Short Answer 
Changing mate selection criteria among female cardinals could lead to a focus on alternative traits like behavior or song quality instead of just feather color, which would increase variation in male color shades. This shift aligns with sexual selection theory, allowing a broader spectrum of colors to thrive and enhancing genetic diversity within the species.
Step 1: Changing Mate Selection Criteria
If female cardinals stop selecting partners based primarily on vibrant red coloration, it opens the door for alternative criteria in mate selection. These criteria could include factors such as behavior, song quality, or other physical attributes. By diversifying the selection process, females may prioritize traits that are not solely based on feather color, leading to shifts in mating dynamics.
Step 2: Impact on Male Color Variation
The removal of focused selection on vibrant red feathers would result in increased variation among the shades of red in male cardinals. Since males no longer need to compete prominently on color, various shades would no longer be filtered out. This could lead to a broader spectrum of colors being advantageous as mates would evaluate males on diverse characteristics.
Step 3: Understanding Sexual Selection Theory
This phenomenon relates to the theory of sexual selection, which discusses how certain traits become common because they appeal to potential mates. When the emphasis on a specific trait, like vibrant red color, diminishes, it allows a wider range of colors to thrive, enhancing the diversity within the species. Consequently, the evolutionary pressure that once favored bright colors weakens, promoting overall genetic diversity.