Final answer:
A trait must be heritable and enhance individual fitness to undergo positive selection. It conveys an advantage in survival and reproduction, becoming more common in the population through adaptive evolution.
Step-by-step explanation:
A trait must be heritable and enhance individual fitness:
In order for a trait to undergo positive selection it must be heritable and increase individual fitness. Traits that are heritable are those that can be passed from one generation to the next, meaning they have a genetic basis. Moreover, for a trait to be positively selected, it must contribute to an individual's evolutionary fitness, which includes their ability to survive and reproduce within their environment.
This is in essence the process of adaptive evolution, where the frequency of beneficial alleles increases due to natural and sexual selection, enhancing the fitness of the population. Conditions like genetic drift, gene flow, and mutation can influence the appearance and distribution of these traits. However, it is the overall contribution to reproductive success and survival that dictates whether a trait is positively selected through the relentless process of natural selection.