 This mechanism of “kin selection” can explain how worker bees evolved such apparently ‘selfless’ traits, focused on aiding the queen’s survival and reproduction. In fact, the situation is even more interesting and extreme: workers have evolved to serve the queen’s reproduction at the complete expense of their own, as they are sterile. This extreme biological altruism, however, may be explained by the same principles, with the addition of the fact that due to a genetic peculiarity of the Hymenoptera (their haplodiploidy), sisters are more closely related to each other genetically than they would be to their own  offspring.[4]  This means that natural selection will actually favor worker traits that help their mother reproduce (thus making more sisters, who are especially likely to carry copies of the gene for that same trait, making it spread), over traits in workers aimed at personal reproduction. This could explain how worker sterility evolved, as traits focused on helping the queen took precedence over personal reproduction, and it explains how even suicidal behavior could have been selected for as propagating the genes that cause it (Dawkins 1989, 174–75). (It is worth noting, however, that the theory of kin selection and inclusive fitness is highly complex and has received renewed critical scrutiny in recent years. For a recent special collection of articles on the topic see  Royal Society Open Science on Inclusive Fitness).