Garduno Paz, Monica Vanessa
The origin of alternative phenotypes in fishes.
PhD thesis, University of Glasgow.
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A basic aim of evolutionary biology is to explain the enormous diversity among animal and plant species. But also within species there is often large genetic and phenotypic variation, and such variation is necessary for evolution to create new reproductively isolated species. The present thesis is directed to explain differentiation within populations highlighting and discussing the significance of phenotypic plasticity as an evolutionary process that leads to the expression of alternative phenotypes within a species. Such phenotypic expressions are particularly interesting, because the process by which new species are formed typically involves a temporary stage within the splitting species, that is, different heritable and distinct types that coexist within the same population. Such phenotypes may be raw material for full species formation, and the study of alternative-phenotype species should therefore be particularly worthwhile in speciation research. When alternative phenotypes are not entirely genetic they may arise as a result of developmental plasticity, when organisms develop in accordance with local abiotic and biotic conditions. Subject to developmental plasticity, alternative phenotypes, take different developmental routes depending on the local selection pressures, or depending on the environmental conditions experienced during development. Here, laboratory experiments showed that three-spined sticklebacks exhibit alternative phenotypes as a plastic response to physical environment and diet, demonstrating and supporting the idea that environmental inputs modulate the expression of traits through phenotypic plasticity during ontogeny. When, morphological differences arise, discrete morphological characteristics are originated and may be reinforced by the continuous presence of same environmental conditions. Here is demonstrated that these discrete morphological characteristics lead the individuals to specialise on specific prey or habitat types. Moreover, it is showed that plasticity may also play a role in the final stages of species formation, when reproductive isolation completes the speciation process. It is shown that diet-induced morphology has an important influence in mate preferences representing a strong potential to generate reproductive isolation via assortative mating, and this mate preferences may be highly efficient to maintaining isolation, thus the hypothesis of ecological speciation is supported. Finally, in this study, two alternative-phenotype lakes are described. It is suggested that the origin of the segregated alternative phenotypes in both lakes is a consequence of ecological traits divergences; however in one of the lakes the alternative phenotypes arose from a founder population, meanwhile in the second lake the alternative phenotypes may arose by the ecological adaptation of the forms in allopatry.
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