The microevolutionary dynamics of a community of Drosophilids has been analysed in a western‐Mediterranean locality (Bordils, NE of Spain) by comparing the daily and seasonal activities of the nine most abundant Drosophila species: D. subobscura, D. melanogaster, D. simulans, D. immigrans, D. phalerata, D. testacea, D. cameraria, D. hydei and D. picta collected during two periods widely distant in time. The resemblance between samples obtained in different months (seasonal component) has been determined and a clear‐cut clustering of the collections according to the season is observed. A possible relationship between the environmental variables temperature and humidity, and the frequencies of the most abundant species in each seasonal cluster has been analysed by a correspondence analysis. This study allows to characterize each species according to its activity in definite intervals of temperature and humidity. Finally, daily and monthly activity patterns of the different species have been compared and found to be equivalent in the first and second periods. The results obtained allow the characterization of the species according to the environmental variables.
The present work represents a natural test with one replicate of the dynamics of a community of Drosophilids, sampled in two periods widely separated in time. The aim has been to detect a possible loss of biodiversity with its evolutionary implications. Monthly collections of Drosophilidae (Diptera) were carried out during a 2-year period (September 1979-August 1981 in Bordils (north-east of Spain). The site was sampled again during a later period to analyse the pattern of changes that had taken place in this community of Drosophilids. As a marked seasonal component of the diversity was detected during the first period as shown by the rarefaction method and the contribution of seasonal variability to the niche component, only one collection per season was performed during the second period. A significant decrease in diversity was detected. An increase in dominance of the most common species in the collections (Drosophila subobscura) was also observed, possibly due to a change in the behaviour of this species associated with an environmental change. Six species collected during the first period, were not collected during the second period. Three of these were already rare in the first period, so their absence might be attributable to a sampling effect. The absence of one of the other three species (Drosophilupicta) can most probably be attributed to a change in environmental conditions. A change in the cosmopolitan species and the unusual behaviour of the species associated with fungi were detected by analysis of the relative monthly niche breadth.
The phenotypic plasticity of some quantitative traits of two colonizing populations of Drosophila subobscura (Davis and Eureka, California) was studied. Temperature effects and the effect of rearing in the laboratory were studied. Laboratory rearing during four generations at 18ºC significantly increased the wing and tibial length. This increase was similar to that obtained when the flies were reared at 13ºC during two generations.The low temperature environment can be considered more stressful for females than for males, as shown by the increase of phenotypic variance. The two populations analyzed had great phenotypic plasticity in spite of the genetic bottleneck during the colonization event. Our study shows that keeping flies for a relatively short time in the laboratory significantly changes some quantitative traits, emphasizing the need to analyze flies immediately after collecting them in order to obtain reliable estimates for the analysis of these traits in natural populations.
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