In a small, isolated population of piranhas, one individual is born with a synonymous mutation in a gene that codes for the beta-globin protein found in hemoglobin. After five generations, 5% of the piranhas carry this mutation. What is a plausible explanation for this evolution?

In any population of any species having two or more alleles encoding for different genes, the frequency of occurrence of each allele among the individuals of a population can vary.
The frequency of one allele may be far more or far less than the frequency of other alleles.
The frequency of any particular allele is dependent on factors like random mating among individuals as well as natural selection, that tends to increase the frequency of those alleles which provide a survival advantage to the individual in a given environment.
Genetic drift can be defined as a phenomenon due to which the frequency of an allele encoding for a particular gene undergoes drastic change (increase or decrease) in a population due to random mating as well as natural selection.
In the given statement, one individual of an isolated population of piranhas underwent synonymous mutation in the beta-globin encoding gene.
A synonymous mutation can be defined as such a change in the nucleotide sequence of a gene that does not affect the sequence, structure or function of the protein encoded by it.
The increase in the frequency of the mutated form of the beta-globin gene after five generations to 5% indicates that due to the impact of random mating or natural selection or both, this has happened.