Simplify the following text for a Grade 15 native English-speaking person.
Original: It most commonly manifests in the appearance of individuals within a group whose behavior ( and sometimes anatomy -- body shape and function ) is modified for group defense , including self-sacrifice ( altruism ) .
Simplified: It can manifest in the appearance of individuals within a group whose behavior ( and sometimes anatomy ) is modified for group defense , including self-sacrificing ( `` altruism '' ) .