Ring Species and Universals: A Contradiction?

[rvilbig]

One proposed definition for a species is the following: "“two individuals belong to the same species if their gametes can unite with each other under natural conditions to produce fertile offspring.”

If this is the universal definition of a species, then it is violated in nature by ring species. Ring species are a population in which neighboring subsets can interbreed while the subsets at the endpoints cannot. In a sense then, the endpoints are simultaneously two distinct species (because they can’t interbreed) and yet one species (because they can interbreed via the neighboring intermediaries.) Does this suggest that species do not really exist? Do all species blur together? Is the universal for “species” merely a nominalist construct?

I’ll offer my proposed solution in the following post, but I’d appreciate others offering alternative explanations if they have them. Thanks,

-Ryan Vilbig
ryan.vilbig@gmail.com

 

[rvilbig]

The solution I offer is the following: no universals are ever perfectly expressed in the known universe, and so the fact that the universal definition of species is not perfectly expressed does not discredit the idea of the universal for species.

Consider a circle: by its universal definition, it is defined as the collection of all points equidistant from a fixed point. However, every circle that can be drawn in our universe only imperfectly manifests this universal form. Circles drawn with a compass will have imperfections, and even circles drawn with atomic arrangements will vary with quantum fluctuations. So no known circle is a perfect instantiation of the universal form “circle.” Nonetheless, we still believe that the universal for circles exists and that all other circles are defined as such by comparison with this unseen universal. Likewise with species, all species are imperfect instantiations of the universal (due to ring-species situations, or even infertile individuals who fail the definition), and yet we still believe that all species are considered as such due to their comparison with the universal form.

I think Newman had some insight on this:

“[T]hese variations imply, instead of discrediting, the archetypal idea, which is but a previous hypothesis or condition, by means of which issue is joined between contending opinions, and without which there would be nothing to dispute about.” (Idea of University, p. 83).

Are there any other solutions to this ring-species problem?

-Ryan Vilbig
ryan.vilbig@gmail.com

 

[JohnDamian]

In the problem of universals; the Genus-Differentia is not solley to do with biology. It is explained by Aristotle that any Genus has sub-species that are divided by differentia in the individuals. For example;

Cats belong to the genus animal; but the differentia rational divides humans from other animals. The genus animal; with the other genera such as plant; belong to categories of substances. The ontological status of these universials is what is problomatic.

It is the account of common natures that teaches us that univesals have an ontological reality. That nominalism is incorrect. Common natures such as humanity and animality exist; even though their existence is lesser than individuals. They are common in themselves and in reality. They combine with contracting individuators to individuals. Of course; this nessecarily means that there must be a unity below a numerical unity; but this is true because if all unity were numerical; all diversity would be numerical; however any numerically diverse thing is equally diverse; the idea that I am as diverse from you as from a dot is absurd. Thus; there is a less than numerical unity present in common natures; which are contracted through individuators to indidivuals.

 

[Just_Lurking]

Two mathematical solutions pop to mind. Suppose that the following interbreedings are possible: A with B, B with C, and C with D.

As the Wikipedia article points out, the key problem is that this relation is not transitive, and thus not an equivalence relation.

First solution: Define the “same species” equivalence relation as the transitive closure of the “able to interbreed” relation. Then A, B, C, and D are all members of the same single species.

Second solution: Extend the definition of species beyond equivalence classes of transitive relations. A species becomes a “maximal set of individuals that are able to interbreed pairwise with each other”. Then there are three species: AB, BC, and CD, with B belonging at the same to both species AB and BC, and with C belonging at the same time to both species BC and CD.

I don’t have enough philosophical intuition to be able to assess either of these solution suggestions from a philosophical perspective.

 

[didymus]

Perhaps I’m being obtuse but I don’t see a “ring” here – it looks like an open loop. The straight line is just as good a representation, did someone twist it to add some mystery?

What I find more interesting is that there are genetically identical populations that no longer interbreed because they have developed separate breeding habits or have moved into different habitats so they don’t interact so they never breed. Are they now different subspecies?

How well does “ability to interbreed” work in the real world? I mean when you breed a horse with a donkey you get a sterile mule.
When these neighboring bird populations interbreed are they fertile, do they contribute hybrid vigor or just die out in the next generation.

Last word on definitions: as Justice Stewart Potter said of pornography, “I know it when I see it”. Perhaps not a high philosophical standard but one I suspect most humans incl. most scientists are guided by.