Final answer:
Genetic drift is the microevolutionary force that primarily causes non-adaptive variation in populations, altering allele frequencies randomly rather than through selectivity. This mechanism, along with gene flow, mutation, and nonrandom mating, influences the genetic and phenotypic variation within populations without any direction towards an ideal form.
Step-by-step explanation:
The force of microevolution that causes non-adaptive variation is primarily genetic drift. Genetic drift is a random process that can lead to sudden changes in allele frequencies within a population, particularly in small populations. Unlike natural selection, which tends to increase the prevalence of advantageous traits, genetic drift can introduce or increase the frequency of alleles in a population that are not necessarily advantageous, which is referred to as non-adaptive variation. Gene flow can also contribute to non-adaptive variation, as it involves the movement of genes between populations, potentially introducing alleles that do not confer a selective advantage.
Evolution is not goal-oriented; it does not strive for an 'ideal' population. It is the cumulative result of several forces working together, affecting the population's genetic and phenotypic makeup. This includes the contributions of mutation, which can introduce new genetic variants into a population, and nonrandom mating, which affects which individuals reproduce and thus the genetic makeup of future generations.