Dispersal: An Ecological Process

Dispersal is an ecological process involved with movement of individual or multiple individuals beyond their previous distribution limits, with chance of gene flow (1).On the basis of dispersal method, an organism can be ‘pagile’ that is distributed passively by dispersal agent like wind, water or another animal that is ‘vagile’, capable of active dispersal (2).It has been hypothesized as a strategy to increase fitness in a heterogeneous landscape by change of habitat. Dispersing individuals can have a selective advantage if the benefits of moving to a new place exceed the energy expenditure of movement and successful settlement at a new habitat. Environmental stochasticity (e.g. weather, species interaction) and local population conditions

Considering this, a set laboratory populations of Drosophila melanogaster had been selected for higher dispersal (6). In absence of food or water the selected flies showed greater dispersal propensity compared to their matched control populations (6).Taking these flies, the objective of this study is to check if dispersal propensity in the laboratory selected population of fruit flies is greater even in presence of food and water, Dispersal propensity is the measure of number of flies leaving the source divided by total number of flies kept in source or native habitat. Providing food avoids crowding, makes the source a no stress habitat for flies. If the selected flies migrate more here, there would be no other reason for them to do so other than their inherent tendency, called propensity, to spread their gene or to expand their population to different parts of its fundamental niche, an organism would have to evolve willingness to move about (4).Food (banana-jaggery medium) is also the egg laying surface, it may be the case where individuals of one sex (female) would stay to their natal site (or group) to lay egg while individuals of the other sex (male) are prone to disperse.

The objective was to find a significant trade-off of mating latency and dispersal capacity, if any. Dispersal involves movement, which requires energy. Energy resources of an organism are allocated to different tasks and the allocation would change with change in environment. Dispersal, therefore involves co-evolution of various other life-history traits (7).An organism is likely to adapt different strategies to alleviate dispersal related risk and stress. In some species of butterflies (9) and spider mites (10) the dispersers choose to mate at a natal habitat before dispersion. As predispersal copulation is likely to reduce chances of prereproductive death in both males and females (8).On the other hand some dispersers pool in all their resources in migration and prefer mating at a destination habitat (11) In that case, dispersive and non-dispersive population is expected to evolve with a difference in mating latency time. Mating latency defined as the time taken by a pair of virgin flies to initiate mating. In previous studies it has been found that mated flies leave the source (natal habitat) at a greater rate than virgin flies (12). Virginity, therefore, can be expected to be maintained for a shorter duration of time in selection regime. If this be established in present study, several other mating related behaviors could be assayed in migrating