Why do apples have skins instead of shells?

Apples have skins instead of shells primarily due to their evolutionary adaptation. The skin serves as a protective barrier against pathogens, pests, and environmental factors while allowing for gas exchange necessary for respiration. Additionally, the skin helps retain moisture and nutrients within the fruit, promoting its growth and development. This structure supports the fruit's role in attracting animals for seed dispersal, as the skin is often colorful and appealing.