Studies of local adaptation provide important insights into the power of natural selection relative to gene flow and other evolutionary forces. They are a paradigm for testing evolutionary hypotheses about traits favoured by particular environmental factors. This paper is an attempt to summarize the conceptual framework for local adaptation studies. We first review theoretical work relevant for local adaptation. Then we discuss reciprocal transplant and common garden experiments designed to detect local adaptation in the pattern of deme · habitat interaction for fitness. Finally, we review research questions and approaches to studying the processes of local adaptation ‐ divergent natural selection, dispersal and gene flow, and other processes affecting adaptive differentiation of local demes. We advocate multifaceted approaches to the study of local adaptation, and stress the need for experiments explicitly addressing hypotheses about the role of particular ecological and genetic factors that promote or hinder local adaptation. Experimental evolution of replicated populations in controlled spatially heterogeneous environments allow direct tests of such hypotheses, and thus would be a valuable way to complement research on natural populations.

/pdf/conceptual-issues-in-local-adaptation-48foi7473m.pdf

Conceptual issues in local adaptation

Whether inbreeding affects the demography and persistence of natural populations has been questioned. However, new pedigree data from field populations and molecular and analytical tools for tracing patterns of relationship and inbreeding have now enhanced our ability to detect inbreeding depression within and among wild populations. This work reveals that levels of inbreeding depression vary across taxa, populations and environments, but are usually substantial enough to affect both individual and population performance. Data from bird and mammal populations suggest that inbreeding depression often significantly affects birth weight, survival, reproduction and resistance to disease, predation and environmental stress. Plant studies, based mostly on comparing populations that differ in size or levels of genetic variation, also reveal significant inbreeding effects on seed set, germination, survival and resistance to stress. Data from butterflies, birds and plants demonstrate that populations with reduced genetic diversity often experience reduced growth and increased extinction rates. Crosses between such populations often result in heterosis. Such a genetic rescue effect might reflect the masking of fixed deleterious mutations. Thus, it might be necessary to retain gene flow among increasingly fragmented habitat patches to sustain populations that are sensitive to inbreeding.

Inbreeding effects in wild populations.

The ecological hypothesis of speciation is that reproductive isolation evolves ultimately as a consequence of divergent natural selection on traits between environments. Ecological speciation is general and might occur in allopatry or sympatry, involve many agents of natural selection, and result from a combination of adaptive processes. The main difficulty of the ecological hypothesis has been the scarcity of examples from nature, but several potential cases have recently emerged. I review the mechanisms that give rise to new species by divergent selection, compare ecological speciation with its alternatives, summarize recent tests in nature, and highlight areas requiring research.

/pdf/ecology-and-the-origin-of-species-38r7yix3b6.pdf

Ecology and the origin of species

Ecological speciation: Ecological speciation

The long history of reciprocal transplant studies testing the hypothesis of local adaptation has shown that populations are often adapted to their local environments. Yet many studies have not demonstrated local adaptation, suggesting that sometimes native populations are no better adapted than are genotypes from foreign environments. Local adaptation may also lead to trade‐offs, in which adaptation to one environment comes at a cost of adaptation to another environment. I conducted a survey of published studies of local adaptation to quantify its frequency and magnitude and the costs associated with local adaptation. I also quantified the relationship between local adaptation and environmental differences and the relationship between local adaptation and phenotypic divergence. The overall frequency of local adaptation was 0.71, and the magnitude of the native population advantage in relative fitness was 45%. Divergence between home site environments was positively associated with the magnitude ...

https://www.webpages.uidaho.edu/~snuismer/Nuismer_Lab/548_readings_files/Hereford%202009.pdf

A quantitative survey of local adaptation and fitness trade-offs.

"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 F1 and F2 hybrids (backcrosses may show some breakdown). We transplanted F1 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 F1 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 F1 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.

Ecological speciation in sticklebacks: environment-dependent hybrid fitness

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.

Local drift load and the heterosis of interconnected populations.

In this study we assessed whether sexual selection against hybrids contributes to reproductive isolation between two sympatric stickleback species. The species are recently diverged and possibly in the final stages of speciation. Our aim was to find whether mating discrimination of the parental species selects against F, hybrids, and what conditions are necessary for such sexual selection to operate. We used conservative no-choice laboratory trials with reproductively naive, lab-reared fish to measure female mating preferences. Females exhibited ranked preferences, preferring in order: conspecific, hybrid, then heterospecific males. However, intermediate attractiveness does not necessarily imply selection against hybrids: two-way ANOVAs suggested that limnetic, benthic, and hybrid males were statistically equivalent when averaged across females. Thus, this experiment found no evidence for a hybrid mating disadvantage. Our interpretation is that if sexual selection against hybrids is present in the wild, then some factor that biases encounter rates between hybrids and parental species (e.g., habitat selection) is necessary to produce it.

A test for sexual selection on hybrids of two sympatric sticklebacks

Identification of critical habitat is a key step in conservation and recovery of endangered and threatened fresh- water fish. Critical habitat under Canadian and US legislation may include habitat that is not directly used by listed fish, provided it is necessary for species conservation or recovery. Riparian habitat meets biological criteria for critical habitat because riparian zones are integral to aquatic ecosystem functions of importance to many fish species and other organisms. These functions include provision of shade for temperature-sensitive species, control of channel complexity and sediment inputs through bank stabilization, input of large wood and allochthonous energy sources, and filtering of nutrients and tox- ins from adjacent land. In response to decades of stream-riparian research, widespread implementation of regulations to protect riparian zones in most developed countries represent a de facto consensus that riparian buffers are essential for aquatic ecosystem health and the maintenance of populations of fish and other species. Consistent with widespread riparian regulations deemed necessary to protect not-at-risk species, riparian habitat adjacent to a body of water containing a listed freshwater species should be considered biologically critical unless the habitat requirements of individual taxa are demon- strated to be insensitive to the ecological functions associated with riparian habitat. Resume´ : L'identification de l'habitat critique est une etape essentielle dans la conservation et la recuperation des poissons d'eau douce menaces et en voie de disparation. Dans les legislations canadienne et americaine, l'habitat critique peut in- clure des milieux qui ne sont pas directement utilises par les poissons concernes, ala condition qu'ils soient necessaires pour la conservation ou la recuperation de ces especes. Les habitats riverains possedent les criteres biologiques d'habitats critiques parce que les zones riveraines sont necessaires pour assurer des fonctions de l'ecosysteme aquatique d'importance pour plusieurs especes de poissons et pour d'autres organismes. Ces fonctions incluent la production d'ombre pour les es- peces sensibles a la temperature, le controle de la complexitedu chenal et des apports de sediments par la stabilisation des rives, l'apport de sources d'energie allochtones et de debris ligneux de grande taille et la filtration des nutriments et des toxines provenant des terres adjacentes. Ala suite de decennies de recherche sur la relation entre les cours d'eau et la zone riveraine, la mise en application tres commune dans la plupart des pays developpesd e reglements pour proteger les zones riveraines represente un consensus de facto reconnaissant que les zones tampons riveraines sont essentielles a la santedes ecosystemes aquatiques et le maintien des populations de poissons et d'autres especes. En accord avec les regle- ments largement repandus sur les zones riveraines jugees necessaires pour la protection des especes non vulnerables, tout habitat riverain adjacent a un milieu aquatique contenant une espece d'eau douce figurant sur la liste des especes en peril devrait etre considerecomme biologiquement critique, amoins qu'on demontre que les besoins d'habitat de ce taxon parti- culier sont insensibles aux fonctions ecologiques associees al'habitat riverain. (Traduit par la Redaction)

/pdf/do-riparian-zones-qualify-as-critical-habitat-for-endangered-5dq1v3gb6u.pdf

Do riparian zones qualify as critical habitat for endangered freshwater fishes

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. © 2013 The Linnean Society of London, Biological Journal of the Linnean Society, 2013, 110, 466‐476.

https://www.zoology.ubc.ca/~schluter/reprints/southcott%20et%20al%202013%20bjls%20-%20weak%20habitat%20isolation.pdf

Weak habitat isolation in a threespine stickleback (Gasterosteus spp.) species pair

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