The problem, as I see it, is this. there is no standard way of defining a
species using genetic analysis. There is no way of differentiating between
the genetic variation within a species to then be able to draw a line and
say that on one side of the line is one species and the other side is a
different species.
And as for visuals, the ways genes are expressed in physical form is
different from having different genetics.
There are plenty of examples where there are different looking individuals
within the one species. If you think birds are hard think again, try
googling Crinia Signifera, the commonest frog in Sydney, they were once
described as separate species based on how they looked but have now been
shown to be the same species, and the opposite end of the scale some frog
species were lumped because of how they looked (Litoria spp), but had
different calls and have since been split. I think what I'm saying is that
just because it looks different (or the same!) does not mean that it is
different (or the same).
And then there is speciation, which comes back to my original point. At what
stage does a diverging population of one species become a separate species.
Sure they look different and maybe breed on different island but does that
make them new species. Like I said there is no standard amount of genetic
difference that defines a species. Without this it is simply not possible to
define where one species ends and the next begins.
And then it comes back to whether you follow the biological species concept
(BSC) or the phylogenetic species concept (PSC). Put simply BSC says that if
they interbreed and produce viable off-spring then they are a species, PSC
says that the genetic difference between individuals is how a species is
defined (but without defining the difference needed to separate species).
Most twitchers seem to prefer the PSC as it tends to split rather than lump,
though this is not always the case.
Or at least this is my understanding.
Graham Turner
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