Chapter 18 – Classification

18-1 Finding Order in Diversity

To study the diversity of organisms, biologists must give each organism a name and organize living things into groups that have biological meaning. To study the diversity of life, biologists use a classification system to name organisms and group them in a logical manner. Taxonomy – scientists classify organisms and assign each organism a universally accepted name. By using a scientific name, biologists can be sure that everyone is discussing the same organism.

Example: this animal is commonly referred to as a mountain lion, a puma, a cougar, or a panther. These are all common names for the same animal. The scientific name for this animal is Felis concolor

Assigning Scientific Names Early names tended to be very long, in order to fully describe the animal. A Swedish Botanist named Carolus Linnaeus, who lived during the eighteenth century, developed a two-word naming system called binomial nomenclature. This system is still used today. In binomial nomenclature, each species is assigned a two-part scientific name. The scientific name is always written in italics. The first word is capitalized, and the second word is lowercased.

Grizzly bear is called Ursus arctos The first word of the scientific name, in this case, Ursus, is the genus to which the animal belongs.

Genus – a group of closely related species. The second part of the scientific name, in this case, arctos, is unique to each species within the genus. It usually describes some important trait of the organism or an indication of where the organism lives. Ursus maritimus is the scientific name of a polar bear. Maritimus refers to the sea and comes from the fact that polar bears often live on pack ice that floats in the sea

Linnaeus’s System of Classification Linnaeus’s classification system consists of levels. It includes seven levels. They are- from smallest to largest – species, genus, family, order, class, phylum, and kingdom. Each of these levels is called a taxon.

Family – group of genera that share many characteristics Family – group of genera that share many characteristics bears (Ursidae), dogs (Canidae), cats (Felidae) Order – a broad taxonomic category composed of similar families Class – composed of similar orders (ex - order Carnivora is placed in the class Mammalia, which includes animals that are warm-blooded, have body hair and produce milk for their young) Phylum – several different classes make up phylum. A phylum includes many different organisms that share important characteristics.

The class Mammalia is grouped with birds (class Aves), reptiles (class Reptilia), amphibians (class Amphibia), and all classes of fishes into the phylum Chordata. All these organisms share important features of their body plan and internal functions.

Kingdom – the largest and most inclusive of Lennaeus’s taxonomic categories. Linnaeus names two kingdoms, Animalia and Plantae. All animals are placed in the kingdom Animalia.

Seven Taxonomic Categories

18-2 Modern Evolutionary Classification

Which Similarities Are Most Important? Linnaeus grouped species into larger taxa, such as genus and family, mainly according to visible similarities and differences. Which similarities and differences are important? For example, would you classify dolphins as a fish because it lives in the water and has fins or as a mammal because it breathes air and feed their young their milk?

Evolutionary Classification Phylogeny- evolutionary relationships among organisms Biologists now group organisms into categories that represent lines of evolutionary descent, or phylogeny, not just physical similarities. Evolutionary classification – grouping organisms together based on their evolutionary history

All members of the same genus share a recent common ancestor All members of the same genus share a recent common ancestor. All genera in a family share a common ancestor, further in the past. Organisms that appear very similar may not share a recent common ancestor. For example, barnacles and limpets were once grouped together because the look similar.

Barnacle: Limpet:

However, barnacles and limpets are different in important ways However, barnacles and limpets are different in important ways. Their free-swimming larvae(immature forms) are unlike one another. Adult barnacles have jointed limbs and a body divided into segments. Barnacles periodically shed their external skeleton. Limpets have an internal anatomy that is closer to snails.

Classification Using Cladograms Many biologists prefer cladistics analysis, which identifies and considers only those characteristics of organisms that are evolutionary innovations – new characteristics that arise as lineages evolve over time. Derived characters – characteristics that appear in recent parts of a lineage but not in its older members

Cladogram – a diagram that shows the evolutionary relationships among a group of organisms Cladograms help scientists understand how one lineage branched from another in the course of evolution.

Similarities in DNA and RNA The genes of many organisms show important similarities at the molecular level. Similarities in DNA can be used to help determine classification and evolutionary relationships. Now that scientists can “read” the information coded in DNA, they can compare the DNA of different organisms to trace the history of genes over millions of years. Even genes of different organisms like humans and yeasts have many similarities. Both have a gene that code for myosin, which is a protein found in our muscles. This shows that humans and yeasts share a common ancestry.

DNA evidence can also help show the evolutionary relationships of species and how species have changed. The more similar the DNA sequences of two species, the more recently they shared a common ancestor, and the more closely they are related in evolutionary terms. The more two species have changed from one another during evolution, the less similar their DNA will be.

Molecular Clocks Molecular clock – a model that uses DNA comparisons to estimate the length of time that two species have been evolving independently. A molecular clock relies on mutations. A comparison of mutations in DNA in two species can sow how dissimilar the genes are. The degree of dissimilarity shows how long ago the two species shared a common ancestor.

18-3 Kingdoms and Domains

Early attempts at drawing life’s universal tree were based on some misguided assumptions. Some of the earliest trees of life were dominated by humans. These models represented vertebrates as the most important and abundant animals. They also implied that “higher” animals evolved from “lower” animals that were identical to modern forms. Biologist now know that this is incorrect.

The Tree of Life Evolves As biologists learned more about the natural world, they realized that Linnaeus’s two kingdoms, Animalia and Plantae, did not adequately represent the full diversity of life. Microorganisms, such as the protist and bacterium, are significantly different from plants and animals. Scientists agreed that microorganisms needed their own kingdom, which was name Protista. Mushrooms, yeasts, and molds were also placed in their own kingdom called Fungi. Bacteria do not have a nuclei, mitochondria, and chloroplasts, so they are placed in a kingdom called Monera.

Scientists then realized that the Monera are made up of two distinct groups. So, Monera have been separated into two groups: Eubacteria and Archaebacteria. There are a total of six kingdoms: Eubacteria, Archaebacteria, Protista, Fungi, Plantae, and Animalia

The Three-Domain System Scientists have come up with a new taxonomic category that is bigger than a kingdom call the Domain. The Domain is more inclusive than any other category.

The three domains are: Eukarya, made up of protists, fungi, plants, and animals Bacteria, which corresponds to the kingdom Eubacteria Archaea, which corresponds to the kingdom Archaebacteria

Domain Bacteria Members of the domain Bacteria are unicellular and prokaryotic. Their cells have thick, rigid cell walls that surround a cell membrane. These bacteria range from free-living soil organisms to deadly parasites.

Domain Archaea Members of the domain Archaea are unicellular and prokaryotic. They live in extreme environments, like volcanic hot springs, brine pools, and black organic mud with no oxygen.

Domain Eukarya Members of the domain Eukarya consist of all organisms that have a nucleus. It consists of Protista, Fungi, Plantae,and Animalia Protista is composed of eukaryotic organisms that can’t be classified as animals, plants, or fungi. They show great variety. Most are unicellular but some are not. Some are photosynthetic and some are not.

Fungi Fungi are heterotrophs. Most feed on dead or decaying organic matter. Most are multicellular, like mushrooms. Some are unicellular, like yeast.

Plantae Members of the domain Plantae are multicellular, photosynthetic autotrophs.

Animalia Members of the kingdom Animalia are multicellular and heterotrophic. Their cells do not have cell walls.