Intraguild predation-competition and predation by the same antagonist-is widespread, but its evolutionary consequences are unknown. Intraguild prey may evolve antipredator defenses, superior competitive ability on shared resources, or the ability to use an alternative resource, any of which may alter the structure of the food web. We tested for evolutionary responses by threespine stickleback to a benthic intraguild predator, prickly sculpin. We used a comparative morphometric analysis to show that stickleback sympatric with sculpin are more armored and have more limnetic-like body shapes than allopatric stickleback. To test the ecological implications of this shift, we conducted a mesocosm experiment that varied sculpin presence and stickleback population of origin (from one sympatric and one allopatric lake). Predation by sculpin greatly increased the mortality of allopatric stickleback. In contrast, sculpin presence did not affect the mortality of sympatric stickleback, although they did have lower growth rates suggesting increased nonpredatory effects of sculpin. Consistent with their morphology, sympatric stickleback included more pelagic prey in their diets, leading to depletion of zooplankton in the mesocosms. These findings suggest that intraguild prey evolution has altered food web structure by reducing both predation by the intraguild predator and diet overlap between species. Food web interactions, such as predation and resource competition, are important agents of natural selection that can drive evolutionary change (e.g., Schluter 1994;Reznick et al. 1997;
An age-at-length relationship was generated for the tiny Indo-Pacific coral reef goby Trimma nasa using presumed daily increment counts of sagittal otoliths from 110 specimens captured in Palau in May 2007. From these data, the estimated maximum age was approximately 87 d, and the average length of the pelagic larval duration was 33.9 ± 3.7 d (SD), nearly 39% of the maximum lifespan. Linear and power regression lines fit the data equally well (r 2 = 0.8458 and 0.8464, respectively). Mixed model analysis of age, length and sex data produced a better fit, with significant effects of age (p < 0.0001), sex (p = 0.0134) and age × sex interaction (p = 0.0059) on standard length. Males were significantly smaller than females and juveniles when corrected for age (p < 0.05). The male age-at-length relationship was also significantly different from those of females (p = 0.0218) and juveniles (p = 0.0066). Daily mortality rate as calculated by Hoenig's equation was 4.7%. These findings are in keeping with an increasing body of data demonstrating that small reef fish lead short lives with linear growth and high mortality, with important consequences for their contribution to the biomass and, hence, energy production of coral reefs.KEY WORDS: Life history · Gobiidae · Trimma nasa · Otoliths · Pelagic larval duration · Cryptobenthic fishes Resale or republication not permitted without written consent of the publisher Editorial responsibility: Hans-Heinrich Janssen,
Analyses of genomewide polymorphism data have begun to shed light on speciation and adaptation. Genome scans to identify regions of the genome that are unusually different between populations or species, possibly due to divergent natural or sexual selection, are widespread in speciation genomics. Theoretical and empirical work suggests that such outlier regions may grow faster than linearly during speciation with gene flow due to a rapid transition between low and high reproductive isolation. We investigate whether this pattern could be attributed to neutral processes by simulating genomes under neutral evolution with varying amounts and timing of gene flow. Under both neutral evolution and divergent selection, simulations with little or no gene flow, or with a long allopatric period after its cessation, resulted in faster than linear growth of the proportion of the genome lying in outlier regions. Without selection, higher recent gene flow erased differentiation; with divergent selection, these same scenarios produced nonlinear growth to a plateau. Our results suggest that, given a history of gene flow, the growth of the divergent genome is informative about selection during divergence, but that in many scenarios, this pattern does not easily distinguish neutral and non‐neutral processes during speciation with gene flow.
Reproductive isolation is central to the study of speciation. Multiple isolating barriers may prevent species from hybridizing, although their individual strength and the interactions between them are rarely measured. We quantified habitat isolation in a recently diverged threespine stickleback species pair (Gasterosteus aculeatus complex) and controlled for any such interactions. Using enclosures in an outdoor pond, we confirm that males of the two species strongly prefer different nesting habitats: limnetic males build nests in open habitats, whereas benthic males nest under vegetation. However, forcing males to nest in their nonpreferred habitat did not reduce the probability of spawning by females. As a result, habitat isolation between the species is estimated to be weak. We compared the strength of habitat isolation estimated in the present study with estimates of other behavioural barriers using previously published data. We discovered that, although total mating isolation between the species is strong, the contributions of differences in body size and male nuptial colour are similarly individually weak. Instead, interactions with other, undetermined species-specific traits were responsible for most of the isolation resulting from differences in body size and, in benthics, colour. This is one of the first attempts to estimate individual isolating barriers at the same time as controlling for interactions.
In sexually reproducing organisms, speciation involves the evolution of reproductive isolating mechanisms that decrease gene flow. Premating reproductive isolation, often the result of mate choice, is a major obstacle to gene flow between species because it acts earlier in the life cycle than other isolating barriers. While female choice is often considered the default mode in animal species, research in the butterfly genus Heliconius, a frequent subject of speciation studies, has focused on male mate choice. We studied mate choice by H. cydno females by pairing them with either conspecific males or males of the closely related species H. pachinus. Significantly more intraspecific trials than interspecific trials resulted in mating. Because male courtship rates did not differ between the species when we excluded males that never courted, we attribute this difference to female choice. Females also performed more acceptance behaviours towards conspecific males. Premating isolation between these two species thus entails both male and female mate choice, and female choice may be an important factor in the origin of Heliconius species.
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