Genetic erosion impedes adaptive responses to stressful environments

Abstract

Biodiversity is increasingly subjected to human-induced changes of the environment. To persist, populations continually have to adapt to these often stressful changes including pollution and climate change. Genetic erosion in small populations, owing to fragmentation of natural habitats, is expected to obstruct such adaptive responses: (i) genetic drift will cause a decrease in the level of adaptive genetic variation, thereby limiting evolutionary responses; (ii) inbreeding and the concomitant inbreeding depression will reduce individual fitness and, consequently, the tolerance of populations to environmental stress. Importantly, inbreeding generally increases the sensitivity of a population to stress, thereby increasing the amount of inbreeding depression. As adaptation to stress is most often accompanied by increased mortality (cost of selection), the increase in the 'cost of inbreeding' under stress is expected to severely hamper evolutionary adaptive processes. Inbreeding thus plays a pivotal role in this process and is expected to limit the probability of genetically eroded populations to successfully adapt to stressful environmental conditions. Consequently, the dynamics of small fragmented populations may differ considerably from large nonfragmented populations. The resilience of fragmented populations to changing and deteriorating environments is expected to be greatly decreased. Alleviating inbreeding depression, therefore, is crucial to ensure population persistence.

Keywords: anthropogenic stress; changing environments; cost of inbreeding; genetic drift; genetic variation; habitat fragmentation; inbreeding depression; population persistence.

Figure 1

Viability of inbred (black circles, broken lines) and noninbred (gray squares, solid lines) populations of Drosophila melanogaster at four different temperatures. For each population, the viability is scaled for each temperature relative to the highest viability observed for that population. The highest viability was set at 1 for each population (from Joubert and Bijlsma 2010).

Figure 2

Mortality during the pupal stage (fraction noneclosed pupae) at 29°C for nine independent inbred lines of Drosophila melanogaster. For each inbred line, mean pupal survival (±SE) is based on five replicates started with 100 eggs each (R. Bijlsma, unpublished data).

Figure 3

Top: Schematic diagram depicting fitness distributions for inbred individuals (left curve with mean x_i) and outbred individuals (right curve with mean x_o). The vertical lines represent the threshold values for the hard selection below which individuals do not survive for four different stress levels: benign (B), low stress (LS), intermediate stress (IS) and high stress (HS). Bottom: Amount of inbreeding depression (δ) expected at the four stress levels, B, LS, IS and HS. From the top figure survival rates were estimated to be 0.95, 0.85, 0.60 and 0.35 for the inbred individuals and 1.00, 1.00, 0.99 and 0.85 for the outbred individuals for the four respective stress levels, and these rates were used to calculate the expected level of inbreeding depression as: δ = (survival outbreds − survival inbreds)/(survival outbreds).

Figure 4

Stress sensitivity of Drosophila melanogaster populations in relation to their inbreeding coefficient for high-temperature stress and ethanol stress. Stress sensitivity is expressed as the decrease in survival probability due to the stress factor corrected for the survival probability observed under benign conditions for the same populations: stress sensitivity = (survival benign − survival stress)/(survival benign) (redrawn after Bijlsma et al. 2000).

Figure 5

Mean adaptive response (±SE) after six generations of adaptation at three stress environments, temperature stress (Temp), salt stress (Salt) and ethanol stress (Ethanol) for fragmented (M) and nonfragmented (P) populations of Drosophila melanogaster. The adaptive response for each population was calculated as the difference in viability of adapted flies minus the viability of nonadapted flies for each population at each stress (from Bakker et al. 2010).