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Classification
 Before
exploring the many groups of
animals we need to need to deal with the
general topic of classification or how we
group organisms into a manageable
framework.
Classification
 Science
of Systematics dates to Linnaeus
in the 18th century who devised the basic
systems of binomial nomenclature and
hierarchical classification in use today.
 All
organisms have a unique binomial
name
 E.g. Humans are Homo sapiens
Classification
 Organisms
are classified into a
hierarchical classification that groups
closely related organisms and
progressively includes more and more
organisms.
Species
 The
species is the basic biological unit
around which classifications are based.
 However,
what constitutes a species can
be difficult to define and there are multiple
definitions of species in use today.
What is a species?
 The
species is a basic biological unit and
humans seem to intuitively recognize
species.
 However, why do species exist?
 Why
don’t we see a smooth continuous
blending of one species into another?
Why do we see discrete species?
 Because
intermediate forms between
closely related organisms are usually
selected against.
 If
they were not selected against, then the
two forms would merge into one as their
gene pools mixed.
Why do we see discrete species?
 Organisms
are very well adapted to their
environments having evolved over millions
of years.
 Each organism has specialized
characteristics such as camouflage,
feeding structures, behavior, and genitalia
that equip it to survive well in its
environment.
Why do we see discrete species?

An offspring that results from a cross between members
of two different species or between members of different
populations that have been evolving in isolation from
each other, will probably have traits intermediate
between its parents.

As a result, it likely will be less well adapted to its
environment than either parental form and be selected
against.

Thus, we see distinctively different species.
What is a species?
 John
Ray (1627-1705) gave first general
definition of a species.
 A species
consists of all individuals that
can breed together and produce fertile
offspring.
A female donkey mated to a male horse
produces what?
A mule (which is sterile)
Hence, donkeys and horses
are separate species.
Biological Species Concept

Ray’s idea was updated into the Biological Species
Concept. Two definitions of the BSC are given below:

“Species are groups of actually or potentially
interbreeding natural populations, which are
reproductively isolated from other such groups.” Ernst
Mayr.

“A species is a reproductive community of populations
(reproductively isolated from others) that occupies a
specific niche in nature.” Ernst Mayr.
Biological Species Concept

The biological species concept emphasizes that
a species is an interbreeding population of
individuals sharing common descent and that
members of that community because they share
a niche constitute an ecological entity in nature.

Members of a species we expect to be similar to
each other but different from other organisms,
Criticisms of the Biological Species
Concept

The BSC has been criticized for several
reasons:
 1. It applies only to sexually reproducing
species.
 2. Distinguishing between species on the basis
of reproductive separation is problematic
because it can be difficult to determine how
much reproductive separation is needed to
distinguish between species.
 3. The definition refers only to current
populations and ignores the species status of
ancestral populations.
Evolutionary Species Concept
 George
Gaylord Simpson proposed the
Evolutionary Species Concept in the
1940’s to add an evolutionary time
dimension to the Biological Species
Concept.
Evolutionary Species Concept
 Evolutionary
species concept “A single
lineage of ancestor-descendant
populations that maintains its identity from
other such lineages and that has its own
evolutionary tendencies and historical
fate.”
Evolutionary Species Concept
 Definition
applies to both sexually and
asexually reproducing species and
emphasizes common descent. As long as
diagnostic features are maintained a
lineage will be recognized as a single
species.
Phylogenetic species concept
 A third
species concept is the phylogenetic
species concept.
 “an irreducible (basal) grouping of
organisms diagnosably distinct from other
such groupings and within which there is a
parental pattern of ancestry and descent.”
Phylogenetic species concept
 The
phylogenetic species concept also
emphasizes common descent and covers
both sexually and asexually reproducing
organisms.
 Under
the PSC any population that has
become separated and has undergone
character evolution will be recognized as a
species.
Phylogenetic species concept

Criterion of irrreducibility requires that no more
than one diagnosibly distinct population can be
included in a single species.

Main difference in practice between ESC and
PSC is that PSC recognizes as species the
smallest groupings of organisms that have
undergone independent evolutionary change.
Phylogenetic species concept

The ESC would group into one species a series
of geographically disjunct populations that show
some genetic divergence, but the PSC would
treat them as discrete species.

Thus, subspecies under the ESC would be
species under the PSC and in general more
species would be recognized under the PSC
than either the BSC or ESC.
Typological Species concept

For historical interest this is the pre-Darwinian idea
that species are defined by fixed and unchanging
features and do not change over time (i.e. evolve).

Biologists discarded the idea after Darwin’s theory
of evolution by natural selection became
established.

Creationist’s still cling to the typological species
concept and you’ll often see “types” referred to in
creationist writings.
Phylogenetic trees
 Systematists
aim to figure out the
evolutionary relationships among species.
 Branching
diagrams called phylogenetic
trees summarize evolutionary relationships
among organisms.
Phylogenetic trees
 In
a phylogenetic tree the tips of the
branches specify particular species and
the branching points represent common
ancestors.
Phylogenetic trees
 Phylogenetic
trees are constructed by
studying features of organisms formally
called characters.
 Characters
may be morphological or
molecular.
 Character
similarity resulting from shared
ancestry is called homology.
Cladistics and the construction
of phylogenetic trees
 Cladograms
are diagrams that display
patterns of shared characteristics.
 If
shared characteristics are due to
common ancestry (are homologous) the
cladogram forms the basis of a
phylogenetic tree.
Cladograms
 Within
a tree a clade is defined as a group
that includes an ancestral species and all
of its descendants.
 Cladistics
is the science of how species
may be grouped into clades.
Shared derived characters

Cladograms are largely constructed using
synapomorphies or shared derived
characters.

These are characteristics that are evolutionary
novelties or new developments that are unique
to a particular clade.

For example, for birds possession of feathers is
a shared derived character and for mammals
possession of hair is.
Shared primitive characters

Shared primitive characters are characters that
are shared beyond the taxon we are interested
in. Among groups of vertebrates the backbone
is an example because it evolved in the ancestor
of all vertebrates.

If you go back far enough in time a shared
primitive character will become a shared derived
character. Thus, the backbone is a shared
derived character that distinguishes vertebrates
from all other animals.
Constructing a cladogram
 Outgroup
comparison is used to begin
building a cladogram.
 An
outgroup is a close relative of the
members of the ingroup (the various
species being studied) that provides a
basis for comparison with the others.
Constructing a cladogram
 The
outgroup lets us know if a character
state within the ingroup is ancestral or not.
 If
the outgroup and some of the ingroup
possess a character state then that
character state is considered ancestral.
Constructing a cladogram

For example, birds, mammals and reptiles are
all amniotes (produce hard-shelled or amniotic
eggs). Birds have no teeth, but mammals and
reptiles do.

An outgroup to the amniotes, fish, possesses
teeth. Therefore, the ancestral state among the
amniotes is to possess teeth and birds have
secondarily lost them.
Constructing a cladogram
 Having
the outgroup for comparison
enables researchers to focus on those
characters derived after the separation
from the outgroup to figure out
relationships among species in the
ingroup.
Constructing a cladogram
 Cladogram
of various vertebrates:
monkey, horse, lizard, bass and
amphioxus.
 Use
amphioxus as outgroup (is a
chordate, but has no backbone).
Constructing a cladogram

In the cladogram new characters are marked on
the tree where they originate and these
characters are possessed by all subsequent
groups.
Cladograms and Phylogenetic
Trees

A cladogram and a phylogenetic tree are similar,
but not identical.
A phylogentic tree’s branches represent real
evolutionary lineages and branch lengths
represent time or amounts of evolutionary
change.
 Cladogram branches contain no such
information. Branching order of cladogram
should, however, match that of phylogenetic
tree.

Early phylogenetic tree of
amniotes based on
cytochrome c gene by
Fitch and Margoliash (1967).
Note numbers on branches.
These represent estimated
numbers of mutational
changes in gene.
Theories of taxonomy
There are two current major theories of
taxonomy:


Traditional Evolutionary Taxonomy
Phylogenetic Systematics (Cladistics)
 Both
based on evolutionary principles, but
differ in the application of those principles
to formulate taxonomic groups.
Theories of taxonomy
 There
are three different ways a taxon
may be related to a phylogentic tree.
 The taxon may be a monophyletic,
paraphyletic or polyphyletic grouping
Monophyletic Group
 A monophyletic
taxon includes the most
recent common ancestor of a group and
all of its descendents.
Paraphyletic group
 A taxon
is paraphyletic if it includes the
most recent common ancestor of a group
and some but not all of its descendents.
Polyphyletic grouping

A taxon is polyphyletic if it does not contain the
most recent common ancestor of all members of
the group.

This situation requires the group to have had
independent evolutionary origin of some
diagnostic feature. E.g. If you grouped birds
and bats into a group you called “WingedThings”
it would be a polyphyletic group because birds
and bats evolved wings separately.
Theories of taxonomy
 Both
traditional evolutionary taxonomy and
cladistics reject polyphyletic groups.
 They
both accept monophyletic groups,
but differ in their treatment of paraphyletic
groupings.
Traditional Evolutionary Taxonomy

TET uses two principles for designating taxa.



Common descent
Amount of adaptive evolutionary change
The second criterion leads to the idea that
groups may be designated as higher level taxa
because they represent a distinct “adaptive
zone” (Simpson) because they have undergone
adaptive change that fits them to a unique role
(e.g. penguins, humans).
Traditional Evolutionary Taxonomy
 Classification
 The
of anthropoid primates.
genera Gorilla, Pan (chimpanzee) and
Pongo (orang utan) are grouped into
family Pongidae and humans (genus
Homo) into family Hominidae even though
humans are phylogenetically closer to
Gorilla and Pan than either of those is to
Pongo.
Traditional Evolutionary Taxonomy
 Under
TET designation of family
Hominidae is because humans represent
a different grade of organization.
 Humans
are terrestrial, intelligent,
omnivores with advanced cultures.
 Members
of Pongidae are arboreal, less
intelligent, herbivores.
Cladistics
 Cladistics
emphasizes the criterion of
common descent. Cladistic approach
proposed by Willi Hennig in 1950.
 Under
cladistic rules all groups must be
monophyletic. Thus, cladists group the
Pongidae and Hominidae into one group
the Hominidae.
Differences between Evolutionary
Taxonomy and Cladistcis

Differences between ET and cladistics become
apparent in considering evolution.

Saying that amphibians evolved from fish or
birds from dinosaurs is meaningless to a cladist
because it implies the descendant group
(amphibians or birds here) evolved from an
ancestral group that the descendant does not
belong to. To an evolutionary taxonomist,
however, amphibians and fish are different
grades.
Current taxonomy
 Current
taxonomy was developed using
evolutionary systematic approaches, but
has been revised in part using cladistic
approaches.
 How
classification may finally be resolved
is unclear, but the issues of paraphyletic
groups and grades remain to be sorted
out.
Animal Architecture
 Levels
of organization in organismal
complexity.
 There
are 5 major grades of organization
each being more complex than the
previous.
Levels of organization

1. Protoplasmic level: occurs in unicellular
organisms. Organelles within the cell carry out
specialized functions. Protozoans are examples.

2. Cellular level: Cells are aggregated and cells
engage in a division of labor, being specialized
for particular tasks. Colonial protozoan groups
with distinct somatic and reproductive cells and
sponges are examples. (Animals that are
multicellular are referred to as Metazoans).
Levels of organization
 3.
Cell-tissue level: Similar cells
aggregate into patterns or layers forming
tissues. Nerve net in Cnidarians (e.g.
jellyfish) is example of a tissue.
Levels of organization
 4.
Tissue-organ level: Organs are made up
of more than one kind of tissue and have a
specialized function. Flatworms
(Platyhelminthes) generally represent this
level having organs such as eyespots and
reproductive organs, but their reproductive
organs are organized into level 5 an organ
system.
Levels of organization
 5.
Organ-system level: Organs work
together to perform functions. Most
complex level of organization.
 Examples
of organ systems include
circulatory, reproductive, digestive,
respiratory. Most animal phyla exhibit this
level of organization.
Animal symmetry
 There



are three types of symmetry.
Spherical
Radial
Bilateral
Animal symmetry

Spherical symmetry occurs mainly among
protozoans.
 Radial symmetry occurs among the Cnidarians
(jellyfish) and Echinoderms (starfish, sea
urchins).
 Bilateral symmetry commonest form of
symmetry. Strongly associated with
cephalization or development of a head with
associated sensory and feeding apparatus.
A variety of descriptive terms are used to
describe orientation in bilateral animals.
Development of body plans
 An
animal’s body results from division of
cells during embryonic development.
 Differences
in developmental patterns
have been used to classify more complex
animals so an understanding of basic
embryology is necessary to follow this.
Process of development

Once an egg is fertilized it becomes a zygote.
This cell divides into a large number of cells
called blastomeres.

Cleavage of cells proceeds until a fluid-filled
hollow ball of cells is formed. This is a blastula.

In multicellular animals other than sponges the
blastula invaginates to begin forming the future
gut. At this stage the embryo is a gastrula.
Process of development

The invaginating layer of cells, which will give
rise to the gut, form a germ layer called the
endoderm. The endoderm surrounds and
defines a body cavity called the gastrocoel.

The cells not involved in forming the invagination
constitute another germ layer the ectoderm. The
ectoderm surrounds a cavity called the
blastocoel.
gastrocoel
Process of development
 When
the invaginating gastrocoel forms a
complete tube by forming a second
opening to the outside it is then called the
gut.
 In
the cnidarians (jellyfish, sea anemones)
no second opening develops.
Process of development
 In
most animals (but not cnidarians, which
are two-layered or diploblastic) a third
germ layer of cells called the mesoderm
develops.
 The
mesoderm gives rise to many internal
organs. Organisms with mesoderm are
called triploblastic having three germ
layers.
Germ layers

Endoderm: innermost germ layer of an embryo.
Forms the gut, liver, pancreas.

Ectoderm: Outer layer of cells in early embryo.
Surrounds the blastocoel. Forms outer
epithelium of body and nervous system.

Mesoderm: Third germ layer formed in gastrula
between ectoderm and endoderm. Gives rise to
connective tissue, muscle, urogenital and
vascular systems and peritoneum.
Process of development
 The
way in which the mesoderm forms
and whether or not a cavity (called a
coelom) develops within it are important
characters in deciphering the relatedness
of animal groups.
Coeloms
 The
coelom is a cavity entirely surrounded
by mesoderm.
 A coelom provides a tube-within-a-tube
arrangement which has many advantages:



Allows flexibility in arranging visceral organs
permits greater size and complexity by
exposing more cells to surface exchange
fluid-filled ceolom can act as a hydrostatic
skeleton
Coeloms

Triploblastci organsims (organisms with three
germ layers including mesoderm fall into one of
three different coelomic states:



Acoelomate: mesoderm fills the blastoceol, no cavity
occurs in the mesoderm. Flatworms and nemerteans.
Pseudocoelomate: mesoderm lines only outer edge
of blastocoel. No peritoneal lining develops.
Nematodes and rotifers.
Eucoelomate: Have a true coelom derived from
mesoderm and lined with peritoneum. Arthropods,
annelids, mollusks, echinoderms, vertebrates.
Both eucolomate
Protostomes and Deuterostomes
 Within
the eucolomates there are two
major evolutionary lineages that split early
in the history of animals and follow quite
different developmental pathways.
These are the protostomes “mouth first” and
deuterostomes “mouth second”.
Important differences in development
between protostomes and deuterostomes

The differences in development that distinguish
the protostomes and deuterostomes include:




Whether cleavage of cells in the early zygote is spiral
or radial.
Whether or not, if the early blastomere is separated,
each cell can develop into a normal larva or not.
Whether the blastopore ultimately forms the mouth or
anus of the organism.
Whether or not the organism possesses a coelom
and how that coelom is formed.
Figure 08.10
Protostomes and Deuterostomes
 Protostomes
include the annelids,
mollusks, and arthropods.
 Deuterostomes
include the echinoderms
and vertebrates.