Letter: How do we define a species these days?

From Robert Milne, London, UK

Colin Barras discusses chimp evolution being shaped by sex with bonobos and describes them as “close relatives” (2 June, p 4).

When I studied biology many years ago, I was taught that organisms belonged to the same species if, and only if, they could produce fertile offspring together. Horses can mate with donkeys, for example, but because the resulting mules cannot get together to make more mules, horses and donkeys were regarded as belonging to different species.

The distinction seems to have been blurred, with talk of chimps and bonobos benefiting from sex with each other and Homo sapiens breeding with other “species”.

Has the definition changed and if so, why?

The editor writes:
• Yes – it seems that the more we discover, the more confused the notion of a species becomes. It is now the subject of intense debate.