The World Around Us

Phylogenic diagram showing the relationships between four subspecies of the trilobite Phacops rana, based on the  punctuated equilibria principle.

Phylogenic diagram showing the relationships between four subspecies of the trilobite Phacops rana, based on the principle of punctuated equilibria. This takes account of geographical and environmental factors and suggests periods of statis between speciation. The numbers relate to the number of columns of lenses, a distinguishing feature of the subspecies; a cross denotes final disappearance.

Credit: Based on Eldredge, N & Gould, S J (1972)