Final answer:
For color brightness to evolve through sexual selection, male coloration must vary within the population, with females preferring to mate with males that have more vibrant colors, thus selectively passing on the more desirable coloration to offspring.
Step-by-step explanation:
In order for color brightness to evolve by sexual selection in a population, there must be variation in the male coloration to begin with. Sexual selection can drive an increase in the brightness of male coloration if females preferentially mate with males that have more vibrant colors. This sexual dimorphism evolves as a result of females choosing males with more striking colors, which are often seen as indicators of good health or genetic quality. The males with the more desired coloration will have higher reproductive success and pass on their traits to the next generation. Additionally, the process can be influenced by negative frequency-dependent selection, where less common phenotypes, such as rarer colors, have a selective advantage, leading to cyclical changes in color prevalence, as seen in side-blotched lizards.