Adaptation, extinction and global change

Abstract

We discuss three interlinked issues: the natural pace of environmental change and adaptation, the likelihood that a population will adapt to a potentially lethal change, and adaptation to elevated CO2, the prime mover of global change. Environmental variability is governed by power laws showing that ln difference in conditions increases with ln elapsed time at a rate of 0.3-0.4. This leads to strong but fluctuating selection in many natural populations.The effect of repeated adverse change on mean fitness depends on its frequency rather than its severity. If the depression of mean fitness leads to population decline, however, severe stress may cause extinction. Evolutionary rescue from extinction requires abundant genetic variation or a high mutation supply rate, and thus a large population size. Although natural populations can sustain quite intense selection, they often fail to adapt to anthropogenic stresses such as pollution and acidification and instead become extinct.Experimental selection lines of algae show no specific adaptation to elevated CO2, but instead lose their carbon-concentrating mechanism through mutational degradation. This is likely to reduce the effectiveness of the oceanic carbon pump. Elevated CO2 is also likely to lead to changes in phytoplankton community composition, although it is not yet clear what these will be. We emphasize the importance of experimental evolution in understanding and predicting the biological response to global change. This will be one of the main tasks of evolutionary biologists in the coming decade.

Keywords: CO2; carbon cycle; environmental variability; evolutionary rescue; rapid evolution; stressful environment.

Figure 1

The effect of the frequency and severity of environmental change on adaptedness. Mutational landscape model with series of 100 alleles; environmental change simulated by shifting back the wild type one rank after a specified period E (1 or 50 time units) with weak or strong selection (Δ₁ = 0.01 or 0.1). From Bell (2008).

Figure 2

Effect of environmental change on mean fitness in relation to mutation supply rate. Based on a mutational landscape model as in Fig. 1. From Bell (2008).

Figure 3

Dynamics of evolutionary rescue. The population has N₀ = 1000, U = 1, Δ₁ = 0.25 and r_{0 =} −0.5. The lines show the flux in frequency of a fixed series of 20 alleles, of which the top three have r > 0. The diamonds are overall population size.

Figure 4

The probability of evolutionary rescue in relation to the severity of environmental deterioration and the mutation supply rate. Output of extreme-value model with 20 alleles, Δ₁ = 0.25, N₀ = 1000; plotted points each based on 100 simulations.

Figure 5

Affinity of net total carbon uptake, shown as K_0.5 in wild-type and high selection (numbered) lines acclimated to 1050 ppm CO₂ (filled bars, H) and air (white bars, A). Net total carbon uptake was calculated as bicarbonate uptake + net CO₂ uptake). Each bar represents averages ± standard error of the mean from independent duplicate or triplicate measurements. Accompanying table shows effect of changes in carbon uptake rates and population size on carbon sink calculations. From Collins et al. (2006).