Imposing distinct separations on the fluid process of evolution inevitably breeds disagreement on what exactly constitutes a species. On Christmas Eve, 1856, Charles Darwin wrote in a letter to Joseph Dalton Hooker, botanist and friend: “It is really laughable to see what different ideas are prominent in various naturalists’ minds, when they speak of ‘species.’…It all comes, I believe, from trying to define the undefinable.”
Delimiting species has challenged biologists since the 18th century, when Linnaeus first sorted and named organisms using only physical characteristics. It is only since the advent of molecular-sequencing techniques that species have become less confounding to researchers, no longer so able to obscure their true identities once their DNA is laid bare. Two or more look-alike organisms previously classified as one “cryptic” species can be split into separate species due to differences in their genetic code.
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