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SPECIES DEFINITIONS, AS APPLIED TO BIRDS


PREAMBLE, 1998
In 1998, bird species around the world seemed to be getting split or lumped, more according to whim than to any scientific parameters, even though definitions of what a species is had long been published, see below (Mayr 1963; Short 1969, Mayr 1970, 1982; Rosen 1979, Cacraft 1983; Paterson 1982; Sibley & Monroe 1984). What was lacking was splitting or lumping using the same rules for all species using any of these concepts.
At the time Sibley & Monroe was all the rage as they had turned ornithology on it's head with their ground-breaking work on DNA as applied to birds. But, on studying their newly-organised list of world birds, families and orders were re-arranged but species still followed the same authorities for each area or country as they had had for some decades. There was no attempt to apply their own Re-modified Biological Species Concept (1984). It was very disappointing.
So in the absence of a lead being taken, anywhere in the world it seemed, here is an attempt to right that situation.

PUBLISHED DEFINITIONS
"No one definition has yet satisfied all naturalists; yet every naturalist knows vaguely what he means when he speaks of a species."Darwin 1859.

   BIOLOGICAL SPECIES CONCEPT (Mayr 1963)
Species are populations that are interbreeding, or potentially can interbreed, and which are reproductively isolated from other such groups by reproductive isolating mechanisms.

MODIFIED BIOLOGICAL SPECIES CONCEPT (Short 1969, Mayr 1970, 1982)
Species are populations that are interbreeding, or potentially can interbreed, and which are reproductively isolated from other such groups by reproductive isolating mechanisms; with separate species recognised on either side of a stable hybrid zone where parapatric populations have areas of sympatry.

PHYLOGENETIC SPECIES CONCEPT (Rosen 1979, Cacraft 1983)
Species are populations in which the members are diagnosably different from those of other populations and share a parent pattern of ancestry and descent.

RECOGNITION SPECIES CONCEPT (Paterson1985)
Species are groups of individuals that share a common fertilisation system or specific mate-recognition system.

RE-MODIFIED BIOLOGICAL SPECIES CONCEPT (Sibley & Monroe 1984)
Species are populations that are interbreeding and which are reproductively isolated from other such groups by reproductive isolating mechanisms, with separate species recognised on either side of a stable hybrid zone in parapatric populations. Allopatric populations that are geographically isolated and for which there is no evidence that they can, or do, interbreed, are allospecies and also count as full species.

It can be seen, by the emboldened MODIFIED BIOLOGICAL SPECIES CONCEPT above, that it is the concept the author prefers. The following is based on that.

SENSIBLE SPECIES DEFINITION (Anderson 1998), 1998 version -
Species are populations that are interbreeding and which are reproductively isolated from other such groups by
  1. at least two reproductive isolating mechanisms, which may include geographic separation, morphology, voice, habitat preference, a pattern of DNA differences; behaviour differences such as nest-building, social grouping or migration; or by
  2. a stable hybrid zone between contiguous populations which are otherwise reproductively isolated (as in the Modified Biological Species Concept).

Comments (from 1998) -
  The Sensible Species Definition (SSD) differs from the Re-modified BSC in that all allopatric population are not necessarily recognised as separate species as in the latter - they would have to show at least one other obvious reproductive isolating mechanism besides geographic isolation to become separate species.

  An example where reproductive isolating mechanisms have been shown to create species is the Australian ravens which are sympatric but reproductively isolated by voice, behaviour, morphology, and probably by genetic differences. More recent  examples from Australia are Bassian and Russet-tailed Thrushes, Halls and White-browed Babblers, and White-browed and Tasmanian Scrubwrens.

  Australian examples of species that have a common and stable hybrid zone, which may or may not have been considered as they were spit or lumped, are Northern and Southern Masked Lapwings, Torresian and Pied Fruit Pigeons, Varied and Mangrove Honeyeaters, Black-throated and Golden-backed Honeyeaters, and all five races of Varied Sitellas. 

  Other species which may be re-split because they have three or more reproductive isolating mechanisms are Plumed from Marbled Frogmouth and Little from White-throated Treecreeper. A recent split on the same basis is Western from Long-billed Corella, and Australian Ringneck has been proposed as a potential split by Pizzey in his latest field guide (1997) into Eastern and Western Ringnecks.

  Richard's Pipit is now recognised in published literature in Europe and North America as being split into Anthus richardi Richard's Pipit, A. rufulus Paddyfield Pipit, A. cinnamomeus Grassveld Pipit, and A. novaeseelandiae Australasian Pipit. Sibley has even further split the latter to australis; the rational extrapolation of this division would mean recognising the sub-species from Chatham, Antipodes, and Auckland/Campbell Islands as three full species too.

Following the Sensible Species Definition the following pairs of species/sub-species can be evaluated as to whether they are full species or not
where two x,or one x and two or three (x), would make a species.  Like this -
  
Subspecies Pairs
 Geographic Isolation /
  Lack of Hybrid
Zone
Morphology
Differences
Voice
Difference
Habitat
Preference
Patterns
of DNA
 Different
Behaviour
Species?
Gannets

American Great Egret
Eastern Egret

Imperial Shag
MacQuarie Shag

Pied Oystercatcher
S. I. Oystercatcher

Eurasian Oystercatcher
S. I. Oystercatcher

Northern Masked Lapwing
Southern Masked Lapwing

Pied Fruit Pigeon
 Torresian Fruit Pigeon

Southern Sooty Owl
Papuan Sooty Owl
Lesser Sooty Owl

White-throated Treecreeper
Little Treecreeper

Black-chinned Honeyeater
Golden-backed Honeyeater

Grey Whistler
Grey-headed Whistler

Grey-headed Robin
Ashy Robin

Torresian Crow
Bismarck Crow

Lawes Parotia
Eastern Parotia

Magnificent Riflebird PNG
Eastern Riflebird
Magnificent Riflebird AUS

Clamorous Reed Warbler
Australasian Reed Warbler

Varied Sitella OZ
Varied Sitella PNG
 
x


x


x


x


x


(x)


(x)

x
x
x


x


(x)


x


x


x


x


(x)
x


x?


x
(x)


x


x


(x)


(x)


(x)


(x)

(x)
(x)
(x)


(x)


(x)


(x)


(x)


(x)


(x)

























(x)









x


no


?


x





(x)






?









x


x
























(x)












x



x

















little
difference



(x)





x


x





(x)



yes


yes


yes



yes
























yes












yes

  Shortly after (maybe 2000 or 2001?) this definition had been trialled, the British Ornithologists Union published on the species dilemma and defined a species as they saw it; this concept was remarkably close to the Sensible Species Definition (the SSD) and encouraged the author to continue with it. Unfortunately there seems to be no lasting record of it on the web but while searching for it in 2012 the paper, "Quantitative Criteria for Species Delimitation", in the Ibis 2010, by Tobias, Seddon, Spottiswoode, Pilgrim, Fishpoole, and Collar, turned up.

  The pair scoring system that has been trialled this last 14 years is very similar to theirs, where pairs of taxon (see above), one a universally accepted species and the other a proposed split or lump, were compared. Scoring in the SSD was thus -
  x was worth one reproductive isolating mechanism and
 (x) was worth â…“ to ½ of a reproductive isolating mechanism.
So two x, or one x and two to three (x) were needed to make a taxon a species, when compared with a known base species.

  Although this has proved effective it is a relatively coarse method and it wasn't until I read Tobias et al, that a more precise method of scoring became apparent, where numbers could replace x and (x). Under the now-adopted numeral system x might have been equal to 3 points and (x) equal to 1 point, and the total needed to make a species would have been 5-6 points.  So thanks to Tobias et al for that inspiration. 
  It should be noted that the Sensible Species Definition remains the authors own without plagiarising the work of Tobias et al who deliberately do not define what a species is, but instead have developed a technique for assessing the taxonomic status of a taxon to see whether it is a species or not, although their criteria are based on the BSC (Biological Species Concept).
  We hope that the methods published by Tobias et al, in their "Quantitative Criteria for Species Delimitation", or something very like it, will eventually become the base by which all authorities in the world judge a species on.
  In the meantime here is a refinement of the Sensible Species Concept - 

SENSIBLE SPECIES DEFINITION (Anderson 1998 and 2012)
Species are populations that are interbreeding and which are reproductively isolated from other such populations by their reproductive isolating mechanisms.
These may include -
 geographic separation / chance of cross-breeding;
   morphological and plumage differences;
   voice;
   habitat preference;
   behavioural differences such as nest-building, social grouping or migration;
  consistent DNA differences.
Each mechanisms can be weighted according to the strength of difference from a recognised species and given a score between 0 and 5; a total of 5 would give the taxon species status.

Andrew P Anderson
Cairns
Australia
November, 2012.

As follows -

A SENSIBLE SPECIES DEFINITION - a minimum of a 5 point difference between taxa to make a different species.


(1)
Geographic separation/Chance

of cross-breeding
(2) Morphological and plumage differences (3) Song/Calls (4)
Habitat preference
(5) Behaviour
including
migration
(6)
DNA differences

Total Points
sp
or
subsp
?

Scoring 0-1-2-3 0-1-2-3 0-1-2-3 0-2-3-4 0-1-2-3 0-2-4-5


0
 
 
 
 
 
1
 
 
 
 
 

2

 
 
  
 
3




4





5




Broad hybrid zone





Narrow and stable hybrid zone,
e.g. (1.1),(1.5
),




Small chance of interbreeding.
e.g.(1.3)


No chance of interbreeding.
e.g. (1.2). (1.4),
Feather tracts and/or bare parts a little different,
e.g. (2-1).

Feather tracts and/or bare parts noticeably different,
e.g. (2-1), (2.2)

Feather tracts and/or bare
parts very different.

Feather tracts and/or bare
parts completely different.
Not so different voice range and/or song phrases.

Noticeably different voice range and/or song phrases,
3.1,3.2,3.3,






Completely different voice range and/or song phrases.
Similar or slightly different habitat preferences.








Noticeably different habitats.







Completely different habitats.
Slightly different behaviour.





Noticeably different behaviour.




Very different behaviour.
DNA differences are barely measurable.




DNA differences are measurable.



DNA differences are larger.

DNA differences are large enough to probably have non-viable offspring.

DNA differences are so great that viable inter-breeding would be impossible.
e.g. (6.1)



Now we can re-evaluate the 1998 data with a little more precision -

    A 2012 Re-evaluation of 'Reproductive Isolating Mechanisms 1998'      
 


Subspecies Pairs

(1)
Geographic separation/Chance

of cross-breeding
(2)
Morphological and plumage differences
(3)
Song/Calls
(4)
Habitat preference
(5) Behaviour
differences
(6)
DNA differences
Total Points sp
or
subsp
?
Sula genus (Brisson, 1760) 2.
Sula b. bassanus
(Linnaeus, 1758)
Sula b. capensis (Lichtenstein, 1823) 3.

Sula b. capensis (Lichtenstein, 1823)
Sula b. serrator (Gray, GR 1843) 3.

Ardea alba egretta (American Egret)
Ardea a. modesta (Eastern Egret) 4

Leucocarbo atriceps  Imperial Shag
Leucocarbo pupurascens  MacQuarie Shag

Haematopus ostralegus
(Linnaeus, 1758)
Haematopus finschi (Martens, GH, 1897)

Haematopus finschi (Martens, GH, 1897)
Haematopus longirostris (Veillot, 1817)

Vanellus m. miles
Vanellus m. novaehollandiae

Ducula b. bicolor (Scopoli, 1786)
 Ducula b. spilorrhoa (Gray, GR, 1858)

Tyto t. tenebricosa
Tyto t. arfak
Tyto t. multipunctata

Cormobates l. leucophaea (Latham 1802)
Cormobates l. minor (Ramsay, EP, 1891)

Melithreptus g. gularis (Gould, 1837)
Melithreptus g. laetior (Gould 1875)

Pachycephala s. simplex (Gould 1843)
Pachycephala s. griseiceps  (Gray, GR 1858)

Heteromyias a. albispecularis (Salvadori, 1875)
Heteromyias a. cinereifrons (Ramsay, EP, 1876)

Corvus o. orru (Bonaparte, 1850)
Corvus o. insularis (Heinroth, 1903)

Parotia l. lawesii (Ramsay, 1885)
Parotia l. helenae (De Vis, 1897)

Ptilorus m. magnificus (Vieillot, 1819) PNG
Ptilorus m. alberti (Elliot, 1871) Aust.
Ptilorus m. intercedens (Sharpe, 1882) E. PNG

Acrocephalus s. stentoreus (Hemprich & Ehrenberg, 1833) 
Acrocephalus s. australis (Gould, 1838)

Daphoenositta c. chrysoptera (Latham, 1802)
Daphoenositta c. papuensis (Schlegel, 1871)

A further 50 pairs of taxon have been evaluated;
these will be shown, in systematic order, on another page.

 

0
1-3

0
1-3

0
3

0
3

0
3

0
2

0
1

0
0

0
3
3

0
2-3

0
1

0
2-3

0
3

0
2

0
1

0
3
0

0
2

0
3

0
0-1

0
0-1

0
0-1

0
2

0
0

0
0

0
1

0
0-1

0
0
0

0
1

0
0

0
1

0
0

0
0-1

0
0-1

0
0
0

0
0

0
0?

0
0

0
0

0
0

0
0

0
0

0
0

0
0

0
0

0
0
0

0
0

0
1

0
0

0
0

0
2

0
0

0
0-1
1

0
0

0
0?

0
0

0
0

0
0

0
0

0
1

0
1

0
0

0
0

0
0
0

0
0

0
0

0
0

0
0

0
1

0
0

0
0
0

0
0

0
3

0
0

0
0

0
1

0
0

0
2

0
2

0
0

0
1

0
0
0

0
0

0
0

0
0

0
0

0
0

0
0

0
0
0

0
0

0
0

0
0

0
0

0
5?

0
0

0
0?

0
0

0
0

0
0

0
0-1
0-1

0
0

0
0?

0
0

0
0

0
0

0
0

0
0
0

0
0

0
0

0
1-4

0
1-4

0
8-9

0
5

0
6

0
5

0
2

0
1-2

0
3-4
3-4

0
3-4

0
2

0
3-4

0
3

0
5-6

0
1-2

0
3-4
1

0
2

0
6

sp
subsp

subsp
subsp

sp
sp

sp
sp

sp

sp

sp
sp

sp
subsp

sp
subsp

sp
subsp
subsp

sp
subsp

sp
subsp

sp
subsp

sp
subsp

sp
sp

sp
subsp

sp
subsp
subsp

sp
subsp

sp
sp

2. Walters. Sula (Brisson, 1760) should take preference over Morus (Veillot, 1818) as there is little difference in the current genera; Walters (1980, p9) says "The generic division seems principally to be one of convenience".
3. All Gannets, it seems, are of one species.
4. Christidis & Boles in their "Taxonomy and Species of Birds of Australia and It's Territories", 1994, point out (p.43) that "Sheldon (1987) found that in DNA-DNA hybridisation distances Ardea alba modesta (from southeast Asia and Australasia) was as distinct from Ardea alba egretta (from North America) as was Ardea intermedia. This genetic distance is consistent with Hancock's (1984) suggestion, based on the presence of an aerial stretch display in A.a.modesta and not in A.a.egretta, that these two populations may represent different species." Also it should be noted that A.a.egretta is 10cm longer on the average.  
  Further to that, this proposed split of Great Egret was accepted by the IOC (Sheldon 1987, Collar 2007, Christidis & Boles 2008) and by BLI, but on re-visitation by the latter, both the BLI and the IOC have reversed that decision based on insufficient evidence. However it can be seen from the above chart that these taxon really are separate species; the question is where do the other two sub-species A.a.alba and A.a.melanorhyncos stand?