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Species definition. Rhododendron, Danish Chapter
What is a species
There are several definitions of species. (Morphological-Evolutionary- biological - Genotypic cluster (DNA sequences))
From http://menura.cse.unsw.edu.au:1080/1997/06/msg00178.html (dead link)
What definition for species is used in the taxonomy of birds?
The notion of a species is a much vexed question of which there
are wildly differing opinions.
One of the best known species defintion is Ernst Mayr's
biological species concept. In short:
"A species is a group of interbreeding natural populations that is
reproductive isolated from other such groups"
This reproductive isolation need not be absolute. I'm not sure how you
decide when interbreeding becomes significant but Mayr believes his
approach is almost unambiguous for birds. Anyway, his book
"Towards a New Philosphy of Biology" is worth reading.
Even for birds, I'm sure you can find plenty of people to disagree with
Mayr. In fact my theory, is that just as (supposedly) no two snowflakes
are alike, no two biologists have exactly the same view of the species
You can get a good taste of this in the introduction to Christidis and
Boles' The Taxonomy and Species of Birds of Australia and its Territories.
They describe various problems delinating species and cite differing
views and never, as far as I can see, say actually what species
concept they have used :-)
If you want a very different views talk to a botanist. Reproductive-based
definitions are much more problematic for many plants. Or if you want
a real headache add the dimension of time and think about defining
If you want an easy life, you can adopt an authority-based species
concept, e.g its a species if Boles & Christidis say it is.
In the old days, the production of fertile(NB) hybrid
offspring would immediately establish that two taxa are races,not distinct
species. But more modern thinking prefers to look at the frequency of events
rather than the yes/no dimension. Consider it this way. 0 is a small
number, and 1 is closer to 0 than it is to 100. So if you had an area of
sympatry, in which two taxa occurred together, and 99 out of every 100
pairings were between the same taxon, with the other 1 between the two
taxa,even if the offspring of that 1 pairing were fertile, modern thinking
would say that the two taxa had effectively developed reproductive isolating
mechanisms and should be considered distinct species.
Associate Professor John M. Penhallurick
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.
As within subgenera they (different species) do freely interbreed in the
wild and in gardens. And they are defined by what they look like and not
whether they breed.