Classification of Organisms

1.         Because there are so many known species, a system of classification is needed for 3 reasons:

            a.         Organize species into groups and discuss them.

            b.         Identify new organisms.

            c.         Show relationships between organisms.

2.         Taxonomy

            a.         The science of classifying organisms.

            b.         The Binomial system

                        i.         Developed by Carolus Linnaeus.

                        ii.        Each organism is given a two part name using Latin as a standard language providing a uniform means of communication for all people. This avoids the confusion caused by organisms with different common names in different areas.

                        iii.       The format is Genus species or G. species. e.g., Castor canadensis

                        iv.       Genus name is capitalized and may be abbreviated by the first initial. Species name is not capitalized and cannot be used alone. e.g., C. canadensis.

                        v.         The 2 part name gives clues about relationships between organisms. For instance, Ursus americanus, U. horribilis, U. arctos, and U. maritimus are all related.

                        vi.       Names were based largely on physical appearances but modern taxonomists use genetic information, molecular biology, and phylogeny (historical evolution) as other criteria for classifying.

                        vii.      The work of Charles Darwin introduced the idea of considering evolutionary history.

            c.         The taxonomic hierarchy

                        i.         The binomial classification system is hierarchal. The levels of organization are kingdom, phylum, class, order, family, genus, species. In plants, fungi and algae phyla also called divisions. Each of these levels is called a taxon.

                        ii.        Note that the genus and species name are italicized because they are Latin. When handwriting, underline the words. Other levels are capitalized but no special print features are used.

            d.         What is a species?

                        i.         Capable of reproducing with one another.

                        ii.        Individuals of one species may appear quite dissimilar.

                        iii.       Offspring may appear different from one another.

                        iv.       Individuals from different species do not generally reproduce with one another.

                        v.         Estimates on the number of species range from 2 and 100 million species on the planet although about 1.4 million species are currently named and described. Note that this is for eukaryotic species only. It is much more difficult to estimate the number of prokaryotic species.

3.         The six kingdoms system

            a.         Originally there were only two kingdoms recognized: animals and plants.

            b.         Later, these two were divided into five: animals, plants, fungi, protists, and bacteria. Each kingdom evolved from different single-celled ancestors.

            c.         Most people now recognize 6 kingdoms:

                        i.         Two prokaryotic (Kingdom Monera) - asexual reproduction

                                    (1)       Archaebacteria are very ancient bacteria.

                                    (2)       Eubacteria are more modern bacteria.

                                                (a)       Inhabit nearly every known habitat

                                                (b)       Prokaryotic

                                                (c)       Consumers, producers, and decomposers

                                                (d)       Some cause disease but most are harmless

                        ii.        Four eukaryotic - sexual reproduction

                                    (1)       Kingdom Protista

                                                (a)       Contains mostly unicellular organisms, including algae, although there are some exceptions. Members have been lumped together in this kingdom because they don’t seem to fit anywhere else.

                                                (b)       Eukaryotic

                                                (c)       Some show characteristics of animals, some plants

                                    (2)       Kingdom Fungi

                                                (a)       Eukaryotic

                                                (b)       Contains multicellular species and single-celled yeasts.

                                                (c)       Have some characteristics of plants but differ in that they are not photosynthetic.

                                                (d)       Decomposers

                                                (e)       Reproduce by spores

                                    (3)       Kingdom Plantae

                                                (a)       Eukaryotic

                                                (b)       Multicellular

                                                (c)       Producers

                                                (d)       Cell wall made of cellulose

                                    (4)       Kingdom Animalia

                                                (a)       Multicellular

                                                (b)       Eukaryotic

                                                (c)       Consumers

                                                (d)       Motile

            d.         There are greater differences between prokaryotes and eukaryotes than between plants and animals. Also, there is greater diversity between the two prokaryotic groups than among all eukaryotic groups.

            e.         Evolution of kingdoms

                        i.         Bacteria first appeared over 3 billion years ago and were the only organisms on Earth for about 2 billion years.

                        ii.        Fungi, plants and animals are well-defined evolutionary groups, each having arisen from different unicellular ancestors.

                        iii.       These groups are mostly multicellular, and derived from protist ancestors.

4.         Viruses - where do they fit?

            a.         Viruses are not included in any of the six kingdoms.

            b.         Viruses are tiny structures and are called particles rather than cells. There is considerable debate as to whether they are alive or not.

            c.         They consist of a protein shell containing some genetic material with no organelles, no membrane of their own, and they do not divide.

            d.         They use the host organelles to reproduce.