1. 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.

2. 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

3. Classification
Organisms are classified into a hierarchical classification that groups closely related organisms and progressively includes more and more organisms.

5. 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.

6. 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?

7. 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.

8. 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.

9. 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.

10. What is a species?
John Ray ( ) gave first general definition of a species.
A species consists of all individuals that can breed together and produce fertile offspring.

11. A female donkey mated to a male horse
produces what?

12. A mule (which is sterile)
Hence, donkeys and horses
are separate species.

13. 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.

14. 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,

15. 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.

16. 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.

17. 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.”

18. 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.

19. 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.”

20. 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.

21. 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.

22. 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.

23. 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.

24. Phylogenetic trees
Systematists aim to figure out the evolutionary relationships among species.
Branching diagrams called phylogenetic trees summarize evolutionary relationships among organisms.

26. Phylogenetic trees
In a phylogenetic tree the tips of the branches specify particular species and the branching points represent common ancestors.

28. 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.

29. 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.

30. 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.

31. 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.

32. 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.

33. 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.

34. 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.

35. 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.

36. 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.

37. Constructing a cladogram
Cladogram of various vertebrates: monkey, horse, lizard, bass and amphioxus.
Use amphioxus as outgroup (is a chordate, but has no backbone).

39. 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.

40. 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.

41. 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.

42. 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.

43. 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

44. Monophyletic Group
A monophyletic taxon includes the most recent common ancestor of a group and all of its descendents.

46. 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.

48. 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.

50. 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.

51. 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).

52. Traditional Evolutionary Taxonomy
Classification of anthropoid primates.
The 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.

54. 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.

55. 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.

56. 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.

57. 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.

58. Animal Architecture Levels of organization in organismal complexity.
There are 5 major grades of organization each being more complex than the previous.

59. 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).

60. 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.

61. 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.

63. 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.

64. Animal symmetry There are three types of symmetry. Spherical Radial
Bilateral

65. 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.

66. A variety of descriptive terms are used to
describe orientation in bilateral animals.

67. 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.

68. 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.

70. 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.

71. gastrocoel

72. 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.

73. 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.

75. 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.

77. 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.

78. 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

80. 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.

81. Both eucolomate

84. 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”.

85. 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.

86. Figure 08.10

88. Protostomes and Deuterostomes
Protostomes include the annelids, mollusks, and arthropods.
Deuterostomes include the echinoderms and vertebrates.