A feral ancestry for the Przewalski's horse

EDIT: A recent study by Taylor et al. 2021 calls these results seriously into question. The Botai horses apparently were not domestic. Therefore the Przewalski's horse remains the only living wild horse and is not feral. 

Sometimes timing is funny. In November 2017 I wrote a post defending the Przewalski's horse's status as a wild animal. A new study by Orlando et al. has challenged the status of the Przewalski's horse as the last living genuine wild horse.
The earliest archaeological evidence of horse husbandry is from the Botai culture of Kazakstan from 5.500 years ago. It has been assumed previously that these Botai horses belong to the earliest strain of domestic horses of the caballine lineage. Surprisingly, the authors found only about 2,7% Botai-related ancestry for all domestic horses from 4.000 years ago, while the authors claim the Botai horses turned out to be the ancestral stock of the modern Przewalski's horses population. I have not read the paper yet because it is behind a paywall, thus I cannot see what led the authors to the conclusion that all modern Przewalski's horses descend from the Botai population and not just that the Botai population was part of the przewalskii clade. But let us assume the former is the case for now.
The authors thus write that Przewalski's horses are feral descendants of these early domestic horses, and not true wild horses. I have problems with the latter part of the conclusion. First of all, the horses of the Botai culture must have been in a very early state of domestication, and not for a long time. After escaping human husbandry, those early domestic horses have been exposed to natural selection for four millennia again. Any changes that might have occurred in the early state of domestication must have adapted to the requirements of living as a wild animal again. Thus, I don't regard the short (in evolutionary terms) episode of domestication as substantial enough to categorize the Przewalski's horse as feral instead of a wild animal. In fact, ever since its discovery, the Przewalski's horse has been used as a model for a wild equine and numerous differences in physiology, development and behaviour have been noted between the Przewalski's horse and domestic caballine horses as much as feral caballine horses. Przewalski's horses are way less tamable and more aggressive than domestic caballine horses, and genomic studies have shown that numerous genes for physiological aspects have been altered through human utilization while that is not the case in the Przewalki's horse[1]. The recent study by Orlando et al., or to be precise the short period of early domestication, does not alter this fact. The situation is not comparable to mustangs and other feral caballine populations at al.
Thus I do not regard the Przewalski's horse as any less wild as before. One could use it as an example for a "post-domestic wildtype", a term I tried to introduce in my posts on dedomestication as opposed to "pre-domestic", a term that is already used. However, I am rather confident that the majority of authors will begin to list the Przewalski's horse as a feral horse now, as the 19th century conception of  nature and evolution as something static and humans as an irreversible altering factor is still prevalent in a lot of experts heads. Unfortunately, in my opinion.

Interestingly, the authors found the alleles for the Leopard spotted colour present in the Botai horses. This colour variant has already been found in Pleistocene wild horses (see earlier studies of Orlando et al.).

Literature

Orlando et al. 2018: Ancient genomes revisit the ancestry of domestic and Przewalski's horses.


[1] Schubert et al.: Prehistoric genomes reveal the genetic foundation and costs of horse domestication. 2014.

Biological basics pt. III: Subspecies

In Biological basics III: The species concept – a complicated issue, I gave a little overview over how a species is usually defined and what a species actually is. Today’s post is going to look at subspecies. The understanding of the concept of a subspecies is necessary for some aspects of “breeding-back”, such as the issue of using zebus in aurochs projects and, more essentially, the Quagga project. Subspecies are sometimes called a cop-out, nit-picking or something for bored taxonomists, and I am going to outline why this critique is not fact-based.

Subspecies – something for bored taxonomists?

The short and definite answer to this is No. As Charles Darwin already noticed absolutely correctly and perfectly subsumed, subspecies are incipient species and the line between varieties, subspecies and species is gradual. As he noted in his opus magnum, On the origin of species, species are basically “strongly marked varieties”, and subspecies are a state in between. And as the previous post on hybridization has shown, even the line between species is blurred because many closely related species are able to hybridize with limited or unlimited fertility. In many cases there is uncertainty on whether two or more species should be regarded as subspecies of one species, or whether two or more subspecies should be elevated to species level. However, for this section on the concept of a subspecies and what justifies it from a biological point of view, I will try to focus on examples for which there is a consensus.

Subspecies are in a beginning state of cladogenesis, a speciation mode that I already explained in part I. It is the case when populations diverge and become more or less isolated, and the allelic frequencies start to change due to genetic drift, mutation and selection. These changes not only concern neutral genetic variations but all biological aspects: morphology, behaviour, ecology. This is especially enforced by the fact that the different populations often experience different selective pressure due to a different environment. This is particularly true in species with a large geographical range, especially when it is disjunct (isolation). Thus, the different populations become regionally adapted additionally to coincidental differences caused by genetic drift. In some cases, the differences between populations or group of populations assigned to different subspecies may concern only single traits like colour variants, but in some cases the differences are well-marked and concern a number of biological aspects. I am going to go over some examples.

The classification of subspecies experiences the typical taxonomical problems of subjectivity and tradition, and therefore is not always consistent. In some cases two subspecies might be as distinct as a pair of species, and vice versa. There is no clear line as taxonomy is an artificial system that aims to classify the complex reality of organismic relationships.

One example clear example of a species that is divided into two subspecies is the American Bison Bison bison. This species is divided into a northern subspecies, the wood bison B. b. athabascae, and a southern one, the plains bison B. b. bison. Both differ ecologically and morphologically; wood bison dwell boreal forest regions and are adapted to lower temperatures, while plains bison live in more open regions and are less (but still very) cold-adapted. Wood bison are larger, have a darker pelage and less hair on the forelegs and beard. Also, the hump is shaped differently.

The African buffalo, Syncerus caffer, is divided into five subspecies. The nominate form, Syncerus caffer caffer, has the southernmost range, is the largest subspecies and very dark in colour. S. c. brachyceros, is smaller, weights only half as much and is lighter in colour. The smallest subspecies, the forest buffalo S. c. nanus, differs considerably ecologically and morphologically. It inhabits the swampy forest areas of Central and West Africa instead of dry grasslands, it is very small (withers height less than 120cm), a bright orangish-red colour instead of greyish to black-brown, small horns and brushy hair on the ears. Some authors have suggesting listing it as a separate species, and since the Caucasian wisent which differs from the nominate form less drastically is considered a separate species now (Bison caucasicus), I definitely list the forest buffalo as a separate species, Syncerus nanus. This is a good example that shows that taxonomy is often subjective and not consistent.

A prime example for a species with a large geographical range and many different subspecies that are morphologically and ecologically distinct is the wolf, Canis lupus. Its large, Holarctic distribution includes more than a dozen subspecies that all differ in colour and morphology, body size, environmental adaptions, and to a certain degree behaviour. For example, large subspecies such as the nominate form C. l. lupus weight twice as much as smaller subspecies such as the Arabian wolf  C. l. arabis or pale-footed wolf and have a stronger and more robust build; smaller wolves live in smaller packs and are less macropredatory. Wolf subspecies can be adapted to totally different climates; the Arabian wolf for example inhabits the hot arid climate of the Arabian peninsular, while the polar wolf C. l. artcos obviously is adapted to arctic conditions. Wolf subspecies also differ in colour nuances.

Another predatory species that has a large range that includes many different subspecies with different morphological characteristics is the lion, Panthera leo. Lion subspecies can be adapted to different ecologic environments and may differ in body size and mane for example, and the diversity in the species becomes even greater if you include Pleistocene forms.

There are many other examples in all possible vertebrate groups that show that subspecies are clear evolutionary lines with distinct morphological, ecological or ethological traits. In some cases the situation is more clear and distinct as in others, but subspecies are definitely not something for “bored taxonomists”. It makes sense to differentiate subspecies not only from a taxonomical standpoint, it also appreciates the evolutionary, ecological, morphological and ethological situation within a species. Therefore, subspecies are a very useful and justified concept, which is followed by all zoologists (quite frankly, I have never heard of any zoologist deeming the subspecies concept as not useful and not using it).

A special case of evolutionary variation within a species is the so-called cline. In this case there is not a clear differentiation into several lineages, but variation along a geographic or ecologic gradient. This is comparable to the concept of a ring species that has been explained in the previous post. The quagga, Equus quagga quagga, has been suggested by some to represent the end of a cline rather than a distinct subspecies, although I think this should be backed up by empirical evidence (see this post on the quagga).

Biological basics pt.II: Hybridization and its role in evolution

Having had a look at how species are usually defined, this post is on the result of the reproduction between two distinct species – hybrids. More precisely, with this post I want to illustrate the role of hybridization in evolution and to show that hybrids are neither Frankenstein creations of bored farmers or zoos, or signs of the apocalypse (as hybrids of polar bears and brown bears are sometimes presented in the media). I also want to go into the role of hybrids in conservation with reference to one particular case, the wisent Bison bonasus.

The role of hybridization in speciation

Speciation is the event of the evolution of a new species. Most species evolve either through anagensis or cladogenesis. Anagenesis refers to the case of one species directly evolving into another. Cladogenesis is the evolution of new species by the split into new evolutionary lines, called clades, caused by reproductive isolation. In both cases, the genotype gets transformed by mutation, selection and genetic drift. But there is a third way a new genotype and a new species can involve, hybridization. In this case a new genotype is formed by the mixing of alleles respectively genes between species that are not too distantly related so that they are not (fully) reproductively isolated yet. Usually hybridization is constrained by pre- or postzygotic isolation mechanisms, and even when two species produce fully fertile offspring the hybrids may have a lower reproductive fitness than their parent species because they are less suited to the ecological niches of their parent species respectively. But in some cases, such as during the shift of environmental conditions or colonizing a new environment, hybridization and the resulting new genotype can be advantageous.

The plant kingdom is rich with such examples, especially polyploid hybrids such as Tragopogon miscellus. Both T. dubius and pratensis have been brought to North America by man, hybridized in nature and formed a stable hybrid via polyploidization in the 1940s, T. miscellus. In this case a novel species has been created. Other plant examples are to be found in the “genera” Brassica and Triticum.

There are not only examples for plant species that evolved through hybridization or introgression (introgression is when hybridization leads to an influx of genes into one gene pool), but also plenty for mammals.

The “genus” of modern Capra, goats, evolved through hybridization between the ancestors of Capra and Hemitragus, a closely related group, and as a result all modern Capra share Hemitragus mtDNA₁.

The phylogenetic tree of Equus apparently also experienced at least four events of hybridization or introgression (the kiang and the donkey lineage, the Somali wild ass and the Grevy’s zebra, the African asses and the mountain zebra)₂.

The Caribbean bat species Artibeus schwartzi is a stable, locally adapted and morphologically distinct hybrid of three congeneric bat species₃.

In elephantid evolution, there must have been some interbreeding as was recently revealed by ancient genomes. Palaeoloxodon antiquus interbred with both the woolly mammoth Mammuthus primigenius as much as with the Asian elephant Elephas maximus₄.

Mutual introgression and hybridization has been confirmed between the Alerigan mouse Mus spretus and three subspecies of the house mouse Mus musculus₅.

Even in marine mammals there is at least one confirmed case of speciation through hybridiziation: the clymene dolphin Stenella clymene is a hybrid species of the spinner dolphin Stenella longirostris and the striped dolphin Stenella coeruleoalba₆.

The phylogeny of modern bears has experienced mutual hybridiziation as well, between the Asiatic black bear and the ancestor of polar, brown and American black bear₇.  Brown bears and polar bears repeatedly admixed ever since they split up phylogenetically₈.

All recent Panthera species evolved under continuous intermixing between their evolutionary lines₉.

Moving to species more relevant for the topics of this blog, bovines, we also have some examples in this group. Cambodian banteng populations share mitochondrial genetic material of the Kouprey, which they acquired probably through introgressive hybridization during the Pleistocene₁₀. It is also well-supported by genetic evidence that the wisent is a hybrid of aurochs and Pleistocene bison, to be precise the maternal lineage was founded by aurochs and the paternal by bison_11,12. 

Last but not least concerning mammals, we humans Homo sapiens are a prime example for hybridization and introgression as well. Human populations north to the Sahara interbred with Neanderthals, and as a result about 1-4% of the genome of non-African people is inherited from Homo neanderthalensis₁₃. In some Asian populations we also find 4-6% inherited from the Denisova people (a yet undescribed species)₁₄, and there is evidence of gene flow from a third archaic population₁₅. So the diverse modern human global population is the result of intermixing with at least even three different species.

Before going too much into detail, there is also evidence from birds and other vertebrate groups. As an example, Darwin finches are known to hybridize, which influences each others’ evolution₁₆.  In 2017, a Science paper announced the rapid speciation of a new Darwin’s finch lineage through hybridization₁₇.

Now I have listed plenty of examples that show that hybridization and introgression played a role in the evolution of many different species and species groups, even and especially us humans, and there would probably be more if more were tested. The role of hybridization in vertebrate speciation should be considered well-supported by these recent studies.

Hybridization in nature

Natural hybridization not only occurs when lineages diverge or environments shift, but also happen in nature on daily basis in certain cases. I am going to go over these examples now.

Precisely I am talking about hybrid zones that exist when neighbouring closely related species interbreed. Classic examples are fire-bellied toads (Bombina), where the species Bombina bombina and Bombina variegata have a hybrid zone through Europe where their habitat overlaps. The hybrids are fully fertile, but have reduced evolutionary fitness. This limits the reproductive success of the hybrids, otherwise there would be a continuum between species. A wild case of natural hybridization in amphibians is Pelophylax. Pelophylax lessonae and P. ridibundus often overlap in range and produce fertile hybrids. These hybrids, known as the edible frog, however, passes on only one complete parental chromosome set and therefore never produces stable hybrids but the backcrossed offspring “reverts” to the parental species. Thus, edible frogs are classified as a kleptospecies, Pelophylax kl. esculentus.

In Canis, there is also frequent hybridization in the wild between the species. Gray wolves hybridize with golden jackals (for the reference, see the Species concept article), coyotes (the hybrid populations have been described as a separate species, Canis rufus, which is now called into question), and, as long as you regard them as a separate species, Timber wolves. Another mammal hybrid zone is that between the polar bear and brown bear, which has been addressed above already. Among megaherbivores, African savannah elephants Loxodonta africana and forest elephants Loxodonta cyclotis hybridize readily in overlap zones and form hybrid populations₄.

Another classic example of a hybrid zone is that between the carrion crow (Corvus corone) and the hooded crow (Corvus cornix), which differ in plumage colour but are otherwise very closely related. Both species are also often listed as subspecies of one species or even only colour variants, but the subspecies issue will be treated in a separate post.

There are also plenty of examples from other animal groups and of course also plant species that form hybrid zones.

A special case that involves hybrid zones is that of a ring species. The ring species concept refers to a group of neighbouring populations that might also be recognized as distinct species (in which case it is more of a species ring instead of ring species), along an ecologic or geographic gradient. Directly neighbouring populations or species can interbreed, but not those at the respective ends of the chain. Examples for a ring species or species ring are salamanders of the genus Ensatina or the bird species Phylloscopus trochiloides.

Hybridization in conservation

Hybridization is also relevant for conservation. It is mostly considered a problem, even called “genetic pollution”. Why is that, considering that hybridization seemingly is a natural element of evolution and occurs frequently in the wild? There are, one the one hand, good reasons for that which I am going to outline now. But there are also examples and circumstances that should allow a reconsideration of regarding hybridization/introgression as “genetic pollution”.  

When cases of hybridization or introgression are referred to as genetic pollution it is because it is of anthropogenic cause either through invasive species, domestic animals or migration caused by climate change. A consistent influx of alien genetic material alters the allele frequency and diminishes the autochthonous genetic material, the genetic integrity, to a level that a species can go extinct on a genetic basis. This is especially a danger in populations or species that are already endangered. One example for a subspecies that has been lost through hybridization is the Barbary lion, Panthera leo leo. It is extinct in the wild, and in captivity zoos did not pay attention on not to mix subspecies. Nowadays, many lions in captivity that are managed without subspecies classification descended from Barbary lions, but there are no pure representatives of the clade anymore₁₉. There has been announcements for a project to genetically breed-back the pure form, but the project seems to have died a silent death.

Mallard ducks Anas platyrhynchos have been introduced in various regions of the world, and they threaten the genetic integrity of numerous autochthonous species, such as A. rubripes, A. fulvigula, A. wyvilliana and A. superciliosa. American ruddy ducks Oxyura jamaicanensis threaten European A. leucocephala₂₀.

The endangered Californian tiger salamander Ambystoma californiense is threatened by hybridization with the introduced Barred tiger salamander A. tigrinum, and the hybrids themselves cause ecological problems₂₁. From the same salamander group, the Axolotl, is very endangered as well, and in aquarist keeping they sometimes are consciously crossed with tiger salamanders to produce more colour variants. Due to the lack of a transparent breeding book, this could become a problem for the species’ genetic integrity.

Moving back to mammals, another well known example of “genetic pollution” is the American bison. When the American bison went through the severe bottleneck at the end of the 19^th century, herd keepers  experimented with cattle hybridization. Nowadays, nearly all herds tested contain genetic traces of domestic cattle₂₂. The phenotypic legacy of this domestic cattle introgression can occasionally be seen in modern bison, it mostly shows in deviant horn shapes or tails longer than the norm for bison (see here, for example) although in most cases the introgression is invisible. However, bison with cattle introgression may have a fitness disadvantage under fully pure bison because they have a lower body mass in the wild₂₃.

Now we come to the animal that concerns me the most, the European bison or wisent. In the 1920s, the species almost vanished due to hunting and habitat destruction, and all modern wisent descend from only 12 individuals. During this massive bottleneck event, there was intransparent hybridization with American bison and also domestic cattle. It was feared that pure wisents are going to disappear, which is why a breeding book was set up. Nowadays, the danger of pure wisents becoming swamped out by hybrids has been banned. Most modern wisent have a confirmed pedigree. They are pure but their extremely narrow gene pool is a drastic danger for the existence of the species. Epidemics and developmental problems cause high mortality rates or miscarriages both in the wild and zoos which is seen as a danger for the long-term survival of the species₂₄. The hybridization with American bison back in the 1930s have mostly been executed in order to increase the genetic diversity and therefore reduce the impact of the severe inbreeding depression. While most of the hybrids have been culled later on, there is a Wisent population in the Caucasus mountains that still contains about 5% American introgression. It is phenotypically recognizable to some degree (see here). These “hybrids” are frowned upon some conservationists for phenotypical and ecological reasons that I cast in doubt in this post, and are the largest connected wild population of wisents with the longest history. Although it has not been directly tested, I think there is good reason to assume this population is healthier and has a higher evolutionary fitness due to the increased genetic diversity that is also under natural selective pressure. Therefore I actually consider this population very valuable, while some conservationists propose its extermination as it might be a danger for the purity (and thus genetic scarcity) of neighbouring wisent populations. In my opinion, the health and fitness of the Caucasus population should be evaluated, and if it indeed turns out to be higher or considerable higher than in pure and thus highly inbred wisents, which I assume to be likely, one might consider creating a third wisent breeding line with controlled transparent hybridization that aims to conserve the genetic integrity of the species but at the same time provide new allelic variation to overcome its sensitivity to diseases and developmental problems. I proposed this in the post linked above, and also donated for the wisent in the Caucasus.

Because it is my fear that the drastically narrow genetic diversity of the wisent might one day lead to animals that are pure on the one hand but not robust enough to establish and maintain wild or even captive populations, and that the wisent one day might not be able to survive on the long-term sight.

The main problem is the academic acceptance. All the studies I have cited in this posts are of recent years, therefore the numerous examples that underline the role of hybridization and introgression in species are known only since a short period of time. It will require some further studies and more years to pass until this knowledge has found its way into mainstream science and consciousness, and once it becomes accepted, it will start to influence conservational practise. Part of the problem also is that the out-dated concept of the 19^th and 20^th century of nature as a stable, balanced system that only changes in once in considerable geological timespans and of species as static entities with a clear, always tree-like phylogeny seems to be prevalent in the mind of many conversationalists, although modern science establishes a concept of nature as a dynamic system of constant changes that can also occur in a short period of time.

Conservationists have good reasons to condemn hybridization and introgression in most cases, especially uncontrolled in the wild, but there are also cases, such as in the wisent, where controlled and transparent hybridization or introgression could actually be beneficial for the survival of the species.

References

1 Ropiquet, Hassanin: Hybrid origin of the Pliocene ancestor of wild goats. 2006.

2 Jonsson et al.: Speciation with gene flow in equids despite extensive chromosomal plasticity. 2014.

3 Larsen, Marchan-Rivadeneira, Baker: Natural hybridiziation generates mammalian lineage with species characteristics. 2010.

4 Ewen Callaway in Nature News: Elephant history rewritten by ancient genomes. DNA from extinct species forces rethink of elephants’ family tree. 2016.

5 Ulrich, Linnenbrink, Tautz: Introgression patterns between house mouse subspecies and species reveal genomic windows of frequent exchange. 2017.

6 Amaral et al. 2014: Hybrid speciation in a Marine Mammal: The Clymene Dolphin (Stenell clymene).

7 Kumar et al. 2017: The evolutionary history of bears is characterized by gene flow cross species.

8 Miller et al. 2012: Polar and brown bear genomes reveal ancient admixture and demographic footprints of past climate change.

9 Figueiro et al. 2017: Genome-wide signatures of complex introgression and adaptive evolution in the big cats. 

10 Hassanin & Ropiquet 2007: Solving a zoological mystery: the kouprey is a real species.

11 Verkaar, Nijman, Beeke, Hanekamp, Lenstra: Maternal and Paternal Lineages in Cross-breeding bovine species. Has Wisent a Hybrid Origin?. 2004.

12 Soubrier et al.: Early cave art and ancient DNA record the origin of the European bison. 2016.

13 Federico Sanches-Quinto et al. 2012: North african populations carry the signature of admixture with Neanderthals.

14 Reich et al. 2010: Genetic history of an archaic hominin group from Denisova cave in Siberia.

15 Hammer et al. 2011: Genetic evidence for archaic admixture in Africa.

16 Grant, Grant 2015: Introgressive hybridization and natural selection in Darwin’s finches.

17 Grant et al. 2017: Rapid hybrid speciation in Darwin’s finches.

18 Gonzalez 2012: The pariah case: some comments on the origin and evolution of primitive dogs and on the taxonomy of related species.

19 Barnett et al. 2006: The origin, current diversity and future conservation of the modern lion (Panthera leo).

20 Rhymer 2006: Extinction by hybridization and introgression in anatine ducks.

21 Ryan, Johnson, Fitzpatrick 2009: Invasive hybrid tiger salamander genotypes impact native amphibians.

22 Derr, 2006: American bison: the ultimate genetic survivor.

23 Derr et al. 2012: Phenotypic effects of cattle mitochondrial DNA in American bison.

24 Mammal Research Institute, Polish Academy of Science: European Bison Bison bonasus: Current state of the species and an action plan for its conservation. 2002

The Breeding-back blog as a book?

Before I continue with my Biological basics series I want to announce an idea that I have been thinking about for some weeks now. A couple of people told me that I gathered so much material on the Breeding-back blog in the form of literature references, information on projects, trips, reports, photos, artworks and own theories that it would be worth to publish all that as a book. I like that idea pretty much and I am confident that I can accumulate enough material to fill a comprehensive book with it, so I have been working on a preliminary table of contents for now.

What is important is that this book would not be a mere copy+paste collection of articles of the BBB, not at all. It will be a well-structured work that collects all the material of the blog topic by topic, goes more in-depth and will be precisely researched and referenced. Of course it will give a precise overview over the history and present of the major breeding-back projects, breeds involved and results. And it will also cover a couple of animals other than horses, cattle and the quagga. I will extend articles with new data and more profound research and I am also going to cover aspects that have not been addressed much on the BBB before. I would also present new artworks and photos that I am currently working on or have not published yet so far. Therefore, even if you have been a rigorous reader of the blog from the first moment on, you would find a lot of new material in it and also find it a clear and structured reference work which is way handier than hundreds of loose single articles published in the web.

The book would probably be written and published as an ebook. This would make it easier for me than looking for a print publisher, because the topic of the BBB is rather special. Now, the question for my readership is: what do you think about my idea, would you be interested in having a comprehensive, in-depth and well-structured ebook version of the BBB that also contains a lot of new information and picture material?

I am highly motivated and looking forward to start with the work for the book. I will keep you up to date and present information on title and the new material that will be found in it during the next weeks or months.

Biological basics I: The species concept - a complicated issue

Todays post is on a rather theoretical topic, namely the definition of a species. It is very basic, but a necessary requirement for other issues such as hybridization or the subspecies concept, which are relevant for several units of “breeding-back”. At first it might seem easy to define a species, but everybody who has a basic deeper knowledge of biology will know that it is actually a very tricky issue. There are several concepts and definitions of a species, and none of them qualifies as the ideal and only one. This is why there is the saying “in a room of n biologists there are n+1 species definitions”.

The reason for this problem is that the taxonomical system that classifies organism is an artificial one made by us humans in order to work efficiently with the diversity of living beings. Classical Linnean ranks, thought to us in school and common to most people having a basic biological knowledge, are painfully artificial and subjective which is why they are usually avoided in modern phylogenetic systematics and cladistics. The basic biological entity that systematics are based on are individuals (even here we have blurred lines, see animal colonies; however, this blog deals only with vertebrates so let us ignore those for now). The next level are populations, and above that, biological ranks begin and so does subjectivity. Strictly spoken, a species is more of a hypothesis than an entity. In this post, I want to outline the different approaches to how define a species, their problems and also give some examples.

We want to focus on vertebrates here. For other organismic groups, such as the asexually reproducing bacteria, there are other approaches on how to define a species. The most common species definition was coined by Ernst Mayr and is widely known as the biological species concept. As “biologic” is rather generic, I refer to as the Mayr’s definition in this post. It defines a group of individuals or populations that actually or potentially reproduce and produce fully fertile offspring. This concept works well in many cases, which is why it is widely used. But there are also examples where it becomes problematic. One problem are hybridization in the wild; many species have hybrid zones with closely related, neighbouring species. Prominent examples are fire-bellied toads (Bombina), or Canis, where wolves hybridize with golden jackals in Eurasia₁ and coyotes in North America, and so do the so-called pariah dogs₂. A special case of hybridization in the wild that makes the Mayer’s definition problematic are the so-called ring species. Another example that is relevant for us are American and European bison (Bison bison and Bison bonasus). Both are geographically isolated because they dwell different continents, but when brought to the same area they reproduce readily and produce fully fertile offspring. This is why some authors have suggested listing them as one species, giving them subspecies status (although both the European and American bison are further divided into two subspecies themselves). However, most zoologists still consider both different species. One reason is another level that is relevant for species recognition, morphology. There is the so-called morphological concept or morphospecies that differentiates species based on phenotypical traits. Palaeontologists mostly have to work with morphospecies only due to a lack of genetic, ethologic and ecologic data. The problem of morphospecies are intraspecific variation, which can also lead to a morphologic overlap between closely related species, as much as phenotypic plasticity. The ambiguity of the morphospecies concept shows for example in the case of lion (Panthera leo) and tiger (Panthera tigris). The skeleton of a female tiger and a male lion are almost indistinguishable₃. However, apart from other morphological differences such as coat colour and hair growth (mane), lions and tigers have different social behaviour, hunting behaviour and despite an overlap in range do not reproduce in nature, which is why they are considered separate species. This, as much as the bison example, shows that in practice not just one species definition but often a mix of several levels (genetic, morphologic, ethologic and ethologic) is used to determine and differentiate species.

The third relevant definition of a species is called the phylogenetic concept, which deals with species that evolved through cladogenesis (that is the split of evolutional lines; the direct evolution from one species to another without cladogenesis is called anagenesis). In this case a species is a monophyletic clade of one or more population that ends either through speciation (the evolution of a new species) or extinction. This concept is only useful under the frame of phylogenetic systematics or cladistics.

Having had a look at what species actually are, we can dive deeper into the issues of hybridization and subspecies, which is directly relevant for “breeding-back” related topics concerning the wisent, quagga and others that have been covered here on this blog.

References

 ₁ Moura et al. 2013: Unregulated hunting and genetic recovery from a severe population decline: the cautionary case of Bulgarian wolves.

₂ Gonzalez 2012: The pariah case: some comments on the origin and evolution of primitive dogs and on the taxonomy of related species.

₃ Weishampel, Dodson, Osmolska, 2004: The dinosauria