Is a species defined by it's DNA or by other characteristics?  In the case of hybrids, when does something stop being one thing and start being another?

Hi Dorid,

The formal system we use to name species has been in place since 1753 (for plants) and 1758 (for animals) and was introduced by the Swede Carl Linnaeus (http://en.wikipedia.org/wiki/Carolus_Linnaeus). The familiar Latin binomials (literally 'two names', referring to the genus and species, e.g. Homo sapiens) that we use today were his invention. The discovery of the structure of DNA didn't come until much later in 1953 and the kinds of 'DNA bar-coding' biologists use today are even more recent.

So basically we have been defining species for a lot longer than we have known about, or been able to use, DNA.

For much of the history of biology we have had to suffice with defining species on aspects of anatomy. In palaeontology we are still hampered in this way (as fossils older than a few thousand years simply won't preserve DNA).

However, the availability of DNA based methods has revolutionised our science and even helped us to discover species we never knew were there - so-called 'cryptic' species. A great example comes from bats, where what was once thought to be one species turned out to be two genetically distinct species. There was no obvious difference in anatomy, but further observations noted differences in the frequencies used in their calls, as well as dietary preferences and even their geographic distribution.

Your question about hybrids is a bit trickier.

As you are no doubt aware hybridisation (the successful breeding of two distinct species) is very common amongst plants. Hybridisation is actually a great way to evolve as you can combine the characteristics of very different species to provide an intermediate which can occupy a whole new environment - and you can do this very quickly, in fact it is effectively instantaneous.

Interestingly as a hybrid is often ecologically distinct it isn't in competition with its 'parents' so the two can live happily side-by-side.

So to answer your question the 'parent' species never stop being species and the hybrid starts being a species straight away.