Final answer:
An allele that has reached a frequency of 1 is said to have reached fixation. This occurs as a result of evolutionary processes, such as natural selection, and is explained in part by the Hardy-Weinberg Principle in terms of allele frequencies in populations.
Step-by-step explanation:
When an allele has reached a frequency of 1, it is said to have reached fixation. Fixation in a population means that every individual in the population carries that allele, effectively making it the only version of the gene present. This can happen through various evolutionary forces, such as natural selection, genetic drift, mutation, and gene flow, with the overall process being part of microevolutionary shifts within a population. The Hardy-Weinberg Principle helps to understand this concept by providing a baseline expectation for allele frequencies in a non-evolving population to which we can compare evolving populations. It states that if no evolutionary forces are acting upon a population, allele frequencies will remain constant across generations. However, since these forces are usually in play, populations are often evolving, thus causing allele frequencies to change.
To observe a new trait appearing and reaching a steady, high frequency within a population, the allele must code for a favorable adaptation, and it may arise through gene mutations or the process of evolution. If, over time, the trait proves to be advantageous for survival or reproduction, its frequency will increase in the population due to natural selection.