Or in other words, why is the species problem a problem? The "mutually interbreeding kinds" definition seems natural enough; in fact, it's as ancient as the Bible, as shown by Leviticus 19:19

You shall keep My statutes. You shall not let your livestock breed with another kind. You shall not sow your field with mixed seed. Nor shall a garment of mixed linen and wool come upon you.

Even the ancients seem to have adopted the biological species concept! What are the philosophical problems with it as compared to other concepts, like morphological species concept, or the typological species concept?

  • ucpress.edu/book.php?isbn=9780520271395 Blurb: This comprehensive work takes a fresh look at an idea central to the field of biology by tracing its history from antiquity to today. Wilkins explores the essentialist view, a staple of logic from Plato and Aristotle through the Middle Ages to fairly recent times... Tracing “generative conceptions” of species back through Darwin to Epicurus, Wilkins provides a new perspective on the relationship between philosophical and biological approaches to this concept.
    – Uticensis
    Jun 8 '11 at 0:44
  • Also note that an entry on species can be found in the Stanford Encyclopedia of Philosophy: plato.stanford.edu/entries/species
    – Uticensis
    Jun 8 '11 at 0:45
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    Sorting out just what account to give of species is one of the central problems in the philosophy of biology. While the question's appeal to the Bible isn't especially helpful, the question is clearly on topic.
    – vanden
    Jun 11 '11 at 21:02

First, not all species are capable of breeding. How are we then to distinguish different species of bacteria?

Secondly, what about a continuum of mammals, of which each animal can breed with the animals close to it in the continuum, but not with others farther off? Where are we then to set the boundary?

For example, I could hypothetically breed with all of my female ancestors down to a certain time in the past, but not beyond. Should that be the point in time when my ancestors became a different species? But my ancestor X, who lived just after that time, could breed both with me and with some of her ancestors, down to our ancestor Y, etc. If all my ancestors are one species, I should be able to breed with Y's sister, which I cannot. If they are not all one species, the boundary of my species must lie somewhere between X and Y—but then, given the above, X and Y would be able to breed across species. This leads to contradictions.

  • 1
    This sounds like a Sorites paradox (in that there really is none). With respect to ancestors, yes, it's pretty continuous, and labeling a group of individuals as a particular species needs an arbitrary vague cut-off.
    – Mitch
    Oct 20 '11 at 15:53
  • 1
    Is there really a continuum of mammals? Do you have any examples?
    – Mitch
    Oct 20 '11 at 16:03
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    @Mitch: I don't know this paradox. I don't know whether there exists such a continuum of mammals, I think we need a more precise definition. But I was wondering about dogs: a Chihuahua and a Great Dane cannot and/or do not interbreed; dogs are said to be a ring species. If you define the boundary as "do these populations exchange genes in practice, yes or no?", then a ring species is somewhere in between; but two populations who could breed but live on different islands would then be two different species.
    – Cerberus
    Oct 20 '11 at 16:28
  • ... If it is just the ability to breed, we'd need to be more precise: does it count if the male is able to mount the female? If she is able to grow a foetus from the mating? If she is able to give birth to it? If the offspring survives? If it is able to reproduce in turn? See the excellent answer by Cirbryn on our competitor's page (scroll down): answers.yahoo.com/question/index?qid=20081106142908AAHTUsZ
    – Cerberus
    Oct 20 '11 at 16:32
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    The Sorites paradox is a difficulty of vagues and naming. Start with one grain of rice. Is it a heap of rice? No. Add a grain. How about now? Keep adding. adding a single grain of rice 'cannot' turn a non-heap into a heap. (the problem with this obviously problematic argument is that 'heap' is vague. 'Species' is vague similarly.
    – Mitch
    Oct 20 '11 at 16:55

My answer is twofold:

One is that the species problem is raised by attempts to give an explanation for species in terms of Mendelian genetics (particularly Johannsen's "pure line" theory), which failed and set a problem up, to which answers were given by Dobzhansky and Mayr, repeating and Mendelising the old reproductive isolation definitions that go back to Blumenbach. Dobzhansky published "The Species Problem" in Philosophy of Science in 1935.

The other is that there were attempts (beginning with Joseph Woodger) to set biology into a logical axiomatisation, that led to questions of species essences and definition.

So on the one hand we had biologists arguing over what caused species and on the other we had philosophers trying to formalise biology.

There was no species problem to speak of before around 1900.


As I see it the problem with using the ability to interbreed as the hallmark of a species is that the ability to interbreed is itself a matter of degree, in a number of different ways. Some have been mentioned above, such as does the male have to mount the female, etc., but I'd add others such as does the offspring have to be fertile? Does it have to be possible for mating to occur naturally in the wild, or is it enough if it's only possible with human intervention? In this context Daniel Dennett, in Darwin's Dangerous Idea talks about chihuahuas and Great Danes, as mentioned above. Both counted as of the same species but clearly in the wild interbreeding is unlikely to say the least and even with human assistance might not be possible. Another aspect of the problem is that we think speciation mainly occurs as a result of reproductive isolation - but that too is a matter of degree. Populations can be more or less isolated (both geographically and in terms of mate choice.) A type of Sorites phenomnon seems to arise but seeing it as a problem seems to me to result from the failure to fully take on board the fact that speciation is not an event but a historical process. We represent it with a branching pattern but if you think about actual branches on real trees where exactly would you say you no longer have one branch, but two? There is no precise point. We're able to say clearly this part is one branch, and these parts are two, but there's a vague area in between where any drawing of the line would be arbitrary. While in that vague area for populations of organisms that a relatively reproductively isolated, there is no definitive answer to the question as to whether we still have one species or now have two. And so as a result the boundaries between species are similarly fuzzy. That is not in itself a criticism - it may just be that many concepts in biology are fuzzy, and so be it.

  • 1
    First off welcome to philosophy.SE. I haven't read you answer carefully for one very important reason -- right now, it's a text wall. You can split it into paragraphs with double enters between each new paragraph.
    – virmaior
    Dec 1 '16 at 7:01

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