The argument for psychological unity is that, as a sexually reproducing species, it is almost impossible for one gene to rise in relative frequency if the genes it depends on are not already nearly universal. So the all the diversity within any species at any given time consists of only one-step changes; no complex adaptations. The one exception of course is that males can have complex adaptations that females lack, and vice versa.
That argument is not right—as fig wasps demonstrate. Organisms can exhibit developmental plasticity, and show different traits under different circumstances. Humans are a good example of this: we show remarkable developmental plasticity due to neoteny. E.g. the phenotypes of the jockey and the sumo wrestler are remarkably different. The jockey is continuously on calorie restriction—and so exhibits a radically different gene expression profile in almost every tissue.
That argument is not right—as fig wasps demonstrate. Organisms can exhibit developmental plasticity, and show different traits under different circumstances. Humans are a good example of this: we show remarkable developmental plasticity due to neoteny. E.g. the phenotypes of the jockey and the sumo wrestler are remarkably different. The jockey is continuously on calorie restriction—and so exhibits a radically different gene expression profile in almost every tissue.