Recombination Facilitates Adaptation from Standing Variation

Poon and Chao (2003) set up a population of bacteriophage φ6, of which 10% was from a population that had adapted to high temperature. They passed this mixed population through a bottleneck of either 100, 1,000, or 10,000 phage and then selected at high temperature for 20 generations. The φ6 genome is in three segments, which segregate independently in mixed infections—in effect, behaving like three genetic loci. After ten generations, one set of populations was propagated with multiple infection, allowing free recombination between the three segments. Recombination leads to faster adaptation in small populations, where random drift is expected to interfere with selection, but not in large populations (Fig. WN23.8). This fits with computer simulations of the effects of drift on three genetic loci under these experimental conditions. Moreover, analysis of the genetic basis of adaptation to high temperature showed no significant epistasis. Both this experiment and Colegrave (2002) support the hypothesis that recombination is advantageous because it breaks up associations generated by genetic drift. (Note that the patterns shown in Fig. 23.22 and Fig. WN23.8, are in opposite directions because the first experiment relies on new mutations, whereas the second depends on standing variation.)