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Which organisms or anatomical structures have been the most difficult to classify using taxonomy?

Katy Silbersak

In my humble opinion, the organisms or anatomical structures that have been the most difficult to classify using taxonomy are those that exhibit a high degree of variation within and between populations. These organisms are commonly referred to as "ring species" and are a real headache for taxonomists.

Ring species are defined as a group of populations that form a ring around a geographic barrier, such as a mountain range or body of water. Each population is able to interbreed with its adjacent populations, but as you move around the ring, the degree of genetic divergence increases until it exceeds the point of reproductive compatibility. This results in two distinct, but closely related species that are reproductively isolated from one another. Ring species thus form a continuum of intermediates between two distinct forms, making it difficult to delineate where one species ends and another begins.

An example of a ring species is the Larus gulls of the Holarctic region. Here, a ring of gull populations extends all the way around the Arctic, with each population interbreeding with its closest neighbors. However, as you move around the ring, the birds become increasingly morphologically and behaviorally divergent. At the ends of the ring, “two” species are recognized: the Herring Gull “complex” in the west, and the Lesser Black-backed Gull “complex” in the east. This suggests that the Herring and Lesser Black-backed Gulls are distinct species, but the intermediate populations mean that there is much debate over how to classify them.

The difficulty in classifying ring species comes from the fact that they blur the line between what we commonly consider to be “species”. Traditionally, species were defined as groups of organisms that could interbreed and produce viable offspring, but ring species challenge this definition. If the populations at the ends of the ring are distinct species, then what about populations that exist in the middle of the chain? Can they still be considered the same species? And if not, then how many species are we dealing with?

The classification of ring species highlights the challenges of taxonomy and the limitations of our traditional definitions of species. It also illustrates the importance of employing multiple lines of evidence, such as genetics, morphology, behavior, and ecology, to classify organisms.