Density-dependent selection at low food levels leads to the evolution of population stability in Drosophila melanogaster even without a clear r-K trade-off

Abstract

Density-dependent selection, especially together with r-K trade-offs, has been one of the most plausible suggested mechanisms for the evolution of population stability. However, experimental support for this explanation has been both meagre and mixed. One study with Drosophila melanogaster yielded no evidence for populations adapted to chronic larval crowding having also evolved greater population stability. Another study, on D. ananassae, suggested that populations adapted to larval crowding evolved both greater constancy and persistence stability, and the data also suggested an r-K trade-off in those populations, though the evidence for the latter was not conclusive. Moreover, theoretical work suggested that density-dependent selection could result in the evolution of greater population stability, even in the absence of an r-K trade-off. Here, we show that populations of D. melanogaster, selected for adaptation to larval crowding at very low food amounts per vial, evolve enhanced constancy and persistence stability. The enhanced population stability in the crowding-adapted populations seems to have evolved through the increased equlibrium size (K) and reduced sensitivity of realized population growth rates to density (α).There was no clear evidence for reduced intrinsic population growth rate (r) in the more stable crowding-adapted populations. Our study adds to the growing evidence in support of the hypothesis that population stability can evolve in response to density-dependent selection through the evolution of certain life-history traits that are associated with higher K and less negative α. We discuss our results in the light of previous work, and suggest that a model-free framework might be of great heuristic value in understanding the evolution of population stability through changes in the density-sensitivity of life-history traits, whether or not these changes result from density-dependent selection.

Density-dependent selection at high food levels leads to the evolution of persistence but not constancy in Drosophila melanogaster populations

“…Our results essentially confirmed the insight of Dey et al (2012) that Drosophila populations adapting to chronic larval crowding at high versus low food amounts are likely to differ in whether or not they also evolve greater population stability attributes as a correlated response to density-dependent selection. In terms of demographic attributes and population stability characteristics, we found that the LCUs, adapted to larval crowding at relatively high food amounts, had evolved a different pattern of responses relative to controls than the MCUs (Pandey and Joshi 2022), which were adapted to larval crowding at very low food amounts. The LCUs did not evolve higher constancy than controls (Fig.…”

“…The MCUs exhibited elevated realized population growth rates, compared to MB controls, across a wide range to medium to high densities, spanning both below and above the equilibrium populations size (Fig. 5 in Pandey and Joshi 2022). As noted by Pandey and Joshi (2022), this kind of change cannot be accommodated within the framework of even the θ -Ricker or θ -logistic models.…”

“…Dey et al 2012, Pandey and Joshi 2022). The empirical estimates of realized population growth rates in the LCUs across densities also revealed a difference from what was seen in the case of the MCUs (Pandey and Joshi 2022). The MCUs exhibited elevated realized population growth rates, compared to MB controls, across a wide range to medium to high densities, spanning both below and above the equilibrium populations size (Fig.…”

“…We carried out the population dynamics experiment for 26 generations in a destabilizing LH food regime (L = low quantity of larval food, and H = high quantity of adult food with yeast supplement: Mueller and Huynh 1994, Sheeba and Joshi 1998). This experiment was carried out with 1 mL of larval food in the LH environment, as this food regime provides a high probability of being able to detect differences in constancy and persistence (Vaidya 2013, Pandey and Joshi 2022). At the time of initiating the population dynamics experiment, the LCU populations had undergone about 75 generations of selection and had diverged from their controls in many traits relevant to fitness under larval crowding (Sarangi 2018).…”

“…We used data from the study in this chapter, and the one described in Pandey and Joshi 2022, and transformed the estimated values of stability indices, mean population sizes and the three Ricker-based demographic attributes ( r , K and α ) into fractional deviations from control population values. For each measure from each single-vial population in the LCU and MCU population dynamics experiments, we calculated Y * ij = ( Y ij – µ j ) / µj ( i = 1…10, j = 1…4), where Y * ij was the transformed response variable, Y ij was the measure of a given attribute in the i th replicate single-vial population (in the population dynamics experiment) of the j th replicate population (from the ongoing selection experiment), and µ j was the mean value of that attribute in the j th replicate population of the control MBs, averaged over all 10 single-vial populations within that replicate.…”