Chapter 15: Classification

Why Classify? To study the diversity of life, scientists use a classification system to name organisms and group them in a logical manner. Two features of biological classification systems: 1. Each organism has a universally accepted name. 2. Organisms are grouped based on shared biologically important traits. Taxonomy - science of naming and classifying organisms.

History of classification Aristotle - first person who grouped plants and animals based on structural similarities. Greeks and Romans continued grouping organisms into basic categories like: horses, dogs, oaks, etc. In the middle ages, each category was called a genus and each was given a Latin name (ex. Cats - Felis). Latin phrases (called polynomials) used to describe the physical details of each species of a genus. Polynomial system was very confusing - up to 20 words were used to describe each species! Ex. “Oak with deeply divided leaves that have no hairs on their undersides and no teeth around their edges” Linnaeus - person who simplified system of naming organisms.

Scientific names are universal Linnaeus developed a 2-word Latin naming system; this system is called binomial nomenclature. When naming using binomial nomenclature, each organism has a scientific name - unique 2-word name for a species.

Scientific names are universal The first word of a scientific name is the genus to which an organism belongs. An organisms is assigned a genus based on its major characteristics. Ex. Oak trees all produce acorns and are put into the genus Quercus.

Scientific names are universal The second word of a scientific name is the species name which identifies a particular organism within its genus. Ex. Red Oak - Quercus rubra White Oak - Quercus phellos Notation alert! The first letter of the genus is always a capital letter, the first letter of the species is always a lowercase letter! The scientific name is always italicized! Dog - Canis familiaris Cat - Felis domesticus

Taxonomy After naming organisms, Linnaeus group them according to their body structures. Organisms that shared important characteristics were classified into the same groups. These groups are called taxa (singular, taxon). Taxonomy - the science of naming organisms and assigning them groups or taxa for classification.

Classification via hierarchies Organisms are classified into seven different taxa. The seven taxa include: Kingdom, Phylum, Class, Order, Family, Genus, Species. Pneumonic: King Phillip Came Over For Great Spaghetti. Kingdom is the largest and most general / inclusive classification group. Species is the smallest and most specific group. Subspecies - least inclusive, not every member of a species is alike. Ex. red squirrel, southern red squirrel

Classification via hierarchies Honeybee: Kingdom - Animalia Phylum - Arthropoda Class - Insecta Order - Hymenoptera Family - Apidae Genus - Apis Species - Apis mellifera

Problems with traditional classification Sometimes organisms that are quite different from each other evolve similar structural characteristics. These similarities make it difficult for taxonomists to decide how organisms should be classified. Organisms share certain traits because they share evolutionary history. Biologists today group organisms into categories that represent lines of evolutionary descent, not just physical similarities.

Classification using cladograms Cladistic analysis identifies and considers only those new characteristics of organisms that arise as lineages evolve over time. Cladogram - a diagram that shows evolutionary relationships between organisms. Clade - groups on cladograms. The larger clade on a cladogram contain increasingly smaller clades that share the traits of the clades before them, but also exhibit distinct changes as the organisms evolves.

Example cladogram Derived characteristics - characteristics that appear in recent parts of a lineage but not in its older members. Primitive characteristic - characteristics shared by all organisms in a lineage.

Similarities in DNA The genes (DNA) of organisms often show important similarities at the molecular level. These similarities can be used as a criteria for determining classification of the organisms. The more dissimilar the genes are between organisms, the more evidence there is indicating how long ago the two species shared a common ancestor.

Six kingdoms of life There are six kingdoms into which living organisms are grouped: Archaebacteria Eubacteria Protista Fungi Plantae Animalia LEARN CHARACTERISTICS OF EACH!

Dichotomous keys Dichotomous key - tool that allows users to identify organisms in the natural world. Consist of a series of questions or choices that lead the user to the correct identification of the organism. Like playing “20 Questions”. Dichotomous means “divided into two parts”. Dichotomous keys always give two choices at each step - follow the directions in the key (answering questions as you go) to identify the organism.

To USE a dichotomous key… Some hints include: Always read both choices, even if the first seems to be the logical one at first. Be sure you understand the meaning of the terms involved. Do not guess! When measurements are given, measure! Do not guess! If the choice is not clear, for whatever reason, try both divisions. If you end up with two possible answers, read descriptions of the two choices to help you decide. Having arrived at an answer in a key, do not accept this as absolutely reliable. Check a description of the organism to see if it agrees with the unknown specimen. If not, an error has been made.

To MAKE a dichotomous key… Some hints include: Use measurements. Make the choice a positive one - something "is" instead of "is not". If possible, start both choices of a pair with the same word. If possible, start different pairs of choices with different words. Precede the descriptive terms with the name of the part to which they apply.