Final answer:
The nonselective mechanism of evolution that is entirely random is genetic drift, which can cause random changes in allele frequencies in a population, especially in small populations due to stochastic events like the founder effect or a bottleneck event.
Step-by-step explanation:
The mechanism of evolution that is absolutely random is known as genetic drift. Unlike natural selection, which is a non-random process where advantageous traits increase in frequency, genetic drift is a stochastic (random) effect that can cause the allele frequencies in a population to change at random. It can have a much larger effect in small populations than in large ones and can lead to the loss of genetic variation within populations.
Genetic drift can occur due to random events, such as the founder effect, where a small number of individuals start a new population, or a bottleneck effect, where a large portion of a population is suddenly wiped out, leaving behind a small random subset of the original population. The randomness of these events can significantly influence allele frequencies in a population and is not influenced by allele fitness.
While mutation is also a random process that introduces new genetic variation into the gene pool, it is distinct from genetic drift, as mutation does not directly change allele frequencies. Instead, mutation serves as a source for potential change, which other evolutionary mechanisms may act upon.