Does fitness erode in the absence of selection? An experimental test with Tribolium

In the absence of natural selection, average fitness in the population is e xpected to decline due to the accumulation of deleterious mutations. Replic ate populations of flour beetles (Tribolium confusum) were maintained for 2 2 generations in the Virtual absence of selection (random mating, favorable environment, excess of food, and mortality of only 3%). Larva-to-adult sur vivorship rates were similar in the stock population and selection-free pop ulations. In contrast, starvation resistance of adult beetles from selectio n-free populations was significantly reduced (by more than 2% per generatio n). When tested in the favorable environment, beetles in one selection-free population had significantly slower development and smaller sizes of femal es than beetles from the stock population. Since such changes in these fitn ess components are usually maladaptive, they indicate possible erosion of f itness under relaxed selection at the rate of 0.1-0.2% per generation. No f itness erosion was detectable in the second selection-free population.