Understanding the processes that drive speciation is a fundamental inquiry of evolutionary biology (Coyne and Orr 2004). Ecological speciation is a key deterministic process involved in speciation and is defined as the evolution of reproductive isolation owing to ecologically driven and divergent natural selection (Schluter 2000). Selection may directly favor reproductive isolation owing to hybrid disadvantage (i.e., via reinforcement) or indirectly as a byproduct of adaptation to contrasting environments (Schluter 2000). Evidence for the former includes instances of enhanced reproductive isolation when two species are sympatric compared to when they are allopatric (reviewed in Coyne and Orr 2004). Examples of the latter include reproductive isolation evolving as a byproduct of adaptation to different feeding substrates in Drosophila (Dodd 1989) or as a byproduct of divergence in body shape or size as an adaptation to distinct predator environments in Neotropical fish (e.g., Langerhans et al. 2007).
One system that has been studied heavily in terms of the role of ecology in speciation is that of the threespine stickleback (Gasterosteus aculeatus) “benthic” and “limnetic” species pairs (McPhail 1994; McKinnon and Rundle 2002). These small, freshwater fish are ecologically, morphologically, behaviorally, and genetically distinct in sympatry in several lakes in southwestern British Columbia, Canada (reviewed by McPhail 1994). They are, therefore, largely reproductively isolated from one another in sympatry and fulfill the criteria generally accepted to define biological species. The benthic form is a solitary, robust-bodied bottom browser of aquatic invertebrates in the littoral zone of lakes, whereas the limnetic is a schooling, slender-bodied planktivore of open waters. Consequently, the two species occupy distinct trophic niches (benthic browsing vs. offshore planktivory) and considerable evidence has accumulated indicating that the species are morphologically and behaviorally adapted to these alternative niches (e.g., Bentzen and McPhail 1984; Schluter and McPhail 1992; Schluter 1993, 1995; Matthews et al. 2010). In addition, laboratory-raised hybrids between the two species are morphologically and behaviorally intermediate relative to the parental species in traits related to foraging ecology (e.g., McPhail 1984, 1992), and are less proficient and grow more slowly when foraging in the niches of either parental species (e.g., Schluter 1995; Hatfield and Schluter 1999). Because increasing growth rate, particularly during early life-history stages of fishes, is typically associated with increased survivorship (by reducing susceptibility to gape-limited predators, increasing the potential prey base, or increasing overwinter survivorship), faster maturation, and in females, greater fecundity at a given age, growth rate is reasonably assumed to be an important component of fitness (e.g., Sogard 1997; Shima and Findley 2002; Berkeley et al. 2004; Morita and Nagasawa 2010). In summary, experimental evidence for fitness trade-offs experienced by one parental species in the trophic niche of the other and for the poor performance of hybrids in both parental niches is consistent with the idea that selection may favor mate discrimination owing to foraging-based hybrid growth disadvantage (e.g., Rundle and Schluter 1998; Schluter 2001) and, therefore, contribute to the evolution of reproductive isolation. In addition, genetic cohort analyses have shown a decline in abundance of hybrid genotypes with age in natural stickleback populations (Gow et al. 2007). This observation, coupled with evidence that survivorship of hybrid genotypes of various ages raised under laboratory conditions is no different than that of parental species (McPhail 1994; Hatfield and Schluter 1999) also implies that there is an ecological component to genetic divergence and reproductive isolation between benthic and limnetic sticklebacks (rather than a prominent role for intrinsic genomic incompatibilities).
Although these data provide compelling evidence consistent with the idea that trophically based natural selection against hybrids has contributed to the evolution of reproductive isolation, reproductive isolation may evolve as an incidental byproduct of trophic adaptation in sticklebacks (Vines and Schluter 2006) and perhaps by sexual selection against hybrids (Vamosi and Schluter 1999). Further, most of the evidence for a role for hybrid growth disadvantage in stickleback speciation is based on laboratory studies or micro/mesocosom field studies and direct tests of hybrid growth disadvantage under fully natural conditions are lacking. Mark-recapture studies of individual free-ranging sticklebacks in nature would be one way to collect such data, but they are logistically challenging. An alternative way to assess growth of wild-caught fish is to infer growth rates from otolith analysis.
Otoliths are calcareous structures found in the inner ear of teleost fishes (Campana and Thorrold 2001). The otoliths occur as three pairs on either side of the head of which the sagittal otoliths are the largest. Otoliths accrete layers of calcium carbonate throughout the life of a fish and the accretion rate and growth of the otolith varies with growth rate of the fish. In addition, accretion rate varies on a daily basis such that variation in daily growth rate of the fish is reflected in variation in daily growth rate of the otolith (Campana and Thorrold 2001). Daily variation in otolith growth appears as a series of alternating dark and light bands radiating out from the center of the otolith with one pair of light and dark bands representing 24 h. These patterns have been validated experimentally in many species, and once validated, it is possible to determine the age of a fish from its otoliths by counting the “daily rings.” In addition, because otolith growth is related to overall fish growth, it is possible to estimate the growth rate of a fish by measuring the size of an otolith over a given time period (i.e., days). Daily growth ring analysis has been widely used in studies of the biology and management of many fishes (e.g., reviewed by Campana 2005). In this study, we examined growth of wild, free-ranging benthic, limnetic, and hybrid sticklebacks inferred from the daily growth rings of otoliths to test the hypothesis of hybrid growth disadvantage—a key prediction of trophic-based ecological speciation in these fish.