How to define species in order to recognise the boundary between two species? Biologists have been debating the best definition for a very long time.

The Biological Species Concept (BSC), proposed by the evolutionary biologist Ernst Mayr in the mid-20th century, is perhaps the most widely accepted species concept (see Encyclopedia of Life). According the BSC species are groups of actually or potentially interbreeding natural populations which are reproductively isolated from other such groups. This includes the possibility that organisms may appear to be alike and be different species or organisms may look different and yet be the same species (see Understanding Evolution). This definition of a species as a group of interbreeding individuals cannot, however, be easily applied to organisms that reproduce only or mainly asexually.

Within the BSC, a species represents a set of individuals connected by gene exchange ("gene flow") that is genetically isolated from all other such sets of individuals. There is gene flow among individuals within a species, but not between different species. This lack of gene exchange means that different species can evolve independently.

Other approaches to define species focus on lineages in branching evolutionary trees, but besides that the two perspectives on the speciation process (the origin of new species) are not mutually exclusive, they also not explain all issues that appear when trying to identify all different species in nature using a universal definition.

Therefore most biologists today are more interested in understanding the process of speciation than in trying to find a strict species definition that is always applicable. Speciation is usually a gradual process, so it is not unusual to encounter populations that are only partly reproductively isolated. This means that individuals from diverged lineages may still exchange genes to a limited degree, perhaps even to the extent that they will merge again. These situations are challenging for both the biological species concepts and lineage species concepts. Although some people may wish for a black-and-white criterion for defining species, this is unrealistic.

Recently, in March 2017, a novel intriguing and controversial idea was published: the mitonuclear compatibility species concept, which is based on the fact that the mitochondrial genotype tends to be very good at showing species boundaries between birds.

Appearances can help and deceive
Although, figuring out the relationships among species is much more complex than simply grouping by similarity in appearance:

For example, two organisms that are not closely related (such as a cactus and a euphorb) may resemble each other because they have evolved similar adaptations for similar environmental conditions (this phenomenon is known as ‘convergent evolution’). Alternatively, two individuals that appear morphologically identical may nevertheless belong to distinct reproductively isolated species – with reproductive isolation based, for example, on different sex pheromones, songs or mating behaviours (distinct species that are morphologically indistinguishable, or nearly so, to human scientists are known as "cryptic species").

Eastern meadowlarkWestern meadowlark

The Eastern meadowlark (left) and the Western meadowlark (right) appear to be identical, and their ranges overlap, but their distinct songs prevent interbreeding.

Furthermore, many plants, and some animals, form hybrids in nature. Hooded crows and carrion crows look different, and largely mate within their own groups – but in some areas, they hybridize. Should they be considered the same species or separate species?

Crow hybridization

In practice careful analysis of physical characteristics has been successfully used to infer which groups are probably reproductively isolated and represent independently evolving lineages, leading to field guides. So, without actual knowledge of gene exchange and genetic differences an educated guess could be quite accurate. But when, in recent decades, DNA sequence data became available a more precise assessment could be made of the evidence of gene exchange in recent generations and estimating the evolutionary relationships among living organisms.


(Source: website Understanding Evolution; website Encyclopedia of Life; Wikipedia; naturesongs.com/birds)


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"Tiger map" (CC BY 2.5) by Sanderson et al., 2006.


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