Biologists are often asked what the definition of a species is. Before dealing with this question it is important to consider the differences between what a species is and how species come into being. The ‘what’ question deals with the main subject of this essay, how do you define a species. The ‘how’ question deals with processes and mechanisms. I would like to write about this part first; partly because it is the really contentious and interesting part of the question and partly to clear up any misunderstanding between definitions (what) and mechanisms (how).
Species, at their most abstract, can be thought of individual historical entities. This is to say, like the most basic story outline, they have a beginning, middle and an end. Or, if you prefer, a birth, lifespan and a death. Just as all people do. So species, like people, exist at certain places and certain times.
How the birth of species comes about is a question of mechanism(s) that I will not go into in very much detail, but the key image to have is the notion of a single homogeneous population (group of individuals) that eventually gives rise to two distinct populations that can no longer combine their genetic material to give rise to more of their kind. The most widely accepted method by which this can come about is the subdivision of the single population into two subpopulations by the appearance of a geographic barrier (a mountain range, a new river) that can no longer exchange genes. As time passes then the genetic make-up of the two subpopulations could evolve enough to generate sufficient differences that would mean they are no longer two subpopulations, but are in fact different species. It should be clear that at least some time has to pass for enough differences to accumulate for the two subpopulations to become separate species, but the amount of time varies, due to a wide range of factors.
Despite what many people believe scientists HAVE witnessed the evolution of new species (http://www.talkorigins.org/faqs/faq-speciation.html), but the difficulty in communicating these findings to non-specialists is that the MECHANISM is often quite difficult to understand (e.g. self-fertilization and chromosome duplication in plants) or involves organisms such as bacteria that don’t conform to most peoples’ ideas of animal species that they are familiar with.
Which species concept is best?
Species concepts are best thought of as tools for a job. As an example, think about your five favourite books. This is a meaningful way of categorizing books, but is relevant to you and has nothing to do with the intrinsic properties of the books themselves. Most scientists contend that species are entities that have a real existence outside of human thought, and our scientific concepts should correspond to real entities in nature that would exist whether or not humans had names for them. Thus concepts such as the biological species concept attempt to mirror nature. Conversely, species could be erected using morphological or genetic data that do not reflect shared evolutionary heritage. An example would be old classifications that included whales and dolphins (mammals) within fish. They do indeed share some similarities with fish (fins, living in water) but not as the result of having a shared evolutionary history. Nonetheless, such classifications can be useful for identifying organisms, but they do not take evolutionary history into account.
Now we will go on to the various species definitions used by different areas of biology. That’s right, definitions plural. Different areas of biology use different species concepts, depending on the organisms and data available . Each of the major species concepts is explained below, along with the advantages and disadvantages of the species concept and which areas of biology use the definition. In several cases below I have combined several finer distinctions of species concepts into a single definition, as I think the distinctions are not so important for an introductory essay. For those who want to know more, some suggested sources are at the end of the essay.
Biological Species Concept (BSC)
This is the species concept that many readers will have encountered and has the benefit of having a short, snappy definition. The BSC was advanced by Ernst Mayr in 1942. Mayr is still active and has worked in many areas of biology, but his seminal work was on birds. The standard wording is along the lines of:
‘A biological species is a group of interbreeding or potentially interbreeding organisms that can produce viable offspring.’
Advantages: This is a species concept based on a mechanism. Either individuals can or cannot produce viable offspring, although failure may be down to individuals. The issue of potential interbreeding arises when members of populations are separated but are brought into contact in a lab, or by chance mechanisms.
Disadvantages: Many living species have never been observed mating and then those offspring mating and raising viable. Fossils certainly can’t mate any longer, although we have to classify them. Some living organisms, e.g. bacteria don’t have sex but can swap genetic material among themselves.
Morphological/Phenetic/Typological Species Concept
A species defined using the morphological species concept is called a morphospecies, and the recognition is based on overall similarity. Individuals do not have to be exactly the same as each other, because there is variation in morphology among most species (think how variable people are) and morphospecies take account of this fact.
Whatever most people tell you, most species are still classified by their anatomical characteristics, even bacteria have distinctive shapes. So nearly all biologists make some use of this species concept some of the time. When I am out birdwatching the species I recognize are based on their morphology. Professional ornithologists may have witnessed all of the evidence for breeding, but I just note the birds’ morphology, watch their behaviour and listen to their songs, none of which proves they are good biological species.
Advantages: Morphology can be readily observed, in many cases without handling or harming the organisms. It is relatively easy to communicate with a whole range of people about morpohology.
Disadvantages: This species concept has several disadvantages. In the past researchers classified male and female individuals of the same (biological) species into separate species. In the fossil record variation in morphology among species may be vast. The opposite problem can occur with living animal species when two species are morphologically very similar, but are genetically distinct (cryptic species). This concept also relies on human interpretation to define species, rather than always allowing natural mechanisms to determine species.
Phylogenetic/ Cladisic Species Concept
The phylogenetic species concept considers the evolutionary relationships among organisms and relies on common ancestry and shared evolutionary history to define species. Think of this as a distinct branch on an evolutionary tree.
Advantages: Recognizes the role of history in generating species explicitly and can be reasonably objective. Can use nearly any sort(s) of data.
(The subdivision of lineages into species can be rather arbitrary and subspecies are not recognized. While the latter point may seem trivial, it has legal ramifications for wildlife protection law.
Genetic Species Concept
The genetic species concept can be thought of as the geneticists’ equivalent of the morphospecies concept, but the measure is genetic similarity or distance. Genetic analyses can uncover cryptic species that morphological studies would not.
Advantages: Can provide independent evidence for morphological and biological species. For bacteria and small organisms genetic species concepts can be very useful and save a lot of time. With automated sequencing and web databases it is now very quick to analyse DNA. It is also possible to get many samples from one individual.
Disadvantages: This species concept also relies, to some extent, on human judgement of how much difference is enough to constitute separate species. Taking DNA can be invasive and unnerving for an animal, although many field studies can now use discarded hair, feathers, skin and so on for sequencing. Situations can arise where DNA samples get contaminated with the DNA of parasites living on or in an organism. Communicating with non-specialists about DNA taxonomy can also be very difficult.
This species concept also uses DNA, but covers organisms that reproduce asexually to give rise to exact genetic copies of themselves.
Advantages: Clear criteria and useful for microorganisms (hence the name) and some plants.
Disadvantages: Some organisms may reproduce asexually, but have high mutation rates and are thus not identical copies.
Cladistic Species Concept
The cladistic species concept relies on the possession of certain shared, derived characters (called synapomorphies) among individuals
Ecological Species Concept
This is used to describe populations that are adapted to certain ecological niches and because of their adaptations will form discrete morphological clusters.
Advantages: Acknowledges the role the environment plays in controlling morphological development.
Disadvantages: Can miss cryptic species. Niches tend to be assumed and are difficult to define completely. Many taxa exploit overlapping resources, or can suddenly switch if a resource becomes scare. This is not a very robust species concept.
This is an interesting species concept that essentially lets the organisms be the guide as to which species they belong to by observing which other individuals they mate with. While this seems like a good idea, we have to remember that other organisms get their ‘species concepts’ wrong too! Many ducks will mate with ducks of other species, so just because mating is observed it can, in some instances, be misleading. In some cases the ducks are still closely related enough to produce viable offspring (hybrids), although the parents are members of fairly distinct species. This is different from the biological species concept, as all that has to occur is a mating attempt. Some species have very clear concepts of members of their species of the opposite sex, while others can get confused about sex and species.
Advantages: Does have a mechanism-based test, albeit not a particularly reliable one. However, this concept can help to outline research questions. I saw a male of species x mate with a female of species y, do they produce viable offspring? Also a non-invasive, observational concept. Other qualifiers can be added to this concept to cover what happens after mating.
Disadvantages: Not very clear what advantages this species concept has on its own. The biological species concept covers more cases and mechanisms. Like the BSC this species concept applies only to sexually reproducing organisms.
This essay is a very brief outline of what species concepts are, how they are defined, and who uses them. Any good undergraduate evolutionary biology textbook will go into most of these concepts in more detail and you can, of course, have a look on Wikipedia (http://en.wikipedia.org/wiki/Species). For those who want to know about the ‘cutting’ edge of research and thinking about species and speciation, I would recommend the book ‘Speciation’ by Jerry Coyne and Allan Orr.