Evidence for variations in the morphometric traits between two sibling species of Drosophila: D. ananassae and D. pallidosa

Abstract

Darwinian theory of evolution states that, evolution occurs through the natural selection. Therefore, demonstration of natural selection in nature is the central aim of many evolutionary studies and selection acts primarily at the phenotypic level because it is well known that phenotypic traits are the primary target of natural selection. While keeping this in view, we have studied certain morphometric traits in the sibling species pair, D. ananassae and D. pallidosa to test intra- and interspecific variations. The traits studied are wing length, thorax length, ratio of wing length and thorax length, sternopleural bristle number, ovariole number and sex-comb tooth number. In females of D. ananassae, significant strain differences were found for all the traits except ovariole number. In males, significant strain differences were found for all the traits. On the other hand, in D. pallidosa, significant strain differences were found for all the traits in both, males and females. The values of all the morphometric traits were significantly higher in females of both the species in comparison to males. The values of all the morphometric traits were higher in D. ananassae. However, the phenotypic variability, expressed in terms of coefficient of variation, was higher in D. pallidosa. Except for ratio of wing length and thorax length, CV was higher in the case of females in comparison to males. Size related traits are least variable while bristle numbers and reproductive traits are most variable. Except few, most of the traits are positively correlated with each other in both the species. Intra- and interspecific variations were found with respect to different morphometric traits. Although sibling species have been defined as morphologically identical, our results show that sibling species may show variations in certain morphometric traits and these quantitative differences in the morphometric traits act as discriminant marker between these sibling species in the lack of any qualitative differences.