Chapter 1 - The Diversity, Evolution, and Classification of Vertebrates

Download an ADOBE version of the lecture outline

return to vertebrate zoology syllabus

WB01342_.gif (412 bytes)  The Vertebrate Story

This section gives a brief overview of the classification of the vertebrates. There are about 50,000 species of living vertebrates; this estimate may vary, primarily depending on how many species of teleost fishes one recognizes.  They range in size from fishes with a body mass of about 0.1 g to adult female blue whales with a body mass of 100,000 kg.

WB01342_.gif (412 bytes)  The Different Kinds of Vertebrates

A Phylogenetic Classification of the Vertebrates

the evolutionary history of a group
Descent from a common ancestor (evolution) is the underlying basis for classification

        Subphylum UROCHORDATA
        Subphylum CEPHALOCHORDATA
        Subphylum CRANIATA
                Infraphylum MYXINOIDEA
                Infraphylum VERTEBRATA
                        Order PETROMYZONTIFORMES
                                Class CHONDRICHTHYES
                                         Subclass Holocephali
                                         Subclass Elasmobranchii
                                Class OSTEICHTHYES
                                        Subclass ACTINOPTERYGII
                                                 Unnamed taxon
                                                                Unnamed taxon
                                        Subclass SARCOPTERYGII

                                                Order ACTINISTIA
                                                Order DIPNOI
                                                Division RHIPIDISTIA
                                                       Eusthenopteron, Panderichthys, etc.
                                                                Ichthyostega and other primitive "amphibians"
                                                                        Class AMPHIBIA
                                                                                        Class MAMMALIA
                                                                                Class REPTILIA
                                                                                                          Class AVES

Numbers of Living Species of Chordates

Urochordata (tunicates)              2000         sea squirts, larvaceans
Cephalochordata                            22         amphioxus, lancelets
Craniata                                    54000         hagfish plus the vertebrates

Craniates (Figure 1-1)

Myxinoidea                                    50         hagfish
Petromyzontia                                41         lampreys

        Elasmobranchii                    850         sharks, skates, rays
        Holocephali                           30         ratfish (=chimaeras)

        Brachiopterygii                        10         reedfish, bichirs
        Chondrostei                            25         sturgeon, paddlefish
        Lepistosteus                             7         gar
        Amia                                        1         bowfin (=choupique)
        Teleostei                          27,000         tarpon, herring, perch, etc.
Actinistia                                           2         coelacanth (Latimeria)
Dipnoi                                               6        lungfish

        Gymnophiona                           165       caecilians
        Caudata                                   500       salamanders
        Anura                                     4800       frogs

        Monotremata                                 6       platypus, echidna
        Marsupialia                                275      opossum, kangaroos, etc.
        Eutheria                                    4700      mice, bats, elephants, horses, etc.

Testudinata                                        300       turtles
        Sphenodontia                                 2        tuatara (Sphenodon)
                lizards                              4800        lizards (including amphisbaenians)
                Ophidia                            2900        snakes

Aves                                                9100        birds
Crocodylia                                           23        alligators, crocodiles, gavials

The Traditional Classification of the Vertebrates



Class AGNATHA - 90 species


Class CHONDRICHTHYES - 950 species

Subclass Elasmobranchii---sharks, rays, skates

Subclass Holocephali---ratfish (chimaeras)

Class OSTEICHTHYES - 27,000 species

bony fishes
skeleton contains bone
single external gill opening covered with operculum
swim bladder or lung
 Subclass Actinopterygii
ray-finned fish
most fish (most vertebrates)
Subclass Sarcopterygii
fleshy-finned fishes
lungfish and coelacanth
fleshy finned fish gave rise to amphibians in the Devonian

Class AMPHIBIA  - 5,500 species

Order Gymnophiona (Apoda)---Caecilians
elongate bodies, limbless, scales in annular folds of skin
Order Caudata (Urodela)---salamanders
tailed, usually with two pairs of limbs
Order Anura (Salienta)---frogs and toads
tailless, elongate hindlimbs, head and trunk fused


Class REPTILIA - 7,800 species

Class AVES -- 9,100 species

Class MAMMALIA -- 4,800 species


Marsupials (Subclass Metatheria):

Placental Mammals (Subclass Eutheria):

WB01342_.gif (412 bytes)  EVOLUTION: the unifying concept of biology

Natural Selection - 1858: Charles Darwin and Alfred Russel Wallace

  1. organisms have great potential fertility [excess reproductive capacity ]
  2. natural populations normally remain constant in size, except for minor fluctuations
  3. natural resources are limited
    1. therefore: members of a population compete with each other for resources
  4. all organisms show variation between individuals
  5. most variation is heritable
  6. some traits increase adaptation
  7. selection - better adapted are more likely to survive and reproduce (differential reproduction)
    1. differential survival and reproduction among individuals of a population leads to
    2. evolution of new adaptations and species in response to changing climate, competition, etc.
  8. change in frequencies of genes
  9. over time, evolution of new species, etc.

Evolution is nonteleological (not directed toward a future goal)

Natural Selection:

Modes of Selection  

  1. disruptive selection
  2. stabilizing selection: fixes genetic frequency
  3. directional selection
  4. sexual selection

Evolution of Complex Systems

Inclusive Fitness

WB01342_.gif (412 bytes)  Variation and Evolution

Variation that is Subject to Natural Selection

Factors Promoting Speciation


  1. Linnaean Species:
    • essentialism:
    • one or a few features that expressed an essence; morphologic discontinuities exist between taxa
  2. nominalism:
    • individuals are the only real and concrete things that exist in nature; taxa are human constructs
    Darwin: natural groups of organisms do exist; they are descendants of a common ancestor.
  3. Biological Species Definition
    • in sexually reproducing organisms, populations or groups of populations that are actually or potentially interbreeding and are reproductively isolated from other such populations under natural conditions - Ernst Mayr (1942)
  4. Morphologic Species Definition
    • members of a species have a similar shape; distinct from closely related species; underlying cause is sexual reproduction and gene flow. paleontologists rely on morphology

The Role of Isolation in Speciation

Allopatric Speciation
vicariant speciation: fragmentation of range
founder event
Sympatric Speciation
polyploidy in higher plants
ecological or behavioral differences in animals
The Importance of Population Size in Evolution
Genetic Drift (chance events)
founder effect : small founding populations; not typical of whole population
Darwin's finches  
adaptive radiation

WB01342_.gif (412 bytes)  Earth History and Vertebrate Evolution

The Geologic Time Scale (inside the front cover)

Paleogeography (Figure 1.5; chapters 7, 13, 18)

WB01342_.gif (412 bytes)  Classification of Vertebrates

Classification and Evolution  

Download Dr. David Cannatella's Adobe Acrobat evolution.pdf   file for a lucid discussion of this topic.
ordering organisms into groups
central concern; heart of systematic biology

Classification of Organisms

WB01342_.gif (412 bytes)  Traditional and Cladistic Classifications

Numerical Taxonomy: measures overall similarity; phenetics, phenograms

Evolutionary Taxonomy: common descent and morphological distance; shared structural and functional characters (e.g. Class Reptilia)

Phylogenetic Systematics

Phylogenetic Systematics (cladistics): common descent only; Willi Hennig (1966)

CLADOGRAMS (Figure 1-2)

Cladistic Taxonomy
  1. Journey into the world of phylogenetic systematics
  2. An introduction to cladistics from Dr. Rebecca Irwin of the University of Tennessee Martin
  3. cladistic analysis from  George Washington University

    Inferring Phylogeny from Morphological Data

Establishing relationships depends on the recognition of homologous characters, which evolved from a common ancestor.  Uniquely derived character states reflect common ancestry; e.g. amnion, feathers

character state: presence, absence, or way the attribute is expressed

  1. apomorphy: derived or specialized character

  2. plesiomorphy: primitive or ancestral character

  3. autapomorphy: characterizes a terminal branch only

  4. synapomorphy: shared derived character inferred to have originated in the latest common ancestor

  5. symplesiomorphy: shared primitive character

Evolutionary Hypotheses

clade: monophyletic group
monophyletic: entire phylogeny; latest common ancestor plus all and only all its descendants
Phylogenetic relationships of the vertebrates (Figure 1-3).
sister groups: two monophyletic groups share a common ancestry with each other more recently than either one does with any other taxa

Grades: group of species at same level of organization: paraphyletic or polyphyletic

paraphyletic: group including the most recent common ancestor of all members of the group; but from which one or more descendent groups have been excluded; can not be defined strictly by synapomorphies; part of phylogeny; e.g., Class Reptilia (Figure 1-10); stem groups--"fish"

polyphyletic: descendents of unrelated ancestors; at least one of the common ancestors of any two taxa is not included; convergent evolution; e.g. Radiata - Cnidaria and Echinoderms have radial symmetry; 

Determining Phylogenetic Relationships

Determining Monophyly
monophyly is simple to determine if no homeoplasy
uniquely derived character states: reflect common ancestry; e.g. amnion, feathers
parsimony: smallest number of changes (reversals) of character states

homologous: character state inherited from common ancestor (e.g. ostrich and robin wings; the actual bones in [analogous] bird, bat, & pterosaur wing)

homology is determined by position relative to other structures; development (embryological tissue, pattern of ontogeny) & evolution

analogous: similar function; arose independently

convergent evolution: similar features from different ancestors; cactus vs. euphorbs; wings in bats, birds, and pterosaurs

parallel evolution: independent modifications from the same ancestor; e.g. elongate body of salamanders by independent evolution of extra vertebrae; ?burrowing rodents.

homoplasy: character similarity that misrepresents common descent as a result of convergence, parallelism, or reversal

WB01342_.gif (412 bytes)  The Effect of Human Population Growth on Vertebrate Diversity

Bird extinctions

Major threats affecting birds and mammals


Return to:

Vertebrate Zoology Syllabus

Baskin Home Page

Biology Home Page