Taxonomy is the way in which we classify and sort the natural world. “Creates order from chaos” is one of the definitions of life according Alvaro’s thesis and since Linnaeus first devised his system of classification, scientists have sought to do just that by trying to decipher how the millions of different species on this planet have evolved in relation to one another.
Using simple diagrams called cladograms, taxonomists can provide neat pictures of how birds separated from reptiles, how different species of monkey have evolved separately from a shared ancestor and how the hippopotamus and the whale are related.
However, the closer you look at this “tree of life,” the more problematic things become. The main branches are mostly pretty solid; it’s when you get to the fine twigs at the end of those branches that things become shaky. It’s here that the very notion of a “species” becomes far from certain.
Once it was thought that separate species could be defined by their inability to breed with one another. However since Darwin’s day it has been known that this definition is unsound – hybridization, or breeding between different species has been shown to be possible and even common, especially among plants.
These thoughts were on my mind as I got talking to a DOC employee working with kaki, or black stilts in Twizel the other day. Kaki are elegant wading birds that were once common in riverbeds and wetlands in New Zealand. They are believed to have arrived naturally in New Zealand about 10,000 years ago from Australia. Like so many other bird species in New Zealand, the introduction of mammalian predators has been devastating to the black stilt. By the mid-1980’s they verged on extinction, with the entire species numbering only about 26 birds. At its breeding centre in Twizel, the Department of Conservation collects eggs from breeding kaki and raises the chicks in safety before releasing them to the wild – where their chances of survival are still not good – the riverbeds they inhabit are stalked by feral cats and ferrets, stoats and possums for whom the birds provide an easy meal. Allowed to breed on their own naturally, kaki would most likely go extinct within a number of years. They are thus wholly dependent on human intervention for their existence.
Their situation is complicated by the relatively recent arrival of another bird, the pied stilt, or kouka, which only landed in New Zealand (again, from Australia) a little over a century and a half ago. Since its arrival, the pied stilt has spread to many habitats, from alpine riverbeds to coastal estuaries. Perhaps due to its greater range, migratory habits and historical association with predators in Australia, this species has proved far more resilient to our introduced predators and has now come to greatly outnumber its black cousin.
Pied stilts and black stilts are very closely related. In fact much of the research into them has centred on whether we can actually describe them as separate species at all. The conclusions, based on physical characteristics and genetic make-up, are that we can…but only just.
The pied stilts in New Zealand are darker in colour that their Australian relations and have been found to have significant amounts of black stilt DNA in their genetic make-up. This is probably because in the years after their arrival, when pied stilts were greatly outnumbered by black stilts, they mated with the other species often. The hybrid offspring of these pairings survived and mated (or “backcrossed) with pied stilts in following generations. The black stilt genes were thus absorbed or “introgressed” into the kouka genome.
Now, the situation is reversed. As the black stilt’s fortunes have waned, there has been a strong statistical tendency for it to mate with the far more common pied stilts. This usually produces a hybrid stilt that is neither totally black, nor carries the same pattern as its pied parent.
From a conservation point of view, this is a problem. In the pairing of two different species, offspring of the heterogametic sex are likely to be sterile or poorly suited for survival (This fact of genetics is known as Haldane’s Rule.) In birds (unlike mammals) the female is the heterogametic sex, so if by chance the offspring of pied and black stilts were mostly female, these offspring would likely not reproduce. In other words each hybrid pairing is not only a lost opportunity for reproduction between the few remaining pure black stilts but also presents a greater risk of being a genetic dead-end.
The conclusion therefore is that efforts should be directed at conserving the remaining black stilts and preventing further hybridization between the species. After all, hybridization in this case is almost certainly aided by human influence – the modification of riverbed environments has provided more favourable habitat for the kouka, while introduced predators have reduced the kaki’s population such that the tendency to hybridize with their more common neighbour is greatly increased.
Of course even if black stilts did become extinct, a tiny portion of their genes would live on within pied stilt bodies, which already contain introgressed kaki genes. And it wouldn’t be the first time this has happened.
The more we learn about hybridization, the more it becomes clear that it is not only common in nature, but in some cases may actually provide an evolutionary advantage. When hybrids backcross with one or both of their parent species, their genes are “introgressed” into the original species. In most cases these hybrid genes are a disadvantage to the animal or plant because they are not well-adapted to the environment they are born into – the hybridization has confused the process of natural selection and therefore individuals with introgressed genes are less capable of survival and will eventually die out. However in many cases, the hybrid genes are not disadvantageous and in some instances may actually provide a benefit to the species. If so, the species will, through natural selection, carry that gene forward through future generations. Natural selection will favour individuals which carry that gene and so its effect will be magnified across the population through the process of “positive selection.”
Humans and Neanderthals, separate hominid species that co-inhabitated Europe during the Pleistocene are believed to have hybridized, and Neanderthal genes introgressed into the human genome, to the extent that most people of European and Asian descent carry up to 4% Neanderthal genetic material. (African-descended people do not, because Neanderthals did not exist there.)
Research does suggest that at several stages of our evolutionary history we have interbred and taken on genetic material from other hominoid species long after we went our separate ways down the evolutionary path. Like artists stealing ideas, we borrowed from our neighbours’ genetic strengths before leaving them in our wake. For example, the microcephalin gene, which relates to brain size during early development is believed to have been introgressed by humans from a separate archaic hominid species around 37,000 years ago and since that time has been subject to strong positive selection. Where the microcephalin gene arose from and what advantage it gave us, remains unclear. It has been theorized that it is a remnant from Neanderthal encounters, but this remains unproven. It has also been suggested that introgressed Neanderthal genes helped humans battle viruses.
The extreme end of this process is where hybridization is so successful that it results in a completely new species. Examples of this, at least as far as is currently known, are rare. The red wolf of north America (now extinct in the wild) is believed to be a hybrid of the grey wolf and the coyote, while the Lonicera fly, which feeds on an imported honeysuckle that has only existed in the United States for 250 years is believed to be a hybrid of two other flies, and has apparently developed as a unique species in that short time.
The closer you look at the genetic distances between closely-related species, the more complex things appear – and this blurriness has real-life implications, especially when it comes to conserving endangered species like the black stilt. In the case of this beleagured bird the question; “what defines a species?” determines whether there is even a species to save.
And now, because I know it’s the only reason you started reading this…here’s a picture of a zonkey: