Bell and Collins 2008

Bell G, Collins S. 2008. Adaptation, extinction and global change. Evolutionary Applications 1: 3-16.

These authors review and synthesize the current theory surrounding evolutionary responses to gradual environmental changes, especially increasing atmospheric carbon dioxide concentrations. In this context, these authors argue that the primary task of evolutionary biologists in the early 21st century is to predict adaptive evolution (or extinction) of populations and species in a gradually changing environment.

Populations are assumed to have high fitness, based on historical events. In a stable environment, beneficial alleles will accumulate, thus environmental changes are likely to be changes to the worse. Additionally, populations that do not adapt will suffer gradually declining mean fitness as pathogens and predators should themselves be selected for the most common target genotype.

Environmental variance increases with increasing time scales, that is, events occurring more distantly in time will be more different from each other. This applies to environmental conditions as much as anything else. The major implication of this in the context of this paper is that lineages surviving over long time periods will experience increasingly variable conditions, which also means generally deteriorating conditions if populations start well-adapted.

Many previous studies have found either strong selection, and / or high heritability. This apparent contradiction can be explained by high variability in the direction and magnitude of selection, suggesting that environmental variability is ubiquitous.

Populations can respond to changing conditions in four ways: phenotypic plasticity, dispersal and migration, adaptation, or extinction. These are arranged in order from short time scales to long.

The current pace of global environmental conditions change is probably higher than in most or all previous episodes. Historical levels of CO2 were about 280 ppm (parts per million); currently they are about 380 ppm, with some projections as high as 1000 ppm within the next 100 years.

These authors analysed the effects of the severity and frequency of environmental change (always change for the worse). They found that frequency is much more important to the probability of adaptation or “evolutionary rescue” than is severity. Frequent changes in conditions is likely to lead to extinction because selection does not have enough time to fix beneficial alleles, such that some fraction of the population is not well adapted to start with when the next change hits. In contrast, rare but severe changes lead to strong selection, and expose a long series of potentially beneficial alleles, increasing the mutation supply rate. The mutation supply rate is the critical parameter in determining adaptation or extinction, specifically the fraction of the supply of mutations that are beneficial and can rescue a declining population, i.e. raise a genotype’s rate of growth from negative to positive. Even rapid environmental change can be adapted to if the (rescue) mutation supply is high enough.

A survey of both simulations and studies of natural and laboratory populations revealed several cases of adaptation to frequent or continuous environmental change, and several cases of failure to adapt, even in species with very large population sizes. This indicates primarily that the rate of rescue mutations is unknown, despite being the single most important variable in predicting the evolutionary fates of populations under global climate change. However, these authors argue that a rescue mutation rate of approximately one per generation, regardless of population size, is probably close to the critical value, but qualify this estimate by stating they intend it as a first guess, and stimulus for more exact research in future.

The current driver of global environmental change is increasing CO2 concentrations in the atmosphere. Unlike most other agents of environmental change, CO2 is not in itself likely to represent a lethal stress to organisms, rather the effects will be either direct and positive (increased photosynthesis), or indirect via either temperature and climate changes or via biotic interactions such as competition.

Experiments and simulations by these and other authors have suggested that the short-term physiological responses of phytoplankton to increased CO2 will be opposite in direction to long-term evolutionary responses; this is at least partly because phytoplankton appear to be not limited by CO2 for growth. More specifically, the CO2-importing pump, which is tightly regulated by CO2 concentrations, will become less efficient, removing much of the productivity gain that would otherwise be predicted. Evidence that this will lead to community-level effects such as succession is currently equivocal, and based on short-term studies or idiosyncratic species compositions.

Our current understanding of evolution and ecology is based on populations subjected to a sudden shift from one stable environment to another, and usually a shift to increased stress or scarcity. In contrast, current global change is gradual and continuous, and is a case of nutrient enrichment.

These authors conclude with three main points. First, rescue mutations are critical to the phenomenon of evolutionary rescue and adaptation, but their rate is almost totally unknown. Second, the evolutionary response to increasing atmospheric CO2 concentrations is likely to be reduced efficiency of photosynthesis, rather than increased productivity. Third, these authors urge a general increase in studies of the evolutionary effects of global change, at rates of environmental change between the very fast rates typical of laboratory experimental evolution, and the very slow rates typical of most of the Earth’s history.