While we agree that citations of the review article of Prasad and Joshi (Prasad and Joshi, 2003) could have been done more perceptively in our recent paper (Yadav and Sharma, 2014), most other comments made by Prasad and Joshi (Prasad and Joshi, 2015) are certainly out of place.
We would like to clarify that citation of the review (Prasad and Joshi, 2003) in lieu of the original papers was done to reduce the number of citations. However, Prasad and Joshi's article does state that there is a long-standing view that ‘…Drosophila populations in the wild were subjected to directional selection for rapid development as a consequence of the larvae inhabiting ephemeral habitats like rotting fruits…’ (p. 55). Similarly, it also mentions that trait combinations under selection are constrained by trade-offs (‘…evolution of body size in Drosophila is constrained by a trade-off between adult reproductive fitness and the fitness costs of increasing larval growth rates…’ on p. 49). However, we concur that Prasad and Joshi (Prasad and Joshi, 2003) do not endorse these views, and therefore it would have been more appropriate for us to either cite the primary references or use ‘reviewed by’ or ‘cited in’ to reflect this.
We agree that Prasad and Joshi do not claim that ‘studies on…organismal fitness’, our intention was to refer to the review only for the latter part of the sentence regarding constraints. However, contrary to their claim, such views are not uncommon in the evolutionary biology literature [e.g. ‘The principle of optimality assumes that there is some measure of fitness that is maximized by natural selection’ (Roff, 1992), and ‘Life history theory posits that natural selection leads to the evolution of mechanisms that tend to allocate resources to the competing demands of growth, reproduction, and survival such that fitness is locally maximized’ (Vitzthum, 2008)].
In the fourth sentence the Introduction (Yadav and Sharma, 2014), we used the term ‘development’ to include adult stages as well (otherwise, we use the phrase pre-adult development). Hence, this is a technicality that is arguable.
Resistance to starvation and desiccation have been considered as traits that impinge upon fitness and hence are considered as fitness-related traits [‘Stress resistance characters are of pervasive importance in evolutionary physiology impinging upon a wide variety of characters, from fitness itself, to….’ (Chippindale et al., 1996)]. The cited papers assay the above two traits and infer to them as being indirectly related to fitness.
Prasad and Joshi take the citation of Sheeba et al. (Sheeba et al., 2000) completely out of the context to which it refers. It is cited in support of the results showing greater lifespan under constant darkness than in constant light or light:dark cycles, and therefore it is perfectly valid.
Indeed, total fecundity is taken as a measure of reproductive fitness. In the three instances (Abstract, Introduction, Discussion) where we suggest enhanced reproductive fitness, it is preceded by a statement that it is a relative measure (egg-output normalized by body weight).
The main question and the framework of our study are quite different both from Chippindale et al. (Chippindale et al., 2004) and that reviewed in Prasad and Joshi (Prasad and Joshi, 2003) where flies were selected for faster pre-adult development and also early reproduction under constant light. Our selection approach was designed precisely to avoid the pitfalls associated with such a regime on the evolution of circadian clocks (Yadav and Sharma, 2014, p. 581). In this context, our study is indeed novel (although nowhere did we claim novelty) and therefore any similarity in correlated adult traits between ours and previous studies is merely coincidental.
The argument that similar correlated responses to selection were seen in two very different regimes and hence would infer a lack of the role of the clock is flawed because, firstly, the clock could easily alter the ‘extent of changes’ in life history traits. Secondly it should be noted that many aspects of circadian organization are not affected by constant light (reviewed by Howlader and Sharma, 2006). Moreover, we and others have quite convincingly demonstrated circadian-clock-mediated regulation of life history traits (citations in Yadav and Sharma, 2014) including pre-adult development time, and therefore any skepticism on this matter is misplaced.
Because of constraints of space, we did not discuss the results on fecundity of control flies as it is only tangential to our study. Our control populations (maintained under constant darkness for over 200 generations) exhibit higher fecundity at later life stages, unlike others, where fecundity decreases after about 6 days. We speculate that this could be because, under constant darkness, flies shift their egg-laying peak towards the age when eggs are collected for regular maintenance.
