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dc.contributor.authorTilak Viegas, F. J.en
dc.date.accessioned2019-02-15T14:24:59Z
dc.date.available2019-02-15T14:24:59Z
dc.date.issued1971
dc.identifier.urihttp://hdl.handle.net/1842/34243
dc.description.abstracten
dc.description.abstract1. The work presented in this thesis is concerned with the study of stabilising selection for body size in D.melanogaster. 2. The method followed in this study was to create differences in body size through selection and observe the changes in fitness associated with these differences. Also, once the differences were created, we observed the effects of natural selection on the selected lines when selection was relaxed. 3. The consideration of this problem involves the choice of a suitable estimate of fitness as well as the definition of a suitable ecological system in which fitness should be measured. We decided that the best indicator of fitness available is the biomass produced per unit of time in competition with a standard competitor, in a live -yeast system. 4. The physiological changes responsible for the changes in body size may or may not affect fitness, tinder the conditions described. Given hypothetical differences in physiological parameters like feeding rate, efficiency of food conversion and critical size we were able to simulate in a computer the outcome of competition between strains differing in one or more of these parameters. This allowed us to make predictions of the changes in competitive ability and the productivity of the cultures. Therefore we were able to analyse our results in terms of a system rather than in terms of individual characters. 5. It was found that the physiological changes responsible for the changes in body size of the selected lines were probably changes in feeding rate associated with changes in the critical size. However, the "nature" of growth is affected in this case since we can detect differences in body size in flies exhibiting the same development time. This change in the "nature" of growth can be explained if we postulate an unbalance of growth in the selected lines. 6. Although the changes iñ body size obtained by selection are quite sùbstantial, it was difficult to detect a consistent change in compeitive ability measured against a standard competitor. It is suggested that this difficulty might be due to changes in the variance of the growth parameters correlated with changes in their mean. Within the range of deviations of body size from the unselected value these changes in variance would counteract the possible changes in fitness which are a consequence of selection. 7. Relaxation of selection for lines selected for large and small body size had little effect in bringing the mean fialue of the character back to the unselected level. One possible exception was verified when selection was relaxed in a population cage. Back selection had an immediate response, similar to that of forward selection. The experiments which tested the competitive ability of the back selected lines were not conclusive. Selection for short development time in the selected lines did not affect body size;,the changes in competitive ability were not well defined. Selection for fast and slow development had some response when development time was measured under pure culture conditions. Under competitive conditions the apparent advantage of the fast line disappeared but the disadvantage of the slow line persisted. The response to selection for fast and slow development was accompanied by a reduction in body size below the unselected level in the fast line and an increase in the slow line above the same level. 8. Selection for large and small body size under competitive conditions showed some response in both directions though less well marked than when selection was carried out under optimal conditions. 9. Inbreeding caused a proportionally equal decrease in body size in all the lines. This decrease was more accentuated when body size was measured under competitive conditions. Competitive ability was affected differentially in the 1st generation of inbreeding, but this difference disappeared subsequently. Viability was reduced below the non -inbred level. 10. The experiments on egg production of the selected and unselected flies grown under different conditions in the larval stage fed different amounts of food in the adult stage revealed a superiority of the unselected over the selected lines. 11. The crosses between the Pacific and the Kaduna populations showed no breakdown or improvement in competitive ability in the Fl or the F2, suggesting that the genes controlling the growth ability which is correlated with competitive ability are the same in the two populations.en
dc.publisherThe University of Edinburghen
dc.relation.ispartofAnnexe Thesis Digitisation Project 2019 Block 22en
dc.relation.isreferencedbyen
dc.titleStabilising selection in populations of drosophilaen
dc.typeThesis or Dissertationen
dc.type.qualificationlevelDoctoralen
dc.type.qualificationnamePhD Doctor of Philosophyen


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