Date of Award
Doctor of Philosophy (PhD)
Professor Rama S. Singh
An essential element in the study of evolution is the knowledge about the origin and dynamics of genetic variation within and between populations, and between species. It is for this reason that the experimental population genetic studies always center around the characterization of genetic variation in natural populations. There are two opposing hypotheses about the nature of genetic variation and its role in evolution and speciation. The balanced hypothesis of genetic structure maintains that there is a large amount of genetic variation in natural populations and this variation is maintained by natural selection. The neutral hypothesis agrees with the balance hypothesis with respect to the amount of genetic variation but disagrees with respect to its role in evolution and speciation. The neutral hypothesis assumes that most of the variants are selectively neutral and their fate is governed by balance between neutral mutation and random genetic drift.
The melanoqaster subgroup of Drosophila, comprising of eight closely related species, has provided unique materials for studies of evolution. Presently, there is an increasing amount of interest in pursuing molecular evolutionary studies with species of this subgroup. In the past D. melanogaster and its sibling species, D. simulans have been extensively studied for their genetics, cytology, ecology and behaviour. These two sibling species have also been extensively studied for gene-enzyme variation. However, all previous studies have been sampled approximately between 10-30 gene loci and also these studies have emphasized mainly enzymes; very little work has been done with non-enzymatic proteins. The experimental approach taken in this thesis was to score structurally distinguishable gene products by gel electrophoresis within species, and compare the identity of variants at homologous gene loci between species. Over a hundred gene loci representing both enzymes and non-enzymatic proteins were sampled.
The natural populations of Drosophila melanogaster and Drosophila simulans were compared for their geographic structure and genic divergence. A total of 114 gene-protein loci were studied in four mainland (from Europe and Africa) and an island (Seychelle) populations of D. simulans and the results were compared with those obtained on the same set of homologous loci in fifteen worldwide populations of D. melanogaster (Singh and Rhomberg, 1987b). The main results are as follows: (1) D. melanogaster shows a significantly higher proportion of loci polymorphic than D. simulans (52% vs 39%, P < 0.05), (2) both species have similar mean heterozygosity and mean number of alleles per locus, (3) the two species share some highly polymorphic loci but they do not share loci that show high geographic differentiation, and (4) D. simulans shows significantly less geographic differentiation than D. melanogaster. The differences in geographic differentiation between the two species are limited to loci located on the X and second chromosomes; loci on the third chromosome show similar level of geographic differentiation in both species. Variation in niche-widths and/or genetic "strategies" of adaptation appear to be the main causes of differences in the genetic structure of these two species.
The comparison of genetic divergence between species produced the following results: (1) The polymorphic loci between D. melanogaster and D. simulans are significantly correlated, i.e., if a locus is polymorphic in one species, it is likely to be polymorphic in the other species also. (2) The various chromosomes show similar proportion of unique alleles within species but differ between species; D. melanogaster shows more unique alleles than D. simulans. (3) All chromosomes show similar proportions of shared alleles and similar genetic identities between species. (4) The loci that are diverged within species, are not the one that are diverged between species, suggesting no role of population structure to the species divergence. (5) While the present estimate of mean genetic distance, D = 0.179, between D. melanogaster and D. simulans is lower than previously reported values, the proportion of loci showing complete divergence between the two species is higher (10%) than all previously reported values.
These results suggest that possibly many genes are involved in species formation but the question remains whether the generalized enzyme loci sampled in the present as well as in past studies have much relevance to the problem of reproductive isolation and speciation. It is proposed that different kinds of genes or genetic systems may underlie adaptation and speciation, and that genetic and molecular analysis of reproductive characters (e.g. male-female genetalia and reproductive behaviours) would shed more light on the nature of genetic variation for speciation.
Choudhary, Madhusudan, "Genetic Structure and Species Divergence Between Populations of Drosophila melanogaster and Its Sibling Species, Drosophila simulans" (1987). Open Access Dissertations and Theses. Paper 2127.