Final answer:
New traits in a population arise through mutations, and for them to become prevalent, the traits must offer an advantage that natural selection can act upon, leading to increased survival and reproduction of individuals with those traits. Genetic drift and gene flow can also influence trait frequencies, but natural selection is key in ensuring a trait reaches a high steady frequency.
Step-by-step explanation:
To answer the question, what must occur for a new trait to appear in a population and then reach a steady, high frequency within that population, we need to understand the mechanisms of evolution. New traits primarily appear through mutations, which are changes in the genetic material of an organism. These can occur due to errors in DNA replication or due to exposure to mutagens such as UV radiation. However, for a trait to become common in a population, it must confer some advantageous or favorable characteristic that natural selection can act upon. Over generations, individuals with the advantageous trait are more likely to survive and reproduce, spreading the trait through the population.
On the other hand, genetic drift, the random fluctuations in allele frequencies, can also lead to changes in trait prevalence, especially in small populations. Gene flow, which is the transfer of alleles from one population to another, can introduce new traits. Still, without natural selection favoring these traits, they may not reach high frequency. To sum up, while mutation is the source of new traits, it is the process of natural selection that drives these traits to become common within a population. This conclusion aligns with the scientific understanding of evolutionary theory.