Romeo and Juliet. Star-crossed lovers from two feuding families. What makes this story so epic, and such a tragedy, is that the Capulet and Montague families are such bitter enemies. The star-crossed lovers have it all in front of them if they want love to triumph. They might ask themselves what’s the harm of two families joining together in marriage?

Well resources for one. Blending a family means bringing resources together. If the Capulets had worked hard for generations they might not want to see their hard earned wealth go, in part, to their main rivals. Another issue is the name. Should Juliet Capulet marry Romeo Montague then her children would become Montagues. If at other times a Montague daughter married a Capulet son then everything would be fine. However, if the Capulets tend to produce daughters then the Capulet name would gradually be lost over a few generations.

Indeed this has happened in my own family where my mother, a Todd, has Paterson children. Her brother has not had children. So no more Todds in this particular lineage. My wife, a Burridge, has Paterson children and her brother has two daughters. Her male Burridge cousins have not had children. So Burridge is also likely to end for this lineage.

Surprisingly, something similar can happen in species that can lead to their extinction. Most closely related species have some barriers that prevent them from interbreeding, at least most of the time. Interbreeding between species, when successful, produces hybrids. The problem with hybrids is that they are not representatives of either of their parents’ species – just a partial mix of both.

Sometimes hybrids are not as evolutionary fit as their parents. They may not be as big, or as good at finding food or mates and so are unlikely to pass on their hybrid genes. Other hybrids may be as evolutionary fit (or fitter) than their parents. And that creates a problem.

Hybrids that are reasonably fit will be able to find mates (potentially from both species) and pass on their hybrid genes creating another generation who are not genetically (and probably morphologically) like either of the original parent species. And so on. Before you know it there can be few to no ‘pure’ individuals from the original species left in an area.

Normally this might not be a problem if there are plenty of pure individuals migrating into an area. But there is a big problem if one species is endangered. This is potentially a problem encountered by black stilts (Himantopus novaezelandiae) and the more numerous pied stilts (Himantopus himantopus). They produce fit hybrids when they mate and a black stilt, as they are a rare species, is more likely to find a pied stilt mate than a black stilt individual. If left alone the black stilts could go extinct. Not because they have all died but because there are no pure black stilts left!

A similar situation occurs on the Chatham Islands where the endangered Forbes parakeet (Cyanoramphus forbesi) produces hybrids with the more numerous Chatham Island red-crowned parakeet (C. novaezelandiae chathamensis). A Forbes parakeet mating with a red-crowned parakeet does not produce little Forbes parakeet chicks. Or even red-crowned parakeet chicks. They produce something else, a mixture of genes that represents neither of the parent species. Extinction through dilution.

Banks Peninsula tree weta (Hemideina ricta) are restricted to the outer eastern end of the peninsula. Their close relative, the Canterbury tree weta (Hemideina femorata) is found through Canterbury and over most of Banks Peninsula, except the outer eastern end of the peninsula. Both species occupy very similar habitats and the Canterbury tree weta is a direct rival to the Banks Peninsula tree weta.

Are the two species also kissing cousins? Some limited genetic work in the mid 90s suggested that these species might sometimes mate and produce hybrids. If that is the case then the Banks Peninsula tree weta might be under threat both from without (competition for habitat with Canterbury tree weta) and within (dilution through hybrids).

Mike Bowie (Lincoln University) with Marie and Roddy Hale from University of Canterbury helped PhD student Rachel van Heugten to investigate this question. Over 400 samples were collected from both species by clipping the tarsus off the end of a leg. This was done in areas with just Banks Peninsula tree weta, areas of just Canterbury tree weta, and the region where the species overlap. The samples were subjected to genetic analyses to look for hybrids.

The research has been published in Conservation Genetics. Out of all of the samples only seven were found to be hybrids. This indicates that the two species can definitely produce hybrids and that extinction by dilution is a possibility for the Banks Peninsula tree weta. However, the very low occurrence of hybrids suggests that either the barriers for mating are reasonably effective or that the hybrids that are produced are not very fit relative to their parents.

So while there are a few kissing cousins in these two weta species , they mainly stick to the rival families mode. This is good news for the Banks Peninsula tree weta. They have enough problems to deal with. At least for now they don’t have to worry about dangerous liaisons. So while there might be the occasional Romeo and Juliweta, they will not bring a plague on both their houses.