Final answer:
The neutral theory of evolution suggests that genetic drift is the primary cause of molecular genetic variation within a species. Allele fixation or loss, influenced by genetic drift, can ultimately lead to genetic variation, though it can also reduce variation in small populations. Mutation also contributes to this genetic variation by introducing new alleles.
Step-by-step explanation:
The neutral theory of evolution posits that most genetic variation at the molecular level is due to genetic drift, rather than natural selection. According to this theory, the majority of mutations are neutral (i.e., they do not affect an organism's fitness), and genetic drift is the primary mechanism by which these mutations become fixed or lost within a population. As a result, allele fixation or loss through genetic drift can lead to genetic variation within species.
New mutations that are neutral can accumulate, altering allele frequencies over time, particularly in small populations where genetic drift has a more pronounced effect. This mechanism provides an array of genetic differences among individuals that, while not necessarily advantageous, can contribute to the diversity of the population.
Events like the 'founder effect' and the 'bottleneck effect' illustrate how genetic drift can lead to a reduction in genetic variation, which may eventually promote new variation if the population subsequently expands. Mutation introduces new alleles into the population, further influencing allele frequencies and contributing to genetic variation. It's important to note that while genetic drift can lead to variation, it can also reduce it, especially if alleles are randomly lost in small populations.