Explain how these two species fit into a general theory of sexual dimorphism and why it has evolved.解释这两个物种如何适应性别上同种二形性的普遍理论和它演化的原因。
Pick two related species, one which exhibits sexual dimorphism and one which does not (e.g., robins and blue jays).选两个有亲缘关系的物种,一物种展现出同种异形，另一个则无(例如,知更鸟和冠蓝鸦)。