Zoology - Theses
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ItemNo Preview AvailableStudies in population genetics(University of Melbourne, 1959)Population genetics and. population dynamics have formed two distinct avenues of approach to a field of study which has become increasingly important over the last thirty years. Each has proved highly susceptible to mathematical analysis and to quantitative, rather than merely descriptive, treatment. It is, however, surprising to find that relatively little has been done to unite these two aspects of population biology, while in each theoretical work has tended to outstrip the small foundation provided by the observations so far recorded in the literature. Thus Birch (1957.P.217) was led to say... "I am quite sceptical of the predictive value of such mathematical models as have to date been proposed for natural populations". More serious still is the fact that the attempt to fit slender experimental data to such models has often blinded the experimenter to the possibility that factors of importance other than those considered in his model may contribute to the observed properties of the population. In particular, there has been little attention given to the genetic control of the form and rate of population growth; the work of Buzzati-Traverso (1955) was the first major experimental analysis to produce useful results along these lines. Much of the work on Drosophila populations has been done on small breeding groups which have been assumed to have reached an equilibrium density, although it seems likely that laboratory populations of Drosophila develop in the same way as those of Lucilia (Nicholson, 1954 and earlier papers), in which the population number fluctuates quite widely about a mean, not absolute, "equilibrium" level. Further, the number of individuals in the populations has not usually been determined with accuracy, so that it has seldom been possible to study small changes in population size. Preoccupation with "competition" experiments involving oligogenic markers has led even experienced workers to ignore the importance of modifying influences, particularly polygenic systems, associated with the genetic background. In this connection Buzzati-Traverso (loc.cit.) stated: "The fact is that the change in frequency of a single gene (or chromosome) during a number of generations means that the individuals carrying its allele will produce more or fewer adult offspring in the next generation than the individuals not carrying it. The factor which is decisive for changing frequency is a "productivity differential" involving of necessity the whole genotype and not one gene alone, for the latter will have different survival values in different genetic milies. Some extreme mutants, like those mostly used in laboratory experiments, may affect specifically the productivity of its carriers to such an extent as to make the effects of the rest of the genotype and of interactions of the mutant with it insignificant. But under natural conditions the commonest ease is very likely that of 'small* mutants where the natural selection mechanism probably involves many genes at one time" Even in the case of 'extreme' mutants erroneous or unsupported conclusions have been published (see later discussion of the relative adaptive values of the alleles at the white locus of Drosophila given by Merrell & Underhill, 1956); while in other instances over-emphasis of one particular factor has often prevented a balanced statement of the factors operating in laboratory population experiments. The discovery of selective mating in Drosophila, for example, resulted in the application of mating test results to population cage results without regard to other factors which might be causing the observed gene frequency changes. The work of Nicoletti & Solima (1956) and Morpurgo & Nicoletti (1956), amongst others, has passed largely unnoticed, although these authors adduced considerable evidence to support their view that selective mating is in fact not a controlling factor in laboratory cage competitions even where it can be demonstrated to occur in mating tests of the genotypes involved. In the same way, a sharp distinction has not always been made between the overall adaptive value of a genotype in a particular environment. The differential migration rates reported by liar land & Jackson (1958) under the title �Advantage of the white eye mutant of Drosophila melanogaster over the wild type in an artificial environment" provides an instance of such confusion. The only real measure of "advantage" or "superiority" is the relative contribution of a genotype to the gene pool of the succeeding generation, but these authors mention no breeding experiments at all. It is, however, clear that they have identified one of the factors which might be of importance in determining the frequency of white relative to that of its wild type allele in populations maintained in a particular environment. The main object of the present work has been to investigate further the relation of the genetic structure of a population to its rate and type of growth. Some attention has been given to problems of the sex-ratio and to change in gene frequency; in each case the underlying mechanisms have been studied. Emphasis throughout has been placed on the determination of the relative importance of the possible component factors under conditions of intense competition such as those found in population cage experiments.
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ItemThe Australian freshwater malacostraca and their epizoic fauna( 1964)Because of their disjunctive distribution, the Parastacidae (Crustacea, Decapoda) and the Phreatoicoidea (Crustacea, Isopoda) have long been of considerable zoogeographic interest. The Phreatoicoidea are known from South Africa, India, Australia and New Zealand; the Parastacidae from Madagascar, New Guinea, Australia, New Zealand and South America. Of potentially great value, then, is a study of the epizoic fauna found closely associated with these fresh-water crustacea, and in some cases, sharing the disjunctive distribution. When new species of the peritrichous ciliate Lagenophrys were discovered on the cuticle of those crustacea, the opportunity was taken to initiate an investigation into zoogeographical implication of the distribution of a host-epizooid complex. The hosts were of established interest. Lagenophrys was of presumed interest because of low vagility during its dispersal phase, and because of the apparently high specificity of northern hemisphere members of the genus. In this thesis attention has been concentrated on the peritrichous epizooids. Although ideally a study should ultimately analyse the whole complex, the difficulties involved in such disparate taxonomies allowed of brief notes only on the other groups. Observations of the genus Temnocephala are included in the systematic section below but the other groups will be merely mentioned. Half the thesis is devoted to problems directly concerning the Parastacidae. For maximum value from the data on the epizoic fauna, precise determinations of host species were essential. In the course of investigation, it became clear that a review of Parastacidae systematics would be a prerequisite. (It will be seen that the Parastacidae form the most important group of hosts for the peritrichs investigated.) This situation arose partly from the lack of facilities at present obtaining in Australia for identification of the Parastacidae, and partly from ambiguities in the currently accepted systematics. These problems are posted in the section on Parastacidae, and their influence on problems of specificity is discussed in the final section.
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ItemFactors influencing egg survival in Aedes eggs with special reference to some Victorian species (Diptera: Culicidae)( 1962)Detailed information on the layers comprising the shell of Aedes eggs is important from an ecological point of view. The eggs in this genus are usually deposited above waterline in a variety of habitats which are subjected to period drying. They are especially adapted to those conditions and appear to withstand long periods of drought. These eggs very seldom hatch except under favourable conditions which occur when the breeding sites are inundated by rising water level during heavy rain or irrigational flooding. Thus the fact that these eggs which are primarily dependent on water for survival are able to withstand prolonged periods of desiccation is of particular significance. It would appear that this adaptation is the feature which ensures the continuance of the species in areas where such long periods of drought are common e.g. Mallee in Victoria. Thus a thorough knowledge of the chemical composition, structural morphology and development changes of the protective coverings of the shell in Aedes is a necessary pre-requisite for any ecological study of these species. As far as Aedes is concerned, little work has been accomplished in this area before. The eggs of these mosquitoes are able to remain dormant for a year or more (Christophers, 1960). During this dormancy a fully developed embryo is present within the egg. This is in contrast to the situation in grasshopper eggs, which may also remain viable for extended periods but in which the embryo is in an arrested stage of development (diapause). Because their embryonic development is complete, conditioned eggs of Aedes may hatch in a matter of minutes after sub-mergence. The term “conditioned” refers to previous exposure to environmental factors favouring a rapid and uniformly high incidence of hatching. It has been established that the removal of oxygen from the surrounding medium must occur before hatching takes place. Presumably in these high temperature regions this occurs very rapidly due to the activity of the microorganisms in the soil after precipitation. However, in addition to this need for a reduced oxygen tension during flooding, a pre-exposure to certain environmental conditions is essential to ensure an uniform and a high rate of hatching. These environmental factors which facilitate hatching vary with each species and have been collectively termed “conditioning” by Borg and Horsfall (1953) and Horsfall (1956). The failure to appreciate the conditioning process has been a serious handicap to work on the genetics and developmental physiology of species investigated for their medical importance.