In the natural world, the concept of distinct species often seems straightforward. A lion is not a house cat – that’s pretty clear.  However, upon closer examination, the boundary between different species can be surprisingly blurred. The Galapagos giant tortoises provide an insight into this ambiguity. 

Inter-species cuddling

Galapagos tortoises are thought to have arrived in the islands a few million years ago, their ancestors having floated over from the mainland. They are renowned for their longevity and distinctive shell shapes, have evolved into numerous distinct populations across the islands. These populations exhibit variations in size, shape, behavior, and even diet, reflecting their adaptation to diverse island environments.  Ever since taxonomists first started describing them, a debate has raged as to whether the different giant tortoise populations were the same species, or comprised several different species. 

Take, for instance, the case of the Española and Santa Cruz tortoises. Despite residing on different islands and exhibiting notable physical differences, genetic analysis has revealed that these tortoises are remarkably similar at the molecular level. This genetic similarity challenges traditional notions of species distinction and raises intriguing questions about evolutionary relationships.

Same species?

The complexity of Galapagos tortoise taxonomy highlights the broader challenge of defining species in the natural world. While traditional classification methods rely on observable traits and geographic isolation, genetic analysis reveals a more nuanced reality. Evolutionary processes such as genetic drift, natural selection, and migration can lead to intricate patterns of divergence and convergence among populations, defying simple categorization.

At the end of the day, and most surprisingly to the taxonomically naïve, we learn that the concept of a species itself is not rigid but rather a human construct aimed at organizing the diversity of life.  At one point, the lion and the housecat had a common ancestor – but when, in their evolutionary divergence, would we have been able to definitively draw a line between the common ancestor species, and the two different species? 

ICZN members vote on new species 

It turns out that the line is drawn by the International Commission on Zoological Nomenclature (ICZN) - the global authority responsible for the scientific naming and classification of animals. It establishes and maintains rules and guidelines for the naming of animal species.  The ICZN provides a framework for resolving taxonomic disputes, clarifying rules for naming new species, and promoting stability and consistency in zoological nomenclature. Its decisions and recommendations are widely respected and followed by taxonomists, researchers, and scientific journals around the world.