• Levels of Selection in a Polymorphic Species

      Korody, Marisa L. (2013-09-05)
      Phenotype is affected by many factors, including but not limited to environment, conspecifics, and genetics. Evidence of phenotypic variation is everywhere, some of which is controlled solely by environment, and others that are fixed by genetics. Genetic polymorphisms are rare, but very useful for the study of selection and genetics. These genetic polymorphisms provide a phenotypic link to the underlying genetics and are even more useful when there are associated behavioral differences. I examine multiple levels of selection that are acting upon a polymorphic passerine, the white-throated sparrow (Zonotrichia albicollis). Males and females of this species occur in two morphs, white or tan, based upon the color of their crown strips. This plumage polymorphism is absolutely correlated with a complex chromosomal rearrangement on the second largest autosome. Within this dissertation I explore how climate needs to be addressed in ecological studies to fully understand the mechanisms behind variation. I explore whether sexual selection is acting within this species and the differences between the morphs through the use of Bateman Gradients. Darwin suggested that sex ratios influence sexual selection, but what about morph ratios? I examine the frequency variation of morphs within this species. Variation in morph production may be favored by a potential tradeoff between the number of males and the number of white offspring produced in a clutch that suggests greater costs associated with producing white morph individuals. Mendelian segregation is inconsistent in this species, and transmission distortion may contribute to morph ratio variation. I show that white male sperm varies in production from 0% - 100% white sperm/individual consistent with transmission distortion. Finally, candidate gene mapping was used to identify the genes sequestered in this rearrangement that may be responsible for the polymorphism and the evolution behind the rearrangement.
    • Olfactory Mate Choice and Potential Chemical Signals of the White-Throated Sparrow (Zonotrichia albicollis)

      Sebastian, Peter (2011-03-15)
      Chemical odor signals are well documented in mammals, and yet almost nothing is known about the use of chemical odor signals in birds due to the traditional view that birds have a no sense or a poor sense of smell. Recent studies have revealed the traditional view to be unfounded, but more work is necessary to 1) expand our knowledge of avian olfaction in passerine species and 2) determine whether birds utilize chemical signals. The aim of this thesis was to 1) test for olfactory-based choice in a passerine species, and examine the chemical composition of preen oil for potential chemical signals. Results suggest that the polymorphic white-throated sparrow does choose between odors from their own bedding and odors from fresh bedding based on their unique disassortative mating, with tan males and white females choosing fresh bedding over their own and white males and tan females choosing their own bedding over fresh bedding. Additionally, a study on captive white-throated sparrows found that multiple preen oil volatile compounds were seasonally elevated during the breeding season, and thus indicate the possibility of these compounds acting as chemical signals. In wild populations, preen oil composition varied by morph-sex classes as well as by year sampled, and some compounds may even change throughout the course of the breeding season. Comparisons between wild populations and captive birds indicate that captive conditions may also alter preen oil composition.
    • Sexual Selection and Plumage in the Polymorphic White-throated Sparrow

      Rathbun, Nathan (2010-07-20)
      Feather coloration has been known to be connected with sexual selection for many years. It also provides an opportunity to study evolution, focusing on sexual selection and natural selection. Plumage is affected by both of these forces and the equilibrium is where these forces balance. The white-throated sparrow gives us a unique opportunity to observe the effects of the different strengths of these forces within a species. First, I established that there were differences in plumage characteristics between the morphs and sexes. White males had the brightest white and darkest black feathers. White females and tan males were the next brightest, with tan females having the dullest white and lightest black head stripes. Using plumage characteristics I was able to predict the morph/sex class of the bird significantly more than by chance. With the exact differences between each morph/sex class now known, I looked at the relationship between fitness and plumage. White males with higher overall contrast (brighter white, darker black) were more successful than duller white males. This was attributed to the males displaying their quality to females. Duller tan males however, were more successful than brighter tan males. With duller plumage, they may reduce predation on their nest while they are feeding their offspring. The differences in reproductive strategy changed the relative strength of natural and sexual selection between the morphs. Observing this interaction in this system will let us judge the relative strength of these forces in other systems.