It might seem surprising for the general public, but every biologist knows that the term species is one of the most difficult to define in biology. In fact, it is argued whether species exist or are a concept we have created ad hoc as a unit for measuring biodiversity and interpreting Nature. There is no consensus and over the last century different definitions have been introduced. There is a long list of definitions, each based on different principles. Darwin himself wrote about this matter extensively (1).
This myriad of options does not mean definitions are wrong, instead all of them can be applicable under certain circumstances to define a species. However, it is this lack of universality that makes this matter a headache. And science is, after all, about finding universal traits.
Probably one of the most invoked methods to delimitate a species is what Dobzhansky proposed and Mayr later on popularized as the biological concept of species. The concept became very popular and was accepted as the solution which went uncontested for decades. It states that different species are unable to produce offspring due to pre-zygotic and post-zygotic barriers. In other words, species are populations of naturally reproducing individuals that are isolated from other such groups (1). Thus, if they can reproduce, they are the same.
But viable hybrids exist after all, we are the living proof of that. Indirect studies have suggested that admixture between archaic humans and anatomically modern humans (AMH) occurred (2) (Picture 1). For example, today we know that people of Eurasian ancestry carry a 1-4% of Neanderthal DNA in their genomes (Figure 1) whereas sub-Saharian human populations are purely Homo sapiens (3). Although AMH and Neanderthals coexisted in what we call Europe, it has been estimated that the first interbreeding event happened in the Middle East 50,000-60,000 years ago, after early Homo sapiens left Africa. It makes sense because our cultures and behaviours were not so different at that time. There is archaeological evidence of the overlap as well. This assumption explains how present day populations in Europe and Asia share these Neanderthal traces, although probably there was more than one hybridization event (3). Suddenly our extinct cousins are not our cousins anymore but our great-great-grandparents to some degree. It does not stop here (Figure 1). Denisovans, a recently discovered hominin group, contributed with a 6% of genetic material to the genome of present day Melanesians (Figure 1), as well as to the Han but in much lesser degree. Interestingly, Denisovans were discovered with DNA extracted from the only existing archaeological evidence: a phalanx found in a Siberian cave. Because of this some refer to them as a “ghost” species.
The acceptance of a hybridization event in our own lineage has been a gradual process during the last decade. However, with the biological concept in mind, this raises many questions. If two species that diverged 500,000 years ago have viable offspring, are we the same species? Or not?
Humans are not the only instance
At the arrival of AHM to Europe various types of wolves existed. The traditional assumption is that nowadays dogs evolved from wolves domesticated by humans in agricultural settlements (12kya) (4). Those ancient wolves were assumed to be the species of modern wolves (Canis lupus) we see nowadays. Based on small fragments of the genome experts filed dogs as a sub-species of wolf (Canis lupus familiaris). Recently, there has been increasing support for an origin of dogs linked to hunter-gatherer populations well before (30-40kya) (4).
The new study suggests an alternative explanation: repeated interbreeding between wolves and dogs, who share the 99.9% of their DNA, confused previous studies (5). Wolf-dog admixture still happens today: black coated wolves received that gene from dogs (Picture 1) and sheepdogs in places like the Iberian Peninsula, with whom wolves mate so often that hybrids are not rare. DNA from a 35kya wolf fossil (Figure 2) from Siberia revealed that this ancient species contributed to the Nordic breeds such as the husky through hybridization (Figure 2) (5). It is truly surprising but according to the most recent evidence, man’s best friend is not a sub-species of the modern wolf. Instead, both species are sister braches that originated from a common ancestor now extinct (Figure 2).
Why does this matter in conservation?
The problem of defining what a species is might seem merely academic but it is not, and has ramifications in conservation. For example, when deciding whether to give or remove a protected status to a species.
It is possible that we have repeatedly made the mistake of viewing species as Linnaeus conceived them: forms perfectly separated from the other forms at the time of its creation. Instead, we are starting to see nowadays that the Tree of Life is more like a web. Branches sometimes merge. This means that the term species is not more real than the concept of genera. In other words, species only become easier to define when the intermediates go extinct. When these intermediates exist, it is hard to set a boundary for species (1). This is the reason why dealing with hybrids has always been a problem for wildlife policies. For example, the dusky seaside sparrow extinction, after genetic rescue by interbreeding with close subspecies was not allowed. Do hybrids destroy species by erasing the limits or create new ones? The complexity of this issue is what has led the scientific community to boost the reality of species and cover up hybrids as freaks of the Nature. It looks like hybridization is an inconvenient truth, but it happens and is fairly common in Nature (red wolves, ducks, bears, dolphins).
We should embrace a more dynamic vision of animal species rather than a static one. From there, we could build a better definition of species improving with genetics Darwin’s morphological approach.
 Mallet J. (1995). A species definition for the modern synthesis. Trends in Ecology and Evolution. 10: 294-299.
 Fu Q, Hajdinjak M, Moldovan OT, Constantin S, Mallick S, et al. (2015). An early modern human from Romania with a recent Neanderthal ancestor. Nature. 254: 216-230.
 Green RE, Krause J, Briggs AW, Maricic T, Stenzel U, et al. (2010). A Draft Sequence of the Neandertal Genome. Science. 328: 710-722.
 Skoglund P, Ersmark E, Palkopoulou E, Dalén L. (2015). Ancient Wolf Genome Reveals an Early Divergence of Domestic Dog Ancestors and Admixture into High Latitude Breeds. Current Biology. 25: 1515-1519.
 Freedman AH, Gronau I, Schweizer RM, Ortega-Del Vecchyo D, Han E, et al. (2014). Genome Sequencing Highlights the Dynamic Early History of Dogs. PLoS Genet. 10(1): e1004016.