| The environment is now changing much faster than in recent geological time, causing increasing population extinctions. Experiments have shown that extinction can be avoided by adaptation through natural selection leading to evolutionary rescue. I first determined the response of Chlamydomonas to stressful environments by growing populations over a range of salinity. The population growth is halved at 5 g/L salt (NaCl), and 8 g/L is lethal. In this experiment, the genetic correlation between environments increases with environmental similarity. I then manipulated the genotypic diversity in experimental populations and cultured them by serial transfer at 5 g/L salt. The outcome of adaptation is not influenced by initial genetic variation. Instead, populations adapted mainly through the spread of new beneficial mutations. These results suggest that populations have a greater chance of adapting when new environments are similar to current conditions and that adaptation is sometimes dominated by the spread of new mutations, even in the presence of a substantial amount of standing genetic variation. |
