Genetic Drift in Large Populations and Coalescence

This chapter considers two circumstances in which drift can play a major evolutionary role in populations of any size, including extremely large populations. The first circumstance in which genetic drift can play an important role in both small and large populations involves the evolutionary fate of newly arisen mutations. The other major circumstance under which drift has a major evolutionary impact irrespective of population size is when the mutant alleles are neutral. The molecular clock challenged the preconceptions of many biologists about how interspecific genetic variation accumulated during the evolutionary process. The neutral theory showed that genetic drift was an important evolutionary force in all populations, regardless of size. The chapter introduces coalescent theory by modeling genetic drift in this time-backwards sense. Coalescent theory teaches us to be humble in our dating of events through intraspecific studies of molecular variation.