The human eye is a conspicuous and unique component of facial appearance. From other mammalian eyes it is set apart mainly by its unusual shape, visible white sclera, and a wide range of iris colours, which is something unique within a single species. Whether alone or in the context of the face as a whole, eye colour is an underexplored area within research into the perception of various personality traits. In this thesis, the author first reviews the physiological factors connected with eye colour and reported correlations between eye colour and different psychological and behavioural conditions. This is followed by an investigation of a possible relationship between eye colour and perceived dominance and attractiveness. The first and second study examines whether and to what extent eye colour is associated with facial morphology responsible for perceived dominance. The results are ambiguous. A cross-cultural comparison in the third study revealed that faces with blue eyes are judged as more attractive only in populations where individuals with darker eyes predominate. It is thus discussed whether this population-specific pattern is the consequence of a negative frequency-dependent selection that may have contributed to the present-day eye colour diversity. In short, the aim of this thesis was to...

La sélection fréquence-dépendante est un mécanisme d’évolution selon lequel l’aptitude d'un type varie en fonction de sa fréquence dans la population. Ce mécanisme joue un rôle important dans de nombreuses interactions autant interspécifiques (parasitisme, prédation, compétition), qu'intra-spécifiques entre les différents phénotypes d'une même espèce. La sélection fréquence-dépendante peut être positive ou négative et favoriser alors les phénotypes communs ou rares, respectivement. Elle a été mise en évidence dans le contexte du choix de partenaire chez plusieurs espèces, notamment chez certaines espèces d'insectes (ex.: demoiselles, drosophiles, cantharide de Pennsylvanie) et de poissons (ex.: guppys, xiphos), mais elle a été aussi récemment découverte chez l’humain. L'importance de la sélection fréquence-dépendante dans le choix de partenaire chez les espèces monogames reste tout de même peu explorée et cette étude vise à combler cette lacune en utilisant le diamant mandarin, un passereau monogame, comme modèle biologique. Nous avons étudié l'importance de ce mécanisme lorsqu'un trait est neutre et lorsque celui-ci constitue un indicateur de qualité. De plus, nous avons tenté de déterminer si la présence de rivales peut modifier la préférence initiale des femelles pour les phénotypes rares ou communs. / Frequency-dependant selection is an evolution mechanism in which the fitness of a type depend of its frequency in the population. This mechanism play an important role in several interspecific (parasitism, predation, competition) and intraspecific interactions within different phenotypes of a same species. Frequency-dependant selection can be positive or negative and favor, respectively, either common or rare phenotype. This selection has been found in context of mated choice of several species, especially in insects (damselfly, drosophila, soldier beetle) and fishes (guppy, swordtail) and it has even been recently discover in humans. The significance of frequency-dependent selection in mated choice of monogamous species is still less explored and this study used the zebra finches (a monogamous passerine) in order to explored this field. We studied the significance of this mechanism when a character is neutral and when it is rather a quality cue. Also, we attempt to establish if rivals' presence affect or modify the initial preference of females for common or rare phenotypes.

The European rewardless orchid Dactylorhiza sambucina commonly produces yellow- and purple-flowered individuals in frequencies that in different populations range from balanced to very unbalanced ones (we can find even monochromatic populations). I studied an effect of abiotic factors (soil pH, hydrolimits, slope and altitude of the locality), biotic factors (weight and viability of the seeds obtained from crossing between and within morphs) and the size of the populations on corolla colour of two morphs in European populations (the Czech Republic, Italy, Austria, Germany, France).