Species definitions are indeed a can of worms that have been discussed on B-A
many times, so I do not want to go there.
However, either I don't understand or don't agree with Nikolas and Mike about
hybridisation. A hybrid is simply the offspring of two different forms. The
parents can be individuals from two different genera, species, subspecies
(races), varieties, breeds or cultivars (but not morphs). It is not within the
domain (or interest) of taxonomy to redefine "hybridisation" as something that
only occurs between "two species nowadays".
Of course hybridisation can and is used to indicate species boundaries in ALL
species concepts. It is a line of evidence. When two forms are sympatric and it
is known that they don't hybridise everyone agrees that they are two species
(like the 2 white-tailed black-cockatoos). When they merge into each
other through hybridisation over a broad front then everyone agrees they are
one species (like green and yellow figbirds). In between there is lots of grey
and disagreement, but there is grey and disagreement in everything to do with
taxonomy. Taxonomists can still use hybridisation as a line of evidence
regardless of the species concept they follow, even if few do. There are at
least two big problems with using hybridisation: 1) to understand it you
need data from lots of individuals across a wide area; and 2) it is not
applicable to allopatric species. Neither is justification to dismiss it
as irrelevant to the process of speciation.
A frequent trend in taxonomy these days is to compare the percentage
differences in the Cytochrome B gene. Isn't this just looking for an indication
of whether two forms continue to share genes through the process of
hybridisation, or how long ago they stopped?
Lastly, nearly all existing checklists are predicated on lines of evidence
originally formed around concepts of speciation based on levels of
interbreeding (or the extrapolation of similar patterns when direct evididence
is lacking). Regardless of contemporary opinions, hybridisation still defines
the bird species painted in the field guides.
David James,
Sydney
==============================
________________________________
From: Mike Honeyman <>
To: ;
Sent: Thursday, 24 November 2011 10:03 PM
Subject: Definition of a species
I hear the sound of a can of worms being opened!
Simon there are many definitions of species, to suit specific 'species
concepts'. There are different species concepts that are preferred for
different phyla.
For birds the two most prevalent species concept are the Biological Species
Concept (BSC) after Mayr, and the Phylogenetic Species Concept (PSC) after
Cracraft.
BSC species = "groups of interbreeding populations reproductively isolated from
other such groups"
PSC species = "the smallest diagnosable cluster of organisms within which there
is a parental pattern of ancestry and descent"
Historically the BSC could use the ability to hybridise or not as an indicator
of species, but I think it's a while since anyone thought that was a reliable
indicator, as Nikolas has pointed out.
Re the owls. It is possible that the morphological differences are a red
herring - there could be an environmental 'switch' (e.g. the climate / habitats
that prevail in Tassie and NZ) that cause a particular morphology that exists
widely within the gene pool of the population to prevail. This could be tested
by moving Qld birds to Tassie and see what they look like after a couple of
generations (I've not looked at any of the papers by the way, just flying a
theoretical kite!)
Cheers
mjh
===============================
To unsubscribe from this mailing list,
send the message:
unsubscribe
(in the body of the message, with no Subject line)
to:
http://birding-aus.org
===============================
===============================
To unsubscribe from this mailing list,
send the message:
unsubscribe
(in the body of the message, with no Subject line)
to:
http://birding-aus.org
===============================
|
