sign in sign up
<<
Home , Evolutionary grade, Invertebrate Zoology

The University of Dodoma

Department of Biological Sciences

BI 101-

TOPIC: Principles of Classification of

Instructor: ASD B.Sc, M.Sc (Applied Zoology), PhD (Cand) Introduction to

The majority of the that inhabit the earth are . Some possess a backbone or , but many do not. Animals throughout the world are divided into two major groups; those that possess a backbone (vertebrates) and those that do not (invertebrates) These groupings are based largely on tradition and convenience rather than natural biological differentiation

Invertebrates are found in most habitats including the depths of the oceans and on glacial ice Invertebrates serve various important ecological roles including consumption and reduction of organic matter, decomposition, and as a food source. In this course, we will study and diversity of the invertebrates; building an understanding of the evolutionary relationships and taxonomic classification of the invertebrates as currently understood  We will also highlight some sub-disciplines of invertebrate zoology such as and

Definition of terms Diversity: Measure of success through adaptation and variation. It is characterized by number of species

Abundance: Measure of the specialization and successful competition, often in particular environmental conditions It is characterized by number of individuals

Biological Species Concept

 The term species was first proposed by John Ray, an English clergyman and scientist, around 1700. He pointed out that all individuals that belong to a given species breed with one another and produce fertile progeny. This concept is still used today

 breeds of dogs are one species, as they can all interbreed, even if some individuals look so different

, an American, the father of population genetics and an evolutionist, has redefined the concept, pointing out that “species are groups of actually or potentially interbreeding natural populations which are reproductively isolated from one another”

 The biological concept works fairly well for animals where there are strong barriers to hybridization between species

Biological Species Concept

Animals regularly outcross, and so the concept works well. Outcrossing is less common in other kingdoms. Species in the other kingdoms-bacteria, fungi, etc do not outcross/interbreed with one another as many reproduce and so cannot be characterized in the same way as outcrossing animals and

Most species can be separated by fairly obvious anatomical, physiological and behavioral characters but the final and key separation criterion is reproduction, i.e. whether or not there is actual or potential gene flow SPECIATION

How can two sets of populations that initially share a common gene pool come to have a completely separately gene pool? Speciation usually involves progressive change. First local populations become increasingly specialized. If they become different enough, may act to keep them that way SPECIATION

There are different isolation mechanisms including: 1) Geographical isolation 2) Ecological isolation –occurs in the same area, but occupy different habitants isolation –reproducing in different seasons 3) Behavioural isolation- differs in their mating ritual 4) Mechanical isolation –structural differences between races/species preventing mating 5) Failure of gametes to fuse Classification of Living things  The earth is inhabited by millions of different kinds of organisms. In order to study them, we need to group (classify) them and give them names  Classification of organisms refers to the process of putting together organisms that bear resemblance to one another

refers to the science of classifying organisms into natural groupings

 Biosystematics is the science through which life forms are discovered, identified, described, named, classified and catalogued, with their diversity, life histories, living habits, roles in an ecosystem, and spatial and geographical distributions recorded. It can also be defined as the study of the kinds and diversity of organisms and the relationship between them

 Organisms were first classified more than 200 yrs ago by (Greek) into plants or animals. Later on, this simple classification was expanded by the Romans who grouped animals into units/groups called genera (sing: genus)  E.g. the cats were assigned the genus Felis, horses to genus Equus etc

Binomial nomenclature Until 250 years ago, biologists used to add a series of descriptive terms to the name of the genus when they wanted to refer to a particular kind of organisms (species). There were so many phrases used and the system was known as Polynomial system Swedish biologist; Carolus Linnaeus(1707-78) simplified the system into a system in which a species is given a name consisting of two words; first word, refers to the name of the genus in which that species belongs and the second word, refers to a designation for that particular species. The combination of the two words constituted scientific name for the species  E.g. Honey bee is scientifically named Apis melifera Binomial nomenclature contd: Scientific names serve the purpose of avoiding confusion since any scientific name is the same regardless of your native language or geographical position and that no more than one can have the same scientific name  This can be accomplished because there are sets of international rules on naming living things; The International Code of Zoological Nomenclature (ICZN) which deals with animals and heterotrophic protozoans, The International Code of Botanical Nomenclature (ICBN) for plants, fungi, algae, and photosynthetic protozoans, and The International Code of Nomenclature of Bacteria (ICNB) for bacteria and archaea. Scientists try to adhere to these codes closely.

Binomial nomenclature contd:  Normally, scientific names are italicized or underlined, with the generic name capitalized and the specific name all in small case letters.

 Upon first mention of a species in any write up, full name must be given, but the name can be abbreviated to the first letter of the genus and the species name in subsequent mention.  However, in some cases, particularly in the mosquito literature, the genus name abbreviation consists of two letters, Ae. for Aedes, An. for Anopheles, Cx. for Culex, Cs. for Culiseta, and so on.

 Sp. and Spp.: When dealing with organisms for which, for various reasons, we know the genus, but not the species, the organism is identified by the generic name followed by the abbreviation "sp“. The plural abreviation "spp." is usually used to refer to all the individual species within a genus. i.e. sp.= single species spp.= plural, several species. These abbreviations can also be used when the names of the specific species is not important to the subject under discussion. Linnaeus Classification

Linnaeus reorganized 7 categories of classification: species, genus, , order, , and in a hierarchical order  Each category in the hierarchy is a collective unit containing one or more groups from the next lower level  A genus is a group of closely related species; a family is a group of related genera; an order is a group of closely related families etc  From evolutionary point of view, species in any one genus are believed to be more closely related than to species in any other genus Taxonomy and  The more similar two taxa are, the more closely related they are likely to be  the more similar two organisms are in their (physical characteristics) the more recently did they last share a common ancestor

 Linnaean () system of classification assigns every organism a kingdom, phylum, class, order, family, genus, and species. This is an approach to taxonomy based on measurable similarities and differences in phenotypic characters, but without consideration of homology, analogy, or phylogeny

 categories defined often group organisms that are not necessarily closely related, just similar in their overall grade of evolution.

 This has the effect of excluding some organisms that have inherited a conspicuous evolutionary novelty and lumping together other organisms based on more primitive traits.  E.g. For example, are now known to be the direct evolutionary descendents of small, predatory . The skeletal similarities between Mesozoic birds and coelurosaurian dinosaurs are remarkable, yet birds are not classified as because they have feathers.

Cladistics system of Classification Because the Linnaean system is not based on evolution, most biologists are switching to a classification system that reflects the organism’s evolutionary history  Evolutionary relatedness among species is referred to as phylogeny  is an approach to taxonomy that uses evolutionary descent as the sole criterion for classification.  It was developed by Willi Hennig, an entomologist, in 1950. Under this taxonomy approach, it is phylogeny rather than evolutionary grade or overall similarity that counts Not all traits are of equal importance in determining evolutionary relationships

Cladistics system of Classification The basis of a cladistic analysis is data on the characters or traits, of the organisms in which we are interested. These characters could be anatomical and physiological characteristics, behaviors, or genetic sequences The result of a cladistic analysis is a tree, which represents a supported hypothesis about the relationships among the organisms Terms used in Cladistics  : A branching diagram that depicts species divergence from common ancestors  They show the distribution and origins of shared characteristics  Apomorphy is a derived or specialised character, unique to a particular species and all its descendants and which can be used as a defining character for a species or group in phylogenetic terms

 An is an apomorphy that is restricted to a single species. An example is speech, which is found solely in ( sapiens) and not in other primates

 Synapomorphy is an apomorphy that is shared by two or more species or groups. Such traits define the strictly , which are the basis of cladistic classification systems. Synapomorphy is evidence that the taxa in question are related Terms used in Cladistics  Plesiomorphy is an ancestral or primitive character. These are primitive for the group in question and cannot provide evidence for the group. For example, vertebrae are found in zebras, cheetahs, and orang-utans, but the common ancestor in which this trait first evolved is so distant that the trait is shared by many other animals. Therefore, possession of vertebrae sheds no light on the phylogenetic relations of these three species

 Symplesiomorphy refers to the plesiomorphy that is shared between two or more taxa. Shared possession of a symplesiomorph character state is not evidence that the taxa in question are related  Monophyletic : A group composed of a collection of organisms, including the most recent common ancestor of all those organisms and all the descendants of that most recent common ancestor. A monophyletic taxon is also called a  Paraphyletic taxon: A group composed of a collection of organisms, including the most recent common ancestor of all those organisms but does not include all the descendants of the most recent common ancestor  These are incomplete groups based primarily on physical characteristics rather than directly on evolutionary relationships  Polyphyletic taxon: A group composed of a collection of organisms in which the most recent common ancestor of all the included organisms is not included, usually because the common ancestor lacks the characteristics of the group  This is an artificial group which is based primarily on physical characteristics rather than on evolutionary relationships Terms used in Cladistics

We often think of primitive things as being simpler and inferior — but in many cases the original (or plesiomorphic) state of a character is more complex than the changed (or apomorphic state). For example, as they have evolved, many animals have lost complex traits (like vision and limbs). In the case of , the plesiomorphic characteristic is "has legs" and the apomorphic characteristic is "doesn't have legs." Constructing phylogenetic trees (phylogenies) An evolutionary (Phylogenetic) tree represents the evolutionary relationships among a set of organisms or groups of organisms, called taxa (singular: taxon)  The tips of the tree represent groups of descendent taxa (often species) and the nodes on the tree represent the common ancestors of those descendents Two descendents that split from the same node are called sister groups Constructing phylogenetic trees contd:

Constructing phylogenetic trees contd:

An — a taxon outside the group of interest. All the members of the group of interest (ingroup) are more closely related to each other than they are to the outgroup A clade is a group of organisms that includes an ancestor and all descendents of that ancestor. You can think of a clade as a branch on the tree of life Constructing phylogenetic trees contd: Constructing phylogenetic trees contd:

Testudines, Squamata, Archosauria, and Crocodylomorpha all form clades

Constructing phylogenetic trees contd:

 In contrast to the traditional Linnaean system of classification, phylogenetic classification names only clades  For example, a strictly Linnaean system of classification might place the birds and the non-Avian dinosaurs into two separate groups. However, the phylogeny of these organisms reveals that the actually branches off of the lineage, and so, in phylogenetic classification, the birds should be considered a part of the group Dinosauria Parsimony Principle in constructing phylogenies Parsimony Principle in constructing phylogenies

Consider the two hypotheses about vertebrate relationships using the parsimony principle:

Parsimony Principle in constructing phylogenies Hypothesis 1 requires six evolutionary changes and Hypothesis 2 requires seven evolutionary changes, with a bony skeleton evolving independently, twice. Although both fit the available data, the parsimony principle says that Hypothesis 1 is better — since it does not hypothesize unnecessarily complicated changes. Basic assumptions in cladistics  There are three basic assumptions in cladistics:

1) Change in characteristics occurs in lineages over time 2) Any group of organisms is related by descent from a common ancestor 3) There is a bifurcating, or branching, pattern of lineage-splitting Advantages of phylogenetic classification 1. Phylogenetic classification tells us about the organism’s evolutionary history 2. Phylogenetic classification does not attempt to "rank" organisms. Linnaean classification "ranks" groups of organisms artificially into kingdoms, phyla, orders, etc. This can be misleading as it seems to suggest that different groupings with the same rank are equivalent

 Biologists deal with phylogenetic classification by de-emphasizing ranks and by reassigning names so that they are only applied to clades. This means that your use of biological names doesn't have to change very much. In many cases, the Linnaean names are perfectly good in the phylogenetic system. For example, Aves, which is the class of birds in the Linnaean system, is also used as a phylogenetic name, since birds form a clade

 Most of the specific names that you are accustomed to using (e.g., Homo sapiens, Drosophila melanogaster) have not changed at all with the rise of phylogenetic classification. However, there are some names from Linnaean classifications that do NOT work in a phylogenetic classification. For example, the reptiles do not form a clade (and cannot be a named group in the phylogenetic system) — unless you count birds as members of Reptilia too.