem by evaluating the targets of selection on S. multiplicata imposed by experimentally imposed competition with a heterospecific competitor. I found, that selection on a single trait drove correlated selection on additional traits in S. multiplicata tadpoles. This finding raises the question of how the genetic architecture of the traits

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under selection affects the evolution of character displacement (Smith and Rausher 2008). For example, if populations of the same species differ in their underlying genetic architecture, character displacement could unfold differently in each population. In addition, my results suggested that selection might directly act on different traits between species during the same instance of character displacement. Therefore, character

displacement could make a greater contribution to the evolution of biological diversity than is currently appreciated.

In conclusion, my research suggests that, 1) Disruptive selection, and thus

intraspecific character displacement may be more common than previously thought; 2) The existence of ecological opportunity, as well as competition, may be critical for character displacement to occur; 3) Environmental effects, especially maternal effects, may be important in the evolution of resource polymorphism, and, in general in the evolution of novel traits; and 4) Selection on correlated traits may often occur during character displacement, and that selection may act on different traits between species during the same instance of character displacement. My research has addressed questions regarding both the proximate and ultimate causes of character displacement. The findings of this work will hopefully spur future lines of inquiry into the causes and consequences of character displacement, and a better understanding of the origins of biological

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LITERATURE CITED

Bolnick, D. I. 2004. Can intraspecific competition drive disruptive selection? An experimental test in natural populations of sticklebacks. Evolution 58:608-618. Darwin, C. R. 1859. On the origin of species by means of natural selection. John Murray,

London.

Dayan, T., and D. Simberloff. 2005. Ecological and community-wide character displacement: the next generation. Ecology Letters 8:875-894.

Pfennig D. W., and R. A. Martin. 2009. A maternal effect mediates rapid population divergence and character displacement in spadefoot toads. Evolution 63:898-909. Pfennig D. W., and R. A. Martin. 2010. Evolution of character displacement in

spadefoot toads: different proximate mechanisms in different species. Evolution (in press).

Pfennig, K. S., and D. W. Pfennig. 2009. Character displacement: ecological and

reproductive responses to a common evolutionary problem. The Quarterly Review of Biology: 64:253-276.

Rice, A. M., and D. W. Pfennig. 2008. Analysis of range expansion in two species undergoing character displacement: Why might invaders generally 'win' during character displacement? Journal of Evolutionary Biology 21: 696-704.

Ripa, J. 2009. When is sympatric speciation truly adaptive? An analysis of the joint evolution of resource utilization and assortative mating. Evolutionary Ecology 23:21- 52.

Rueffler, C., T. J. M. Van Dooren, O. Leimar, and P. A. Abrams. 2006. Disruptive selection and then what? TRENDS in Ecology and Evolution 21:238-245.

Schluter, D. 2000. The Ecology of Adaptive Radiation. Oxford University Press, Oxford. Smith, T. B., and S. Skúlason. 1996. Evolutionary significance of resource

polymorphisms in fishes, amphibians, and birds. Annual Review of Ecology and Systematics 27:111-133.

Smith, R. A., and M. D. Rausher. 2008. Selection for character displacement is constrained by the genetic architecture of floral traits in the ivyleaf morning glory. Evolution 62:2829-2841.

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