species definitions

What is a "species"?


This question is important. The scientific system of naming "kinds" of plants and animals revolves around the species level. A scientific name, such as Turdus migratorius (American robin), is always written in italics, and contains first the genus and then the species names. A name like Pinicola enucleator californica (California pine grosbeak) also contains a subspecies name (= californica). For many species (ones that vary in size and/or color over geography), subspecies names are used for distinctive geographic forms.

For a long time, ornithologists almost universally used the biological species concept (BSC). This definition of "species" is based on species being reproductively isolated from each other. Under this definition, distinctive geographical forms of the same "kind" of bird are usually lumped as one species. This is because the geographic forms interbreed (or probably would, if they had the chance) where they intersect on the map. The problem with this definition is slightly different, geographically-isolated, forms rarely present us with "tests" of their willingness to interbreed. According to to adherents of the BSC, if the forms are only slightly different, they would probably interbreed if given the chance. Thus, they should be considered the same species. However, proponents of the BSC also say that because two things rarely interbreed (and produce viable hybrids) doesn't mean they belong to the same species. For example, wolves and coyotes (there's no educated disagreement that these are different species) can mate and have fertile and healthy pups.

The phylogenetic species concept (PSC) says that diagnosable geographic forms of the same basic "kind" of bird should be treated as distinct species. This is because these forms have evolved separately, and have unique evolutionary histories. The PSC is gaining favor because there is no worry about whether slightly-different geeographic forms might interbreed. If they don't, for whatever reason (for example, migration to different breeding areas), they are full species. Obviously, the PSC is less restrictive than the the BSC. There would be many more species of birds under the PSC than under the BSC.

The debate over how species should be defined will continue as long as people are allowed to think freely. Perhaps the argument is rhetorical, because every kind of organism presents a unique situation. It is possiblew that neither definition can be applied consistently in nature.


Are "types" of red crossbill different species?

The different types of red crossbill, Loxia curvirostra, are separate biological species. This is the more restrictive concept, so I'm not dividing crossbills simply upon changing to the more liberal PSC. Crossbill species demonstrate their reproductively isolated status in the many areas in which different forms overlap. Individuals only mate with their own kind, even though they have plenty of chances to interact socially with other kinds. The different forms cannot be called subspecies, because their geographic ranges overlap.

The different kinds of crossbills are also phylogenetic species. They are geneological/evolutionary units. They should not be lumped into one species becuase they have diagnostic vocal and morphological differences.