Sir

In his Words essay “Yes, but what's it for?” (Nature 412, 771; 2001), Steve Blinkhorn reminds us that the current state of language can make it difficult to discuss evolution in an accurate way. We must be constantly alert to invocation of teleological explanations when describing evolution.

I believe that another (related) problem lies in the inherent ambiguity of the term 'related', used to depict similarity between biological objects such as nucleic acid or protein sequences. It is common to see very similar sequences described as “closely related”, less similar ones as “distantly related” and dissimilar ones as “unrelated”. Of course, the truth is that related (similar) sequences are not always related in terms of shared evolutionary origin (homology), and vice versa.

This ambiguity in meaning of the word 'related' differs from that which some believe exists for 'homology'. It is generally agreed that the only useful definition of homology between biological objects is “having a common evolutionary origin” (G. R. Reeck et al. Cell 50, 667; 1987). But when using the term 'related' in a biological context, it would be helpful to readers if authors could be explicit about which meaning is appropriate.