Abstract.-"Ecological" speciation occurs when reproductive isolation evolves as a consequence of divergent selection between populations exploiting different resources or environments. We tested this hypothesis of speciation in a young stickleback species pair by measuring the direct contribution of ecological selection pressures to hybrid fitness. The two species (limnetic and benthic) are strongly differentiated morphologically and ecologically, whereas hybrids are intermediate. Fitness of hybrids is high in the laboratory, especially F) and F 2 hybrids (backcrosses may show some breakdown). We transplanted F 1 hybrids to enclosures in the two main habitats in the wild to test whether the distribution of resources available in the environment generates a hybrid disadvantage not detectable in the laboratory. Hybrids grew more slowly than limnetics in the open water habitat and more slowly than benthics in the littoral zone. Growth of F 1 hybrids was inferior to the average of the parent species across both habitats, albeit not significantly. The contrast between laboratory and field results supports the hypothesis that mechanisms of F 1 hybrid fitness in the wild are primarily ecological and do not result from intrinsic genetic incompatibilities. Direct selection on hybrids contributes to the maintenance of sympatric stickleback species and may have played an important role in their origin.

We use Wright's distribution of equilibrium allele frequency to demonstrate that hybrids between populations interconnected by low to moderate levels of migration can have large positive heterosis, especially if the populations are small in size. Beneficial alleles neither fix in all populations nor equilibrate at the same frequency. Instead, populations reach a mutation-selection-drift-migration balance with sufficient among-population variance that some partially recessive, deleterious mutations can be masked upon crossbreeding. This heterosis is greatest with intermediate mutation rates, intermediate selection coefficients, low migration rates and recessive alleles. Hybrid vigour should not be taken as evidence for the complete isolation of populations. Moreover, we show that heterosis in crosses between populations has a different genetic basis than inbreeding depression within populations and is much more likely to result from alleles of intermediate effect.

"Ecological" speciation occurs when reproductive isolation evolves as a consequence of divergent selection between populations exploiting different resources or environments. We tested this hypothesis of speciation in a young stickleback species pair by measuring the direct contribution of ecological selection pressures to hybrid fitness. The two species (limnetic and benthic) are strongly differentiated morphologically and ecologically, whereas hybrids are intermediate. Fitness of hybrids is high in the laboratory, especially F and F hybrids (backcrosses may show some breakdown). We transplanted F hybrids to enclosures in the two main habitats in the wild to test whether the distribution of resources available in the environment generates a hybrid disadvantage not detectable in the laboratory. Hybrids grew more slowly than limnetics in the open water habitat and more slowly than benthics in the littoral zone. Growth of F hybrids was inferior to the average of the parent species across both habitats, albeit not significantly. The contrast between laboratory and field results supports the hypothesis that mechanisms of F hybrid fitness in the wild are primarily ecological and do not result from intrinsic genetic incompatibilities. Direct selection on hybrids contributes to the maintenance of sympatric stickleback species and may have played an important role in their origin.

The core assumptions of critical habitat designation are a positive relationship between habitat and population size and that a minimum habitat area is required to meet a recovery target. Effects of habitat on population limitation scale from (i) effects on performance of individuals (growth, survival, fecundity) within a life history stage, to (ii) limitation of populations by habitats associated with specific life history stages, and (iii) larger-scale habitat structure required for metapopulation persistence. The minimum subset of habitats required to achieve a recovery target will depend on the extent, quality, and spatial configuration of habitats available to sequential life history stages. Although populations may be limited by available habitat for a single life history stage, altering habitat quality for subsequent stages will also affect individual survival and population size, providing multiple leverage points within a life history for habitat management to achieve recovery targets. When habitat-explicit demographic data are lacking, consequences of uncertainty in critical habitat assessment need to be explicit, and research should focus on identifying habitats most likely to be limiting based on species biology. Résumé :Les présuppositions fondamentales qui sous-tendent la désignation des habitats critiques sont l'existence d'une relation positive entre l'habitat et la taille de la population et la nécessité d'un habitat minimal afin de pouvoir atteindre un objectif de récupération. À différentes échelles, les effets de l'habitat sur la limitation des populations vont (i) d'effets sur la performance individuelle (croissance, survie, fécondité) pendant une étape particulière du cycle, à (ii) la limitation des populations par des habitats spécifiques à certaines étapes du cycle et finalement à (iii) la néces-sité d'une structure d'habitat à plus grande échelle pour la persistance de la métapopulation. Le sous-ensemble minimal d'habitats requis pour atteindre un objectif de récupération va dépendre de l'étendue, de la qualité et de la configuration spatiale des habitats disponibles aux étapes successives du cycle. Bien que les populations puissent être limitées par la disponibilité d'habitats à une seule étape de leur cycle, une modification de la qualité de l'habitat pour les éta-pes subséquentes affectera aussi la survie individuelle et la taille de la population, ce qui fournit de multiples points d'intervention au cours d'un cycle biologique pour atteindre les objectifs de récupération par l'aménagement des habitats. Lorsqu'il n'existe pas de données démographiques spécifiques aux habitats, il faut que les conséquences de l'incertitude dans l'évaluation des habitats critiques soit explicite; la recherche devrait alors se concentrer sur l'identification des habitats qui sont les plus susceptibles d'être limitants d'après la biologie de l'espèce.[Traduit par la Rédaction] Rosenfeld and Hatfield 698

This study explored the genetic basis of phenotypic differences between two sympatric species of ecologically and morphologically divergent sticklebacks (Gasterosteus aculeatus complex). The aim was to understand how many loci determine the differences and to what extent the differences are due to additive or nonadditive gene action. I reared the two parental species, F1 and F2 hybrids, and both backcrosses in the laboratory and measured the following quantitative characters: gill raker number and length (both involved in feeding), lateral plate number and pelvic spine length (both involved in predator defense), and growth (a fitness component). I then applied joint-scaling regression models to estimate composite additive, dominance and epistatic effects, and their contribution to divergence of parental lines. A simple additive model was sufficient for gill raker number and growth; additive and dominance effects contributed significantly to divergence in plate number and pelvic spine length; and additive, dominance, and epistatic effects contributed significantly to divergence in gill raker length. Wright's estimator for the number of loci for the four morphological characters ranged from 1 to 50. My results suggest that adaptive divergence between limnetic and benthic sticklebacks has taken place through a variety of genetic mechanisms specific to different traits. Though interspecific hybrids are completely fertile and viable in the laboratory, they are selected against in the wild. The pattern of inheritance for the traits examined here directly influences how well hybrids can exploit the two major resource environments in the wild.