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Lecture 3

BISC 316 Lecture Notes - Lecture 3: Ostracoderm, Polyphyly, Stout

Biological Sciences
Course Code
BISC 316
Tammy Mc Mullan

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o The naming and classification of species
o The evolutionary history of a species or group of related species
o The study of biological diversity in an evolutionary context
Why are we concerned with classification of organisms?
Museums (shoot em, stuff em, show em)
Conservation biology and biodiversity
Understanding biology of vertebrates requires an appreciation of the diversity of the organism
that make up this group
Systematics / taxonomy
o Concerned with the diversity of organisms
Originally are grouped into taxa
o Originally based upon similarities
o Now based on evolutionary relationships
Hierarchy of Classification
o basic unit; a group of naturally interbreeding populations that are genetically isolated
from other groups
Rules for classification
1. Narrow taxes subsets of broader taxes
a. Each higher taxa is an assemblage of evolutionary related lower taxa
b. At Order level and below share basic structural features (appendages, teeth type)
2. Broader taxa have fewer characteristics common to all members of the taxon
a. Ancestral features
3. Broader taxa originated earlier in evolutionary time
Phylogenetic Trees
The closer together are more closely related
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Two significant structural features
1. Location of branch point
Relative time of origin of different taxa
2. Extent of divergence between two taxa
Emergence from the common ancestor
If classification is based upon evolutionary history of the group, which property (location or
divergence) should be given the most weight?
There are two different approaches to classification
1. Phenetics
2. Cladistics
1. Phenetics or Numerical Taxonomy
Less subjective
No phylogenetic assumptions
Taxonomic affinities based entirely on measurements
Uses many anatomical characteristics
a. Reduced bias
b. Homology vs. analogy
Computer analysis of multiple quantitative comparisons
Important tool
Molecular comparisons
Critics say morphological similarities does not mean there are genetic similarities
2. Phylogenetic systematics or cladistics
Classifies organisms based upon the branching pattern in the Cladogram
Each taxon evolved by dichotomous splitting from a sister group
Objective identify a series of nesting sister groups
a. Increasingly exclusive levels of evolutionary hierarchy
Each branching point is a novel feature unique to that taxon
Feature that establishes ancestry
Features used to split are called character-states
o Ancestral (or plesiomorpic) or derived (apomorphic)
Evolutionary sequence to character-states
o Cladograms constructed to express probably ancestry
Cladograms not scaled to geologic time
In cladistics
All taxa must be monophyletic
o Each taxon must contain all the descendants of the common ancestor
Other classification systems:
May have taxa that are polyphyletic
o More than one ancestor for members of the taxon
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OR paraphyletic
o May exclude some species that share the same common ancestor
Overview of Vertebrates
9 classes
o All the jawless fish are ebers of Class Agatha
Class Agnatha jawless fish
All extinct for 360 million years except for lampreys and hagfish
Most extinct forms were ostracoderms (ostra= shell ; derm= skin)
Extinct agnathans possess important vertebrate features
Head with cranium, a brain, paired eyes
No true vertebrate
Cartilaginous elements on surface of notochord
Bone present as scales; armor in some species
Mouth, but no jaws; no true teeth
No pectoral or pelvic girdle
Most have no appendages (fins)
Have pectoral spikes or folds
Gills in pouches
Adults predators that sucked in small preys and detritus
Living agnathans (lampreys and hagfish)
have cartilaginous skeletons
lack true teeth, pectoral and pelvic girdles, paired appendages
many lampreys are parasitic
larval form of lamprey very similar to ancestral body plan of vertebrates
2. Class Placoderms = plate skin
Extinct group of fishes
Covered with bony armor
Especially anterior part of body
Head (joined to body by hinge in armor)
Persistent notochord
Novel features included:
1. Jaws enlargement and adaptations of a visceral arch
Allows them to catch larger and harder food
2. Paired appendages
Greater mobility and more efficient locomotion
3. Gas bladder
3. Class Chondrichthyes
Skates, rays, sharks, chimeras (ratfish), and extinct species
Arose at the same time as Acanthodii and Osteichthyes
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