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 expected to decline due to the accumulation of deleterious mutations. Replicate populations of flour beetles (Tribolium confusum) were maintained for 22 generations in the virtual absence of selection (random mating, favorable environment, excess of food, and mortality of only 3%). Larva-to-adult survivorship rates were similar in the stock population and selection-free populations. In contrast, starvation resistance of adult beetles from selection-free populations was significantly reduced (by more than 2% per generation). When tested in the favorable environment, beetles in one selection-free population had significantly slower development and smaller sizes of females than beetles from the stock population. Since such changes in these fitness components are usually maladaptive, they indicate possible erosion of fitness under relaxed selection at the rate of 0.1-0.2% per generation. No fitness erosion was detectable in the second selection-free population.