What’s a species 1: Horizontal species concepts

by Piter Kehoma Boll

What’s a species?

Maybe to you that may sound like something too obvious to think about, but actually the concept of species is one of the most intriguing and controversial topics in biology. Sometimes it’s not so easy to distinguish one species from another and that may lead to problems in several fields of biology, including not only systematics, but also ecology, conservation and evolution.

Many times, of course, two individuals can easily be recognized as different species. Nobody would doubt that a giraffe and a banana tree belong to different species, right?

A giraffe and a banana tree, surely not a couple. Pictures by Anna Cervova and Andrew Schmidt.

Now look at these two butterflies. Are they different species?

Two very similar butterflies, Danaus erippus (left) and D. plexippus (right). Photos by Gabriela ruellan* (flickr.com/people/56823778@N00) and David Wagner.

As you can see now, sometimes it’s not so easy to say if a group of individuals forms one or more species.

Trying to solve this problem, many concepts of what’s a species have arisen through the decades. As far as 22 concepts were elaborated, trying to cover all kind of situations related to species differentiation. Those concepts can be divided in two different approaches of the problem: horizontal and vertical. When analyzing species in a horizontal way, we look at them how they are in the present, comparing the populations in the way they look like, behave and are distributed. On the other hand, a vertical approach considers how species happen through time, putting priority on historical and evolutionary aspects.

Here I’ll make a quick review of the three main horizontal species concepts, which are the most practically used as we try to define what a species is.

1. Biological species concept

Probably the best known concept and the most highly applied. It defines a species as a set of organisms where individuals recognize one another and seek each other for mating, so maintaining the intercommunication of their genes. A biological species is isolated from other species by intrinsic or extrinsic features that prevent interbreeding.

In other words, a biological species is a set of organisms able to interbreed and have fertile offspring. A classic example of two biological species is the horse Equus ferus caballus and the donkey Equus africanus asinus which can breed to produce a mule, but that’s sterile.

A mare, Equus ferus caballus (left), a donkey, Equus africanus asinus (right) and a mule (center). Photos by ‘Little Miss Muffit’* (flickr.com/people/42562654@N00)(mare), Adrian Pingstone (donkey) and Dario Urruty (mule).

Even though this concept applies well to most plants and animals, it doesn’t fit to bacteria and other microorganisms that reproduce only asexually.  Despite that, some plant species can easily form fertile hybrids between distinct species. Orchids are champions on that, with lots of hybrids species arising from crossing species of the genus Cattleya.

Cattleya forbesii (left), C. guttata (right) and their hybrid, Cattleya x dayana (center). That’s a naturally occuring fertile hybrid. Photos by Dr. Volkmar Rudolf (C. forbesii), Wikimedia Commons’ User Orchi (commons.wikimedia.org/wiki/User:Orchi)(C. guttata) and Adilson A. Filho (www.flickr.com/people/adilsfilho/)(C. x dayana).

2. Ecological Species Concept

An ecological species is a set of organisms belonging to a single or closely related lineages that basically occupy the same niche in an ecosystem, i.e., have the same habitat and the same habits and needs for physical resources and conditions to survive.

Due to the fact that different species use ecological resources differently, they use to become divergent in aspect, behavior and location, so leading to isolating from one another. Maybe they would be able to interbreed, but it doesn’t use to happen because of their different locations or time of mating.

Two species of giant roundworms, Ascaris lumbricoides and A. suum, are very closely related and similar in shape, but the first is a parasite of humans and the latter a parasite of pigs, so that they are isolated from each other for using different habitats.

Pictures of Ascaris lumbricoides and A. suum. Almost identical, but living in different hosts. Pictures by the Centers for Disease Control and Prevention, U.S. federal government and by nematodes.org, respectively.

Another example are the grizzly bear Ursus arctos and the polar bear Ursus maritimus. Even though living in different habitats and having different behaviors, including the fact that the grizzly tends to mate on land while the polar mates in the water, several hybrids have been reported, including wild ones, and they are fertile, so that by the biological concept, they would belong to a single species, even though by ecological aspects they are quite different ones.

Taxidermied “prizzly bear”, a polar-grizzly bear hybrid, at Rothschild Museum, Tring, England. Picture by Sarah Hartwell*, extracted from wikipedia.

3. Phenetic species concept

This concept defines basically that a species is a set of organisms that look similar enough, i.e., similarity is the primary criterion for defining a species. Different from the two previous concepts, this one simply considers that species exists, but doesn’t justify how they came to be so.

Despite of its apparent inaccuracy to define natural species, it’s actually the primary method used to differentiate species. When a new species is described, it is usually so defined by comparing it to species already known, highlighting morphological and behavioral aspects.

Two beautiful species of macaw, the blue-and-yellow macaw Ara ararauna and the scarlet macaw Ara macao, are quite different in coloration, so considered different species. Their habitats overlap in nature, but they do not produce hybrids except in captivity, and those are not fertile.

Blue-and-yellow macaw, Ara ararauna (left), Scarlet macaw, Ara macao (right) and their sterile hybrid Catalina macaw (center). Photos by Wikimedia Commons’ user Fiorellino* (A. ararauna), Matthew Romack* (www.flickr.com/people/stoichiometry/) (A. macao) and Wikimedia Common’s user Arkansas Lad* (catalina macaw).

However, this concept doesn’t work many times. A typical example is when considering two fruit fly species, Drosophila persimilis and D. pseudoobscura. They look almost identical to each other, but when put together, they never breed, so indicating that they actually do not belong to the same species.

Two different species that look almost identical, Drosophila persimilis (left) and D. pseudoobscura (right). Photos by BIO Photography Group*, Biodiversity Institute of Ontario, extracted from boldsystems.org

I hope that now you are starting to see why is not so easy to tell where one species begins and the other ends. Perhaps there is no such thing as a species after all, at least not like we think about it.

– – –

* These images are licensed under Creative Commons license, hold by their respective owners.


Mayden, R. L. 1997. A hierarchy of species concepts: the denoument in the saga of the species problem, in M. F. Claridge, H. A. Dawah and M. R. Wilson (eds.), Species: The units of diversity, London: Chapman and Hall, 381-423

Ridley, M. 2004. Evolution. Blackwell Publishing. ISBN 1-4051-0345-0.

Smith, D., Lushai, G., & Allen, J. 2005. A classification of Danaus butterflies (Lepidoptera: Nymphalidae) based upon data from morphology and DNA Zoological Journal of the Linnean Society, 144 (2), 191-212 DOI: 10.1111/j.1096-3642.2005.00169.x



Filed under Ecology, Evolution, Systematics, taxonomy

5 responses to “What’s a species 1: Horizontal species concepts

  1. HI Piter, really thought provoking post. Personally, I think that the concept of a species, however one may define it, is important especially in the area of conservation biology. Knowledge and awareness of species is crucial in protecting biodiversity and in maintaining a vibrant (and as yet, somewhat unexplored) bank of genetic material.

    • Piter Keo

      Hi. Thanks for your comment!
      I agree with you that the knowledge of species is really important for conservation biology, along with, as we have been learning in the last years, the knowledge of genetic diversity. However, there are much debate about the main aspects to be highlighted in conservation, especially concerning the fact that natural changes caused by human influence cannot be separated from those that occur by the influence of nature only, leading to many dilemmas in conservation. For example, here in Rio Grande do Sul, Brazil’s southernmost state, we have a biome characterized mainly by steppes and other of mixed rain forest. Despite the high rates of forest removal in the last century, currently, with more strict laws preventing it and actually protecting some tree species from being cut down, there’s a tendency of the forest to invade steppes, and that leads to troubles concerning conservation, since there’s always that doubt about how much of it should be repressed to conserve the steppes and their unique species.

  2. Pingback: What’s a species 2: Vertical species concepts | Earthling Nature

  3. Excellent post. I open my intro biology course with this question – well, it’s one of several questions I bring up on day one. What is it to be alive? How and why do we categorize life? What’s a species? oh yeah, and What’s science anyway?

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