Final answer:
The changes observed in the prevalence of spiral galaxies at higher redshifts and their rarity in dense environments like galaxy clusters can be explained by galactic mergers.
Step-by-step explanation:
The changes observed in the prevalence of spiral galaxies at higher redshifts and their rarity in dense environments like the centers of galaxy clusters can be explained by the theory of galactic mergers and the transformation of spiral galaxies into elliptical galaxies.
Collisions and mergers between galaxies may dramatically change spiral galaxies into elliptical galaxies over billions of years. Even isolated spirals can change their appearance as they consume their gas, leading to a decrease in star formation and the gradual fading of spiral arms. Spirals eventually begin to resemble elliptical galaxies.
In dense environments like the centers of galaxy clusters, spiral galaxies are rare because mergers tend to convert spirals into ellipticals. The high concentration of giant elliptical galaxies in these regions indicates their formation through top-down and bottom-up mechanisms, which occurred early and quickly in the history of the universe.