12.1 The Invention of the Linnaean System To name organisms, biologists use a multilevel grouping of individuals called classification Organisms were first classified more than 2,000 years ago by Aristotle Living things were either plants or animals Later groups started to be formed and referred to as genera (singular genus)

The classification system of the Middle Ages was known as the polynomial system Polynomials were a string of Latin words or phrases consisting of up to 12 or more words This system was cumbersome and confusing In the 1750s, the Swedish biologist Carolus Linnaeus developed the binomial system Binomials are two-part names They have become our standard way of designating species

Fig. 12.1 How Linnaeus named two species of oaks

12.2 Species Names A group of organisms at a particular level in a classification system is called a taxon (plural, taxa) Taxonomy is the branch of biology that identifies and names organisms Organisms are named using Latin and in such a way that no two have the same name Using common names can be misleading

Fig. 12.2 Common names make poor labels Bears Robins Corn

12.2 Species Names By convention, the binomial name consists Genus The first word and is always capitalized Epithet The second word which refers to the species and is not capitalized The two words are written in italics Together, they form the scientific name Example: Apis mellifera The honeybee

Information gets more and more general 12.3 Higher Categories Taxonomists use a hierarchical system to classify organisms spaghetti green for over came Philip King Species Genus Family Order Class Phylum Kingdom Information gets more and more general

Fig. 12.3

12.4 What Is a Species? John Ray (about 1700) A species is a group of individuals that can breed with one another and produce fertile offspring Fig. 12.4 Sterile

12.4 What Is a Species? Ernst Mayr (1940s) Proposed the biological species concept Species are groups that are reproductively-isolated This concept works well for animals, but poorly for other organisms Problems Assumes regular outcrossing Assumes strong reproductive barriers

12.4 What Is a Species? Since the time of Linnaeus, about 1.5 million species have been named However, scientists estimate that at least 10 million species exist At least two-thirds of these occur in the tropics

12.5 How to Build a Family Tree Taxonomy also enables us to glimpse the evolutionary history of life on earth The evolutionary history of an organism and its relationship to other species is called phylogeny The reconstruction and study of phylogenetic trees is called systematics

12.5 How to Build a Family Tree A clade is a group of organisms related by descent Cladistics is the construction of phylogeny based on similarities derived from a common ancestor Examination of these derived characters allows the construction of a branching cladogram Cladograms are not true family trees They convey comparative relationship information Each cladogram must contain an outgroup to which the ingroup is compared

Fig. 12.5 A cladogram of vertebrate animals Ingroup Outgroup

12.5 How to Build a Family Tree Modern cladistics attempts to assign extra weight to the evolutionary significance of key characters Weighting characters lies at the heart of traditional taxonomy Phylogenies are constructed based on a large amount of information about the organism gathered over the years To construct their trees traditional taxonomists use both ancestral and derived characters Cladists use only derived characters

Fig. 12.6 Two ways to classify terrestrial vertebrates Birds have their own Class Birds are lumped with reptiles

12.5 How to Build a Family Tree So which approach is better? Traditional taxonomy, when a lot of information is available to guide character weighting Cladistics, when little information is available about how the character affects the life of the organism

Fig. 12.7 The cat family tree

Fig. 12.7 The cat family tree

12.6 The Kingdoms of Life The designation of kingdoms has changed over the years Originally there were only two kingdoms As more information about organisms was obtained, the number of kingdoms increased! Indeed, a taxonomic level higher than kingdom has been recognized Domain

12.6 The Kingdoms of Life Fig. 12.8 Different approaches to classifying living organisms

More closely related to each other than either is to bacteria Fig. 12.9 A tree of life More closely related to each other than either is to bacteria

12.7 Domain Archaea Consists of only one kingdom, the Archaebacteria Archaebacteria inhabit some of the most extreme environments on earth They share certain key characteristics Cell walls lacking peptidoglycan Unusual lipids and unique rRNA sequences Some archaebacterial genes possess introns

Archaebacteria are grouped into three general categories Methanogens Obtain energy by using hydrogen gas to reduce carbon dioxide to methane gas Extremophiles Grow under extreme conditions Thermophiles (heat) Halophiles (salt) pH-tolerant Pressure-tolerant Nonextreme archaebacteria Grow in the same environments as bacteria do

12.8 Domain Bacteria Consists of only one kingdom, Bacteria Bacteria are the most abundant organisms on earth They play critical roles throughout the biosphere Most taxonomists recognize 12-15 major groups Bacteria are as different from archaebacteria as they are from eukaryotes

12.9 Domain Eukarya Appeared about 1.5 billion years ago Consists of four kingdoms Animalia Plantae Fungi Protista Well defined evolutionary groups Largely muticellular Very diverse Unicellular

TABLE 12.2

TABLE 12.2

12.9 Domain Eukarya The hallmark of eukaryotes is complex cellular organization This is highlighted by the presence of organelles Mitochondria and chloroplasts most likely entered early eukaryotic cells by endosymbiosis Mitochondria are descendants of purple bacteria Chloroplasts are descendants of cyanobacteria

Fig. 12.10 Diagram of the evolutionary relationship among the six kingdoms Symbiotic events

Interestingly, other types of photosynthetic protists are endosymbionts of some eukaryotic organisms Zooxanthellae algae Fig. 12.11 Coral animal