Species
From SkepticWiki
Contents |
[edit] Introduction
The definition of species is problematic. The reason for this is that we have an intuitive idea of species as discrete distinct biological groups into which all organisms can be partitioned, with each species being interfertile among itself; and this does not correspond to what we actually find in nature. The upshot of this is that if we want "species" to mean something precise and meaningful, we have to abandon some of the intellectual baggage that goes along with the concept.
The question is, then, which parts of the intuitive species concept we retain, and which we get rid of, in order to make the species concept meaningful. You will notice that this is not a dispute about the facts, which are agreed on by all parties. Rather, it is a dispute about how the word species can most sensibly be defined; it is a semantic or even a philosophical question.
[edit] Problems with the intuitive species concept
Now, suppose we make successful interbreeding the criterion for being in the same species (as in the Biological Species Concept, below). As you can see, B. nigra crosses happily with B. rapa, and so are the same species; and B. rapa crosses with B. napus, so they are the same species. But B. rapa will not cross with B. napus, so they are different species. We have a situation where nigra is the same species as rapa, and rapa is the same species as napus, but nigra is not the same species as napus. To use mathematical jargon, the relation "is the same species as" is not transitive. This means that if we take interbreeding as our criterion, species do not partition organisms into groups. The same may be said of the criterion of shared or separate gene pools.
We might consider putting this situation right by taking the transitive closure of the Biological Species Concept --- that is, we add a rule that says that if A is the same species as B, and B is the same species as C, then we will consider A to be the same species as C. Some creationist criteria for "Created Kinds" appear to correspond to this approach. The problem is that now, although we have restored the partition property, we find that Brassica is now one big species, consisting of varieties some of which are completely genetically incompatible and unable to breed. We have restored the partition property of species only at the cost of removing the fertility of a species with itself.
This solution has other strange consequences. Imagine that some virus (call it Brassica Blight) wipes out B. carinata, B. juncea, and B. oleracea. So far, Brassica remains all one species. But now let the Brassica Blight render B. rapa extinct also. Then B. nigra and B. napus would be different species according to the stated rules, and an extinction event would have produced two species where there was only one before.
On the other hand, suppose we try focussing on the character traits by which we distinguish the species. Then we find that there are six species of Brassica again, each of which can clearly be distinguished from the others. Only now we have different species which are capable of interbreeding, which again doesn't fit with our intuitive species concept.
The existence of evolution presents another problem for the species concept. Except in the case of genetic oddities such as polyploid speciation, each organism is the same species as its parents, and yet an evolutionary line of descent can turn one species into another. Again, we see that the species concept is not transitive: we can have a line of descent in which organism 1 is the same species as organism 2, organism 2 is the same species as organism 3, and so forth, and yet organism 10,000 is a different species from organism 1; this is so whether our criterion for species is by interfertility or by character traits.
We might attempt to restore the partition property by drawing an arbitrary line, and saying that organisms 1 - 5,000 belong to one species, and organisms 5,001 - 10,000 belong to a second species. The problem there is that by any genetic or morphological criteria, organisms 5,000 and 5,001 would in fact be judged the same species.
The reason paleontologists are able to split fossils up into distinct species is because the fossil record is incomplete: it does not contain every organism that ever lived. It is only the discontinuities in the available fossil data that allows the partition of fossils into distinct species, and this breaks down when we have intermediate forms between species (as opposed to intermediate forms between higher taxa), and such transitions are to be found in the fossil record. [1] So the study of evolution shows that the question of what consistutes some given species is not black and white: it must have some gray areas.
To confuse matters further, we might make mention of cryptic sibling species: closely related species which cannot interbreed for genetic reasons (such as a change in chromosome number) but which look absolutely identical, and which by eye we wouldn't even identify as distinct varieties, let alone species.
It is difficult, then, to make the word "species" mean what we feel it ought to mean. In some cases, this doesn't matter: we can say that giraffes, for example, are a species without any ambiguity. In cases where there is ambiguity, as with the relationships of the Brassica genus, it is possible simply to state the facts. You are not obliged to summarise their relationships by deciding whether they are one species or six; you can state, as we have stated, exactly what the interbreeding potential is between the various groups without taking a position on what species should mean. Or, again, discussing a fossil intermediate between two species, you are not obliged to categorise it as one or the other: you could give details of the mixture of traits it exhibits.
Finally, to avoid all ambiguity, we may use one of the species concepts listed below, and say which one we're using. These do not match our intuitive idea of what a species is, but they are well-defined.
[edit] The Evolutionary Species Concept
The Evolutionary Species Concept defines species as follows:
- An evolutionary species is a lineage (an ancestral-descendant sequence of populations) evolving separately from others and with its own unitary evolutionary role and tendencies. (Simpson, 1961)
This is an important and useful concept, but the term "population" is already in use for it. It does not in the least satisfy our intuitive concept of what "species" should mean, since in principle it can require us to recognise as different species animals which are merely geographically separate, which are not reproductively isolated, and which don't even look different from one another.
[edit] The Phylogenetic Species Concept
In the Phylogentic Species Concept, species is defined as follows:
- The smallest aggregation of populations (sexual) or lineages (asexual) diagnosable by a unique combination of character states in comparable individuals. (Nixon and Wheeler, 1990)
In effect, this adds to the Evolutionary Species Concept the idea that we should be able to distinguish between two species according to their traits.
This species concept runs counter to our intuitions because it identifies as different species two distinguishable varieties which may in principle be able to breed, but which for reasons of purely geographical separation do not do so. If the human race consisted only of the Masai and the Cantonese, and they never met, they would be two species by this definition.
The Phylogenetic Species Concept has been growing in popularity lately, at the expense of the next candidate, the Biological Species Concept.
[edit] The Biological Species Concept
The Biological Species Concept of Ernst Mayr has long been the most popular formal concept of species: it is defined by the capacity to interbreed.
- Species are groups of actually or potentially interbreeding natural populations, which are reproductively isolated from other such groups. (Mayr, 1940)
We emphasise that mere geographical isolation is not reproductive isolation: according to the Biological Species Concept it is required that two organisms should have, as Mayr says, the potential to breed rather than the practical ability to do so, in order to be classified as the same species.
The Biological Species Concept requires us to abandon some intuitive concepts of what "species" should mean, in particular, as we have noted above, it does not satisfy the requirement that it should partition organisms into distinct non-overlapping subsets.
