Convention (4) – Biological species

by Neil Rickert

This series of posts on convention originated with my comment to a post by John Wilkins, that I see species as being determined by convention.  See the first post in this series for links.  John disagrees with me, and gave reasons for his objections.  I plan to discuss those objections in the next in this series.  Today’s post will discuss why I take the designation of species to be conventional.


Biological classification is an example of categorization.  I take categorization to be a dividing up of the world into manageable parts.  This is often described as “carving the world at the seams.”  However, there aren’t enough seams to account for how we carve up the world.

As an example, consider the dividing of the USA into fifty states.  Some of the state borders are along rivers.  Some are survey lines.  We could perhaps think of rivers as natural diving lines, or seams, except that we often don’t use them even when rivers are available.

I find it better to take this dividing up to be done on a pragmatic basis.  Sometimes rivers might be useful.  At other times there are different considerations.  So we end up dividing the country in a way that does not fit “carving at the seams.”  I take this dividing up to be an example of the use of social conventions.

Biological classification

With that introduction to categorization, let’s look at how biologists classify.  They classify into a hierarchy, with phyla subdivided into classes, subdivided into families, subdivided into genera and subdivided into species.  I have not attempted to list all of the components there.

Many biologists say that this division is arbitrary.  By “arbitrary”, they do not mean random.  They only mean that it could have been different.  Or, otherwise said, nature does not dictate how to do this classification.  In the terms of the previous section, the dividing is not all along seams.  Or, in the terminology of my previous post in this series, there is no canonical way to divide up the organisms into groups.

The species level is a bit different from the other levels.  There is at least a semblance of a principle for dividing into species.  That principle is that a species should consist of a population that interbreeds.  This is probably why John thinks that division at the species level is not conventional.  And there are probably quite a few biologists who would agree with considering species an exception.

I see the interbreeding principle as a bit like the use of rivers as division marks between geographical states.  It can sometimes be a good pragmatic choice.  But it isn’t always useful, and we then have to make different choices.

Let’s examine some cases where the interbreeding principle doesn’t seem to work.

Ring species

A ring species is a group of related species that form a ring.  An example might be species of related frogs around a lake.  What often happens is that there is some breeding between two adjacent species in the ring,  but not between species that are far enough separated.

The question that then presents itself is:  Why are there seven species in the ring?  Why not eight, or six?

This question is what illustrates that there is something a tad arbitrary about the division into species.  Apparently there is no canonical way to divide, so we must instead establish conventions.

Asexual species

The interbreeding principle cannot be easily applied for species that reproduce asexually.  And asexual reproduction is quite common, particularly at the micro-organism level.  So here is another case where the interbreeding principle does not determine the choices.

Separated species

There are some cases of species that could interbreed, as shown by lab testing.  But they just don’t even though the live near enough to each other that they could.  These are usually taken to be separate species.

There are other case where the species are physically separated, so could not interbreed in practice.  But, perhaps if they were brought together they could and would.  That makes for a difficult decision, and biologists often use other criteria (morphological distinctions) to decide whether they are separate species.


I’ll mention the example of baraminology.  The proponents of baraminology are mostly creationists and mostly not biologists, so perhaps they should be ignored.  They reject the whole idea of species.  Instead, they want to divide into traditional kinds, as mentioned in the Bible or perhaps in other classical literature.

Most biologists won’t have any interest in baraminology.  But it does at least raise the question of whether the interbreeding principle is itself a convention.  I’m inclined to think that it is, though it is no doubt a very good and useful convention.


My view, in summary, is that nature does not determine how it we should divide up (or categorize) biological organism.  The way that we currently do it is pretty good.  But that only says that scientists are pragmatists.  If nature does not dictate our choices, if there is no canonical way of dividing up,  then we should see our actual dividing as a matter of developing useful categorization conventions.

My next post in this series will plan to answer some of the objection from John Wilkins.

One Trackback to “Convention (4) – Biological species”

%d bloggers like this: