The article discusses two theories in community ecology: niche-assembly theory and neutral theory. Niche-assembly theory suggests species coexist based on resource differences, while neutral theory proposes species are similar and diversity is maintained by extinction, immigration, and speciation. Neutral theory predicts specific statistical distributions for species abundance. Recent research has focused on testing assumptions, predictions, and speciation modes of neutral theory. The future goal is to combine niche-assembly theory and neutral theory to better understand species diversity.