What is a species anyway?

Seeing “species as individuals” helps to understand taxonomic disagreements.

More than ten million years. It has been more than ten million years since the Rose-breasted Grosbeak (Pheucticus ludovicianus) and the Scarlet Tanager (Piranga olivacea) diverged. Despite this significant evolutionary gap, these species still managed to produce a hybrid (see this paper for the complete description). This unusual hybrid attracted some media attention – including this piece in National Geographic – prompting some Twitter-accounts to ask the age-old question: what is a species?

Most people still think about the Biological Species Concept, defining species as “a group of organisms that can successfully interbreed and produce fertile offspring.” A very strict application of this species concept will merge any two species that produce the occasional fertile hybrid. If the cross between the Rose-breasted Grosbeak and the Scarlet Tanager turns out to be fertile, should we consider these drastically different birds as members of the same species? No, because the reality is more nuanced and complicated than blindly following the Biological Species Concept.

As I have explained in a previous blog post, most biologists adhere to the General Lineage Concept or the Evolutionary Species Concept. Both of these concepts emphasize the independent evolutionary trajectory of a species. The General Lineage Concept talks about “separately evolving metapopulation lineages”, whereas the Evolutionary Species Concept mentions “the independent evolutionary fate and historical tendencies” of a species. An occasional hybrid – such as the one described above – will not impact the evolutionary trajectory of both species and we should thus not worry about the species status of the Rose-breasted Grosbeak or the Scarlet Tanager. But what about species that regularly hybridize?

Photographs of the hybrid from (a-c) and the putative parental species: Rose-breasted Grosbeak (d) and Scarlet Tanager (e). From: Toews et al. (2022).

Species as Individuals

The situation becomes more complicated when we consider species that regularly interbreed. Think of the Golden-winged Warbler (Vermivora chrysoptera) and the Blue-winged Warbler (V. cyanoptera) in North America, or the Hooded Crow (Corvus [c.] cornix) and the Carion Crow (C. [c.] corone) in Europe. Does the production of hybrids influence the evolutionary trajectories of these lineages? Here, it is important to consider that the origin of species (or speciation) is a gradual process. Before the development of “separately evolving metapopulation lineages”, these lineages might engaged in a complicated and intricate dance of merging and diverging. Due to the continuous nature of the speciation process, it can thus be difficult to establish clear species boundaries.

To understand this issue, I have always found it useful to consider species as individuals (a philosophical perspective introduced by David Hull). An evolutionary lineage can be regarded as an individual that is born (i.e. start of the speciation process) and will die (i.e. extinction). Some individuals will reach adulthood (i.e. become species) while others will not. However, at what point does an individual become an adult? When I look at the children of my nieces, I am confident that they are not adults yet. And when I meet my uncle or aunt at a family gathering, they are clearly adults. But somewhere between the transition from child to adult, there is a gray zone. Just ask any cashier that needs to check the age of her costumers when they buy alcohol.

Seeing species as individuals. But where do you draw the line between child and adult?

Species Criteria

What characteristics would you use to define an adult? You could focus on particular morphological features, such as secondary sexual traits (e.g., the development of a beard in men or breasts in women). Or you could pay attention to particular behaviors that you consider typical for adults. You could even devise a genetic test to measure the length of telomeres. But when you apply these criteria to a group of people – aged 16 to 25, for example – you will probably come to drastically different conclusions depending on the features you focus on. Different traits – whether morphological, behavioral or genetic – will develop at different rates in different people.

The same reasoning applies to species: during the speciation process, different criteria will evolve at different times in the speciation process. The order in which these criteria evolve will be contingent upon the speciation process. In some cases, morphological differences might emerge before genetic differentiation (see for example Redpolls). In other cases, lineages might be genetically distinct despite little morphological change (i.e. cryptic species, such as in the Warbling Vireo). The result is a taxonomic grey zone where different species criteria lead to different conclusions.

This simplified diagram represents a single lineage splitting into two independently evolving lineages (or species). The horizontal lines represent the times at which the lineages acquire different species criteria. This results in a taxonomic grey zone where alternative species criteria come into conflict. Adapted from De Queiroz (2007) Systematic Biology.

Labeling Life

From the perspective of “species as individuals”, it becomes clear where most taxonomic disputes come from. Lineages that are still in the process of speciation – or even subject to reverse speciation, such as American crows and bean geese – end up in a taxonomic grey zone where species criteria come into conflict. Classifying the inhabitants of this grey zone can be extremely difficult because personal preferences of certain taxonomists and political issues (e.g., protection of endangered species) come into play. This will inevitably lead to some man-made “species” that are not strongly supported by biological data. And the ensuing debate can become heated and unfriendly.

Personally, I prefer to acknowledge the fact that some lineages cannot be easily divided into distinct species. It might be better to just refer to them as taxa – not trying to label them as “species” or “subspecies” – and focus on understanding their ecology and evolution. These resulting insights will be more interesting and fulfilling compared to putting an arbitrary label on an individual.


Hull, D. L. (1976). Are species really individuals?. Systematic zoology, 25(2), 174-191.

Ottenburghs, J. (2019). Avian species concepts in the light of genomics. In Avian Genomics in Ecology and Evolution (pp. 211-235). Springer, Cham.

Toews, D. P., Rhinehart, T. A., Mulvihill, R., Galen, S., Gosser, S. M., Johnson, T., … & Latta, S. C. (2022). Genetic confirmation of a hybrid between two highly divergent cardinalid species: A rose‐breasted grosbeak (Pheucticus ludovicianus) and a scarlet tanager (Piranga olivacea). Ecology and Evolution, 12(8), e9152.

Featured image: Birds of North America infographic © Pop Chart | Trendhunter

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