Final answer:
Most substitutions in the neutral theory of molecular evolution are fixed by genetic drift, a random process that alters allele frequencies in a population, which contrasts with the selective role of natural selection in traditional evolutionary theory.
Step-by-step explanation:
If the neutral theory of molecular evolution is correct, most substitutions are fixed by genetic drift. This is a departure from the traditional view that emphasizes natural selection as the main driver of evolutionary change. Genetic drift is a random process that can lead to changes in allele frequencies in a population, particularly in small populations.
Unlike natural selection, which selects for alleles that confer a survival or reproductive advantage, genetic drift can cause alleles to become more or less common in a population simply by chance.
Mutations introduce new genetic variation into a population, while gene flow can alter allele frequencies as individuals migrate into or out of the population. However, neutral theory posits that most of the variation within and between populations is not caused by differential survival and reproduction due to selection, but by random drift of mutational variants that are selectively neutral.
It is important to recognize that while some evolution is adaptive, much of it is random, and can lead to the fixation of neutral traits that do not necessarily provide a reproductive advantage.