THE TREE OF LIFE: STEPHEN JAY GOULD’S CONTRIBUTIONS TO SYSTEMATICS

In his writings, Stephen Jay Gould often turned to the metaphor of the Tree of Life, Etz Chayim, as the best representation of the richly complex and historically contingent evolution of life on Earth. This image reflects both the taxonomic structure of biodiversity and the essential evolutionary processes of branching and divergence. Gould’s own systematic work always addressed this tension between the static configuration of named objects and the dynamic evolutionary processes that produced them. He developed a multivariate morphometric method to study the complex patterns of covariation found in the notoriously variable Caribbean pulmonate snail Cerion, and then methodically applied this approach from more simple to more complex cases within the genus. Gould’s careful systematic work on Cerion and other fossil and living gastropods challenged conventional selectionist explanations for evolutionary patterns, highlighting the importance of developmental constraints and contingent history on patterns of morphological variability and taxonomic diversity.

Gould’s views on cladistics, that great 20th century innovation of systematic method, were often misunderstood. He did take issue with the marginalization of unique traits and similarities in the biological role of taxa, and he saw major pitfalls in basing taxonomic classifications on cladograms. However, Gould valued cladistics as the best method for reconstructing branching order, and therefore for working out the historical patterns of the origin of traits. In his writings, he cited cladistic studies that challenged notions of gradual, predictable, progressive evolutionary change. Most importantly, Gould saw the cladistic emphasis on branching as essential to recognizing species as discrete evolutionary individuals with a reality in nature, a notion that is central to his hierarchical view of macroevolution.

Gould had great respect for traditional systematic work, speaking and writing with reverence about the authors of major taxonomic monographs from Linnaeus to modern scientists. He always argued that systematics was a theory-driven and vibrant field; the best systematic works, including his own, are both creative and transformative, altering or even revolutionizing our views on evolution.