birding-aus

Definition of a species

To: Mike Honeyman <>, "" <>, "" <>
Subject: Definition of a species
From: David James <>
Date: Thu, 24 Nov 2011 23:31:37 -0800 (PST)
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


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