The frequency of a character increases when it is:

The frequency of a character increases when it is adaptable because adaptability is a key factor in the survival and reproductive success of an organism. If a particular trait helps an organism to better survive in its environment, that character is likely to be expressed more frequently in subsequent generations. Organisms with traits that confer an advantage are more likely to pass those traits onto their offspring. For example, traits that enhance an organism's ability to find food, evade predators, or withstand environmental stresses will be selected for in the population, leading to an increase in their frequency.

The adaptability of a trait allows organisms to thrive in varying conditions, which enhances their chances of survival and reproduction. As these organisms reproduce, the adaptable traits become more common within the population, illustrating the principles of natural selection.

In contrast, a character being mutant refers to an alteration in the genetic material, which may not always provide a survival advantage. Dominance and recessiveness relate to how traits are expressed but do not inherently influence their frequency in a population without considering environmental pressures and advantages. Hence, while mutant, dominant, and recessive traits can all play roles in inheritance, it is the adaptability of a trait that fundamentally drives its increase in frequency within a population.