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ABSTRACT: Growing knowledge of the molecular basis of adaptation in wild populations is expanding the study of natural selection. We summarize ongoing efforts to infer three aspects of natural selection--mechanism, form and history--from the genetics of adaptive evolution in threespine stickleback that colonized freshwater after the last ice age. We tested a mechanism of selection for reduced bony armour in freshwater by tracking genotype and allele frequency changes at an underlying major locus (Ectodysplasin) in transplanted stickleback populations. We inferred disruptive selection on genotypes at the same locus in a population polymorphic for bony armour. Finally, we compared the distribution of phenotypic effect sizes of genes underlying changes in body shape with that predicted by models of adaptive peak shifts following colonization of freshwater. Studies of the effects of selection on genes complement efforts to identify the molecular basis of adaptive differences, and improve our understanding of phenotypic evolution.
Philosophical Transactions of The Royal Society B Biological Sciences 08/2010; 365(1552):2479-86. · 6.40 Impact Factor
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ABSTRACT: Divergence in habitat use among closely related species is a common characteristic of adaptive radiations. Large differences in the size structure of prey between habitats could strengthen disruptive selection on generalist predators and lead to a divergence in trophic position among species in an adaptive radiation. Using threespine stickleback (Gasterosteus aculeatus) in freshwater lakes as a model system, we examined whether divergence in habitat use coincides with shifts in trophic position. We examined the habitat use and trophic position of individual sticklebacks from divergent lake environments that have only one stickleback species (allopatric lakes) and from lakes that have a pair of benthic and limnetic stickleback species (sympatric lakes). In two sympatric lakes, the limnetic species had a higher trophic position than the benthic species, and in both allopatric and sympatric lakes, sticklebacks specializing on pelagic prey had a higher trophic position for a given size than sticklebacks specializing on benthic prey. Furthermore, the trophic position of pelagic specialists was correlated with individual variation in their gill raker length. Our results indicate that gill raker length is an important trait that underlies differentiation in both habitat use and trophic position among stickleback species, populations, and individuals.
Ecology 04/2010; 91(4):1025-34. · 4.85 Impact Factor
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ABSTRACT: Parasites can strongly affect the evolution of their hosts, but their effects on host diversification are less clear. In theory, contrasting parasite communities in different foraging habitats could generate divergent selection on hosts and promote ecological speciation. Immune systems are costly to maintain, adaptable, and an important component of individual fitness. As a result, immune system genes, such as those of the Major Histocompatibility Complex (MHC), can change rapidly in response to parasite-mediated selection. In threespine stickleback (Gasterosteus aculeatus), as well as in other vertebrates, MHC genes have been linked with female mating preference, suggesting that divergent selection acting on MHC genes might influence speciation. Here, we examined genetic variation at MHC Class II loci of sticklebacks from two lakes with a limnetic and benthic species pair, and two lakes with a single species. In both lakes with species pairs, limnetics and benthics differed in their composition of MHC alleles, and limnetics had fewer MHC alleles per individual than benthics. Similar to the limnetics, the allopatric population with a pelagic phenotype had few MHC alleles per individual, suggesting a correlation between MHC genotype and foraging habitat. Using a simulation model we show that the diversity and composition of MHC alleles in a sympatric species pair depends on the amount of assortative mating and on the strength of parasite-mediated selection in adjacent foraging habitats. Our results indicate parallel divergence in the number of MHC alleles between sympatric stickleback species, possibly resulting from the contrasting parasite communities in littoral and pelagic habitats of lakes.
PLoS ONE 01/2010; 5(6):e10948. · 4.09 Impact Factor
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Kerry B Marchinko
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ABSTRACT: Predator-driven divergent selection may cause differentiation in defensive armor in threespine stickleback: (1) predatory fish and birds favor robust armor, whereas (2) predaceous aquatic insects favor armor reduction. Although (1) is well established, no direct experimental evidence exists for (2). I examined the phenotypic and genetic consequences of insect predation using F(2) families from crosses between freshwater and marine stickleback populations. I measured selection on body size, and size-adjusted spine (dorsal and pelvic) and pelvic girdle length, by splitting juvenile F(2) families between control and insect predation treatments, set in pond enclosures. I also examined the effect of insect predation on Ectodysplasin (Eda), a gene physically linked to quantitative trait loci for lateral plate number, spine length, and body shape. Insect predation resulted in: (1) significant selection for larger juvenile size, and shorter dorsal spine and pelvic girdle length, (2) higher mortality of individuals missing the pelvic girdle, and (3) selection in favor of the low armor Eda allele. Predatory insects favor less stickleback armor, likely contributing to the widespread reduction of armor in freshwater populations. Because size strongly influences mate choice, predator-driven divergent selection on size may play a substantial role in byproduct reproductive isolation and speciation in threespine stickleback.
Evolution 10/2008; 63(1):127-38. · 5.15 Impact Factor
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Kerry B Marchinko
Biological Bulletin 09/2007; 213(1):12-5. · 1.70 Impact Factor
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ABSTRACT: Recent work has revealed the molecular mechanisms governing one of the most dramatic examples of parallel evolution in nature: the repeated loss of lateral plate armor in freshwater populations of threespine stickleback. Yet, the ecological mechanisms responsible for armor loss remain unclear. Using a balanced experimental design, we examined Heuts' (1947) hypothesis that selection due to differences in salinity indirectly drive the reduction of lateral plate armor in fresh water while maintaining armor in the sea. We measured two fitness-related traits, hatching success and juvenile growth rate, in offspring of reduced (low and partial) and complete lateral plate morphs from two polymorphic populations when raised in either fresh water or salt water. In contrast to Heuts' results, there was little difference among morphs in hatching success. However, salinity strongly influenced juvenile growth: offspring of reduced lateral plate morphs grew substantially faster (up to 65%) than offspring of completely plated morphs in fresh water, but there was little difference in salt water. We suggest that the parallel loss of lateral plates in fresh water has arisen through a correlated response to selection for faster growth during lateral plate development, but the effect of salinity on hatching success and juvenile growth rate cannot explain the predominance of completely plated morphs in marine populations.
Evolution 06/2007; 61(5):1084-90. · 5.15 Impact Factor
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Kerry B Marchinko
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ABSTRACT: The precise dependence of barnacle leg form on flow suggests the wave-swept environment imposes strong selection on suspension feeding limbs. I conducted three experiments to determine the mechanism, age dependence, and response time of cirrus variation in the acorn barnacle Balanus glandula. (1) To test whether cirrus variation arises via genetic or environmental mechanisms, I transplanted juvenile barnacles from one wave-exposed and one protected population into high and low flow conditions. Both populations exhibited similar abilities to modify cirri in response to experimental velocities: transplanted barnacles grew legs up to 84% longer in low flow. A small (up to 24%), but significant difference between source populations suggested slight genetic divergence in leg form. (2) Because flow is heterogeneous over space and time, I tested whether cirrus plasticity was limited to juveniles by transplanting both juveniles and adults from exposed and protected shores into quiet water. Remarkably, both juveniles and adults from the wave-exposed population produced legs over 100% longer than the original population, whereas protected barnacles remained unchanged. (3) A third transplant of adults into quiet water demonstrated that wave-exposed B. glandula modified cirrus form very quickly-within 18 days, or one to two molts. Results from these experiments suggest that variation in cirrus form is largely environmentally induced, but genetic differences may account for some variation observed among field populations; spatial and temporal flow heterogeneity appear to have selected for extreme flexibility of feeding form throughout a barnacle's life; and flow heterogeneity in the wave-swept environment appears to have selected for rapid ecophenotypic responses in the form of feeding structures.
Evolution 07/2003; 57(6):1281-90. · 5.15 Impact Factor
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ABSTRACT: Wave-exposure influences the form of many organisms. Curiously, the impact of flow extremes on feeding structures has received little attention. Barnacles extend feather-like legs to feed, and prior work revealed a highly precise association between leg length and water velocity in one species. To assess the generality of this flow-dependence, we quantified variation in four leg traits (ramus length, ramus diameter, seta length, and intersetal spacing) in four intertidal barnacles (Balanus glandula, Chthamalus dalli, Semibalanus cariosus, Pollicipes polymerus) over a wave-exposure gradient in the North-Eastern Pacific. All species exhibited a negative allometric relation between leg length and body mass. Proportionally longer feeding legs may permit smaller barnacles to avoid lower flow and particle flux associated with boundary layers. Although coefficients of allometry did not vary with wave-exposure, form differences among wave-exposures were substantial. Depending on the species, acorn barnacles of the same size from protected shores had feeding legs that were 37-80% longer and 18-25% thinner, and setae that were 36-50% longer and up to 25% more closely spaced, than those from exposed shores. Differences were less pronounced for the gooseneck barnacle, P. polymerus. Moreover, in situ water velocity explained an impressive percentage of overall leg-length variation: 92% in B. glandula, 67% in C. dalli, 91% in S. cariosus, and 92% in P. polymerus. Clearly, both size and shape of barnacle feeding legs respond to local flow conditions. This response appears widespread--across two orders of thoracican barnacles, Pedunculata and Sessilia, and two superfamilies of acorn barnacles (Balanoidea and Chthamaloidea)--and likely adaptive. Longer rami and setae would yield a larger feeding area in low flow, whereas shorter, stouter rami with shorter setae would be less vulnerable to damage in high flow. Finally, the proportionally most variable species was abundant in the widest range of habitats, suggesting that increased plasticity may permit a wider niche breadth.
Zoology 02/2003; 106(2):127-41. · 1.50 Impact Factor
