Lecture 9 Wednesday, March 18, 2009
What is a species1?
• Morphological species concept (MSC): The MSC classifies organisms into
species based on their morphology. Individuals in the same species are similar to
one another in morphology. Individuals in different species are different in
morphology.
The Linnaean system of classification and most traditional methods of
classification use the MSC.
The MSC emphasizes morphology rather than behavior in part because
classification was and still is often done with preserved specimens or fossils.
One problem with the MSC is that it is difficult to say what differences in
morphology are important for distinguishing species and how different two
groups organisms have to be in order for them be classified as different species.
Some species are polymorphic, but the traits that are polymorphic are not used to
distinguish species. Males and females of sexually dimorphic species may be
very different from each other, as in the birds of paradise, yet they are classified
as members of the same species.
In practice, the application of the MSC to a group of organisms takes experience
and practice.
In some species, it is difficult even for specialists to know whether a group is a
single polymorphic species or several separate species. Adults of the tropical
butterfly, Astraptes fulgerator, are monomorphic but the larvae are polymorphic.
• The biological species concept (BSC): The BSC groups organisms into species
that are reproductively isolated from one another, meaning that members of
different species do not interbreed under natural conditions or, if they do, they do
not produce viable or completely fertile offspring.
The BSC is regarded as the correct definition of a species: The BSC fits well with
the idea that species rather than individuals evolve. If members of the same
species freely interbreed, then they are part of the same evolutionary unit.
Different species evolve independently of one another.
If two groups are known to interbreed, then they will be classified as one species
regardless of any differences in morphology. If two groups do not interbred under
natural conditions, then they classified as two species regardless of how similar
they are in morphology.
In a single geographic location, the MSC and BSC usually distinguish the same
species. Native New Guineans distinguish the same species of birds of paradise as
European ornithologists. Sometimes, close observation will allow splitting of
what was thought to be a single species into two or more species.
1 Species definitions other than the MSC and BSC will not be covered.
