Species Divisions are Complicated

Zoologists have traditionally divided the vertebrates into classes: major divisions with names like mammals, birds, reptiles and amphibians. Some zoologists, called 'cladists',* insist that a proper class must consist of animals all of whom share a common ancestor which belonged to that class and which has no descendants outside that group. The birds would be an example of a good class. All birds are descended from a single ancestor that would also have been called a bird and would have shared with modern birds the key diagnostic characters - feathers, wings, a beak, etc. The animals commonly called reptiles are not a good class in this sense. This is because, at least in conventional taxonomies, the category explicitly excludes birds (they constitute their own class) and yet some 'reptiles' as conventionally recognized (e.g. crocodiles and dinosaurs) are closer cousins to birds than they are to other 'reptiles' (e.g. lizards and turtles). Indeed, some dinosaurs are closer cousins to birds than they are to other dinosaurs. 'Reptiles', then, is an artificial class, because birds are artificially excluded. In a strict sense, if we were to make reptiles a truly natural class, we should have to include birds as reptiles. Cladistically inclined zoologists avoid the word 'reptiles' altogether, splitting them into Archosaurs (crocodiles, dinosaurs and birds), Lepidosaurs (snakes, lizards and the rare Sphenodon of New Zealand) and Testudines (turtles and tortoises). Noncladistically inclined zoologists are happy to use a word like 'reptile' because they find it descriptively useful, even if it does artificially exclude the birds.

But what is it about the birds that tempts us to hive them off from the reptiles? What is it that seems to justify bestowing on birds the accolade of 'class', when they are, evolutionarily speaking, just one branch within reptiles? It is the fact that the immediately surrounding reptiles, birds' close neighbours on the tree of life, happen to be extinct, while the birds, alone of their kind, marched on. The closest relatives of birds are all to be found among the longextinct dinosaurs. If a wide variety of dinosaur lineages had survived, birds would not stand out: they would not have been elevated to the status of their own class of vertebrates, and we would not be asking any such question as 'Where are the missing links between reptiles and birds?'


The ancestors are birds are reptiles, but in the fossil record where do we draw the line between them?

Folksonomies: evolution species taxonomy

/pets/reptiles (0.816323)
/pets/birds (0.253992)
/family and parenting/children (0.192088)

birds (0.926295 (negative:-0.015910)), reptiles (0.825239 (negative:-0.472286)), immediately surrounding reptiles (0.708970 (negative:-0.254471)), Cladistically inclined zoologists (0.632316 (negative:-0.444117)), key diagnostic characters (0.586615 (positive:0.253530)), truly natural class (0.568703 (positive:0.352579)), dinosaurs (0.519266 (negative:-0.480635)), fossil record (0.471919 (negative:-0.262673)), Species Divisions (0.469333 (neutral:0.000000)), modern birds (0.468737 (positive:0.253530)), major divisions (0.467515 (neutral:0.000000)), longextinct dinosaurs (0.467325 (negative:-0.505184)), common ancestor (0.466778 (positive:0.343508)), conventional taxonomies (0.464480 (neutral:0.000000)), single ancestor (0.464136 (negative:-0.309631)), closer cousins (0.463623 (neutral:0.000000)), proper class (0.457371 (positive:0.343508)), rare Sphenodon (0.457301 (neutral:0.000000)), e.g. lizards (0.452218 (neutral:0.000000)), artificial class (0.448619 (negative:-0.591714)), strict sense (0.447688 (negative:-0.394022)), good class (0.446284 (negative:-0.589110)), Zealand) and Testudines (turtles and tortoises). (0.446074 (neutral:0.000000)), wide variety (0.442798 (neutral:0.000000)), closest relatives (0.442043 (negative:-0.505184)), close neighbours (0.441453 (neutral:0.000000)), dinosaur lineages (0.439889 (neutral:0.000000)), vertebrates (0.379681 (positive:0.364245)), crocodiles (0.377845 (negative:-0.401250)), turtles (0.367187 (neutral:0.000000))

New Zealand:Country (0.672588 (neutral:0.000000)), Testudines:City (0.670453 (neutral:0.000000)), Archosaurs:City (0.644645 (negative:-0.328400))

Reptile (0.989870): dbpedia | freebase | opencyc
Dinosaur (0.678189): dbpedia | freebase | opencyc
Bird (0.644210): dbpedia | freebase | opencyc
Evolution (0.457388): dbpedia | freebase | opencyc
Animal (0.408534): dbpedia | freebase | opencyc
Diapsid (0.402833): dbpedia | freebase
Chordate (0.397138): dbpedia | freebase
Paleontology (0.379651): dbpedia | freebase | opencyc

 The Greatest Show on Earth: The Evidence for Evolution
Books, Brochures, and Chapters>Book:  Dawkins, Richard (2010-08-24), The Greatest Show on Earth: The Evidence for Evolution, Free Press, Retrieved on 2011-05-19
Folksonomies: evolution science


06 SEP 2011

 The Difficulty of Defining Species

Species Divisions are Complicated > Similarity > Are There Bacteria Species?
Dawkins describes the difficulty of defining any species, with missing links making it possible at all; while Frederick William Andrewes describes the difficulty of classifying bacteria, where rapid evolution and gene swapping magnify the issues Dawkins describes.