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

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  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.

  The Different Kinds of Vertebrates

A Phylogenetic Classification of the Vertebrates

phylogeny:

the evolutionary history of a group

Descent from a common ancestor (evolution) is the underlying basis for classification

PHYLUM CHORDATA
        Subphylum UROCHORDATA
        Subphylum CEPHALOCHORDATA
        Subphylum CRANIATA
                Infraphylum MYXINOIDEA
                Infraphylum VERTEBRATA
                        Order PETROMYZONTIFORMES
                        GNATHOSTOMATA
                                Class CHONDRICHTHYES
                                         Subclass Holocephali
                                         Subclass Elasmobranchii
                                Class OSTEICHTHYES
                                        Subclass ACTINOPTERYGII
                                                Brachiopterygii
                                                 Unnamed taxon
                                                        Chondrostei
                                                        Neopterygii
                                                                Lepisosteus
                                                                Unnamed taxon
                                                                        Amia
                                                                       Teleostei
                                        Subclass SARCOPTERYGII
                                                Order ACTINISTIA
                                                Order DIPNOI
                                                Division RHIPIDISTIA
                                                       Eusthenopteron, Panderichthys, etc.
                                                        EOTETRAPODA
                                                                Ichthyostega and other primitive "amphibians"
                                                                TETRAPODA
                                                                        Class AMPHIBIA
                                                                                 Gymnophiona
                                                                                 Batrachia
                                                                                       Anura
                                                                                       Urodela
                                                                        AMNIOTA
                                                                                SYNAPSIDA
                                                                                        Class MAMMALIA
                                                                                                 Monotremata
                                                                                                 Theria
                                                                                                        Marsupialia
                                                                                                         Eutheria
                                                                                Class REPTILIA
                                                                                         Testudines
                                                                                         Sauria
                                                                                                 Lepidosauria
                                                                                                        Sphenodontia
                                                                                                         Squamata
                                                                                                Archosauria
                                                                                                         Crocodylia
                                                                                                          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

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

Actinopterygii
        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

Amphibia
        Gymnophiona                           165       caecilians
        Caudata                                   500       salamanders
        Anura                                     4800       frogs

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

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

Aves                                                9100        birds
Crocodylia                                           23        alligators, crocodiles, gavials

The Traditional Classification of the Vertebrates

PHYLUM CHORDATA

• see chapter 2 for characteristics

SUBPHYLUM CRANIATA

• see chapter 6 for characteristics

Class AGNATHA - 90 species

• a possibly paraphyletic group of jawless fishes, including the first vertebrates
  • see chapter 6 for characteristics
• Living forms are elongate, scaleless, slimy parasites and scavengers - include
  1. Myxinoidea:  the hagfish
  2. Petromyzontia: lampreys, parasitic fish that suck nutrients from circulatory system of host fish

Gnathostomes

• vertebrates with jaws that are modified gill supports
• includes the cartilaginous fish, bony fish, and tetrapods
• see chapter 7 for characteristics

Class CHONDRICHTHYES - 950 species

• the cartilaginous fish
• see chapter 7 for characteristics

Subclass Elasmobranchii---sharks, rays, skates

• see chapter 7 for characteristics

Subclass Holocephali---ratfish (chimaeras)

• see chapter 7 for characteristics

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

• Amphibians today include salamanders, toads and frogs  (the Lissamphibia)
• Paleozoic amphibians gave rise to the amniotes
• see chapter 11 for characteristics

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

AMNIOTES

Class REPTILIA - 7,800 species

• turtles, crocodiles, lizards, snakes, etc.
• Reptile lungs are efficient, don't use skin for gas exchange
• They have internal fertilization and produce eggs with leathery shells
• ectotherms

Class AVES -- 9,100 species

• Characteristics
  1. Birds have internal fertilization and lay hard-shelled eggs
  2. Endotherms
  3. Nearly every anatomical feature is related to ability to fly
     1. The only animals with feathers (modified reptilian scales)
  4. Many types of birds exist:
     • flightless; web footed; divers; waders; vegetarian; etc

Class MAMMALIA -- 4,800 species

• mammals evolved in the late Triassic, the time dinosaurs first appeared.
• mammals diversified greatly following the extinction of the dinosaurs during the Cenozoic
• Characteristics
1. hair - protection from heat loss
2. mammary glands
3. differentiated teeth
4. endotherms; 4 chambered heart; etc.
5. Greatest size range
   1. pigmy shrew to blue whale

Monotremes

• egg layers
• 3 species from Australia and New Zealand

Marsupials (Subclass Metatheria):

• brief gestation period, embryo continues development in pouch
• prolonged lactation and parental care
• Marsupials include: opossum, kangaroo, koala, Tasmanian devil, wombat, etc.

Placental Mammals (Subclass Eutheria):

• placentals: true viviparous
• placenta: organ of gas and nutrient exchange between mother and fetus
• 18 of the 26 orders of mammals are placentals
• Orders in this subclass have adapted to life on land, in water, and in the air

  EVOLUTION: the unifying concept of biology

• John Baptiste Pierre Antoine de Monet, Chevalier de LAMARCK (1744-1829)
  1. chain of being
  2. inheritance of acquired characteristics
  1. direct influence of environment, use and disuse
  2. transformational process - individuals change own appearance (vs. Darwin's variational process)
• CHARLES DARWIN (1809-1882)
1. Voyage of the Beagle 1831-35

2. Lines of Evidence that influenced Darwin's theory of evolution
   1. uniformitarianism

      Charles Lyell

   2. biogeography

      Galapagos: tortoises, mockingbirds, finches

   3. artificial selection
      1. Darwin used artificial selection as a model for his theory
      2. heritability of variation, adaptive characters,  variation
   4. Malthus
      1. fecundity - rate at which a female produces offspring
      2. excess reproductive potential

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)
   2. differential survival and reproduction among individuals of a population leads to
   3. 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:

• the forces causing differential reproduction shown by some members of a population as a result of differences in fitness (a measure of the relative survival and reproductive success of a given individual)
• differential reproduction causes changes in allele frequencies and genotypes in populations
• cycles of infectious diseases which rise and fall in human populations are evidence of 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

  Variation and Evolution

• Nature of Variation in Species
  1. ontogenetic variation
  2. genetic differences
  3. non-genetic differences
     • environmental effects

Variation that is Subject to Natural Selection

• genetic differences
  1. individual variation
  2. polymorphism:
     • presence of different allelic forms in a population
     • can differ geographically  - discontinuous variation 
  3. sexual dimorphism 

Factors Promoting Speciation

WHAT IS A SPECIES?

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

• Gene flow: migrating individuals reproduce with members of different populations
• Geographic Variation: speciation in progress
• reproductive barriers: isolating mechanisms
• an introduction to SPECIATION-- from Dr. T. Herbert, Biology 160, University of Miami

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

bottlenecks

Darwin's finches  

adaptive radiation

  Earth History and Vertebrate Evolution

The Geologic Time Scale (inside the front cover)

• Geologic Time from the USGS
• Geologic Time Scale from the UCMP
• Detailed Geologic Time Scale from GSA (enlarged version): Learning from the Fossil Record.

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

• Global Earth History from NAU. Plate tectonic reconstructions showing the broad patterns of Phanerozoic Earth history
• Animations of plate movements from Christopher Scotese and the UCMP
• A truly excellent Gondwana animation from the International Institute for Aerospace Survey and Earth Sciences (ITC)

  Classification of Vertebrates

Classification and Evolution  

Download Dr. David Cannatella's Adobe Acrobat evolution.pdf   file for a lucid discussion of this topic.

 

Classification:

ordering organisms into groups

central concern; heart of systematic biology

• taxonomy:

  theoretical study of classification; rules, etc

• systematics

  scientific study of the kinds and diversity of organisms and of any and all relationships among them

• from classification further information is possible

Classification of Organisms

• Carolus Linnaeus - 1758
  • Linnaean System: hierarchy of taxa (taxonomic hierarchy)
  • higher categories are more inclusive groups
  • kingdom, phylum, class, order, family, genus, species
  • plus many other categories: e.g. cohort, superfamily
• taxon (taxa): any group(s) in the classification
• nomenclature
  • binomial nomenclature

  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

  The Effect of Human Population Growth on Vertebrate Diversity

Bird extinctions

Major threats affecting birds and mammals

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