IDENTIFICATION OF ORGANISMS

Transcription

1 reflect Take a look at the pictures on the right. Think about what the two organisms have in common. They both need food and water to survive. They both grow and reproduce. They both have similar body organs such as lungs and a heart. Are these similarities enough to classify the organisms into the same group? If you answered yes, you are correct! Both organisms are classifi ed as animals. If you answered no, you are also correct! The organisms have differences that scientists use to further classify these animals into smaller, different groups. The elephant is classifi ed as a mammal, and the lizard is classified as a reptile. What process do scientists use to classify all forms of life? Organizing Life Forms Scientists organize living things using taxonomy, a systematic approach to categorizing organisms into a hierarchy. Taxonomy creates groups based on similarities and differences between organisms. All living things share the trait of life. The fi rst major division among living things is the Domain, which is the broadest classification group. There are three Domains: Bacteria, Archaea, and Eukarya. Organisms within the same Domain have some shared traits, or characteristics. For example, all organisms in Domains Bacteria and Archaea are prokaryotes. All organisms in Domain Eukarya are eukaryotes. Many organisms in Domain Archaea live in extreme environments (high heat, salt, acid, etc.), such as hot springs, while organisms in Domain Bacteria are found all over Earth. prokaryote: an organism lacking a nucleus eukaryote: an organism that contains genetic material within a nucleus Within each Domain, there are six smaller groups called Kingdoms. The Domain Bacteria has only one kingdom, the Kingdom Bacteria. This is also true for the Domain Archaea. Its kingdom is called Kingdom Archaea. The Domain Eukarya has four kingdoms: Protista, Plantae, Fungi, and Animalia. 1

2 Kingdom Protista includes eukaryotic organisms that are primarily unicellular ( one-celled ) and simple, multicellular ( many-celled ) organisms such as algae. Eukaryotes that get their nutrients by decomposing organic matter, such as mushrooms, are in the Kingdom Fungi. Multicellular eukaryotes that make their own food (autotrophs) are in the Kingdom Plantae. This kingdom includes mosses, trees, and flowering plants. The Kingdom Animalia includes multicellular, heterotrophic eukaryotes. Insects, reptiles, birds, and mammals are in Kingdom Animalia. heterotrophic: a characteristic of an organism; it cannot make its own food and feeds on other organisms These four kingdoms are broken down into smaller and smaller classification groups. The entire system for organizing living things from biggest to smallest groups is: Domain, Kingdom, Phylum, Class, Order, Family, Genus, and Species. The smaller the group, the more characteristics the organisms in that group have in common. For example, two organisms that belong to the same genus have more in common than two organisms that belong to the same family, but not the same genus. what do you think? Look at the chart below. It lists the taxonomic groups for three different organisms, beginning with Order. Which two organisms are most similar to one another? Explain your reasoning. Common Name Order Family Genus Species Human Primate Hominidae Homo sapiens Eastern gorilla Primate Hominidae Gorilla beringei Ring-tailed lemur Primate Lemuridae Lemur catta Dichotomous Keys Identifying an organism s taxonomic group or common name sometimes requires a dichotomous key. A dichotomous key is a key used to sort organisms. The key is based on questions about the traits and structures of the organisms. The word dichotomous means cut in two. When using a dichotomous key to identify an organism, cut in two means that a choice must be made about a characteristic at each two-fork branch in the key. 2

3 Basically you have two options, and you have to decide one way or another. The characteristics get more specific at each branch of the key. Dichotomous keys may be either written, as shown below on the left, or graphic, as shown on the right. Let s take a closer look at the graphic dichotomous key. It shows the method for identifying four different organisms by common name: duck, hen, lizard, and snake. The fi rst step of the key, at the top, refers to the question, Does the organism have feathers? If the answer is yes, move to the left side of the key; if the answer is no, move to the right side of the key. Following the left side of the key, the next branch refers to the question, Does the organism with feathers swim? If the answer is yes, the organism is a duck. If the answer is no, the organism is a hen. Following the right side of the key, the next branch refers to the question, Does the organism without feathers have legs? If the answer is yes, the organism is a lizard. If the answer is no, the organism is a snake. Any type of dichotomous key must be detailed enough in order for the user to be able to observe an organism and determine which branch choice is the best match to the organism. The dichotomous keys shown above are very basic. However, some dichotomous keys are quite complex and contain numerous questions to help identify an organism s binomial classifi cation. Binomial classification is the system of naming an organism (starting with its kingdom) by its genus and species. So many different species exist within a particular kingdom or phylum. Therefore, the best dichotomous keys focus on a specifi c bioregion. For example, a dichotomous key may be developed only for the types of organisms that live in oceans. You can see an example of binomial classifi cation for a single species on the next page. 3

4 The binomial classification of the animal shown here is Panthera pardus. Panthera is the genus name, and pardus is the species name. look out! Organisms have two main types of physical traits: those that are inherited and those that are not inherited. An inherited trait is a trait that is passed from a parent to its offspring during reproduction. Eye color and leaf shape are examples of inherited traits. On the other hand, a torn leaf and a scar from an injury are not inherited traits. They were not passed from a parent to offspring. It is important to only use inherited traits when using a dichotomous key to identify an organism. Many groups of organisms demonstrate unique variations and inherited traits, which have gradually changed over time as they were passed from one generation to the next. Therefore, it is also important to remember that dichotomous keys must refl ect these variations. For example, a type of mouse called a deer mouse typically has dark brown fur. physical trait: typically an external characteristic of an organism that can be observed The tear on the left side of this leaf is NOT an inherited trait. The general leaf shape and vein pattern are inherited traits. 4

5 However, in an area within the state of Nebraska, deer mouse have evolved to have a lighter, sand-colored fur. The lighter coloration helps the mice blend in with the sandy environment so they can hide from predators. This trait for light coloration has been passed from parents to offspring over many years and eventually led to an entire population of light-colored mice. Career Corner: Tropical Botanist A botanist is a scientist who studies plants. Some botanists work in education as professors or as museum instructors. Many botanists work in the field conducting research about different plants and how the plants play a role in their environment. These botanists work to protect wildlife and educate the public about the importance of conserving Earth s diverse plant-life. Tropical botanists focus on plants that grow in tropical regions, such as Costa Rica. They collect plants and use dichotomous keys to identify them. These areas Tropical areas are rich with plant-life! are so rich and diverse with plant-life that many botanists believe there are still many plant species that have yet to be discovered. Tropical botanists work to discover new plant species. With help from other scientists, new species are classified and given a species name. The new discovery is then published in various scientific journals. This is one of the reasons that dichotomous keys are always changing! 5

6 What do you know? Study the pictures of the insects at the right. Then use the dichotomous key below to identify the binomial classification of each insect. Write your answers on the line next to each insect. Ant: Butterfl y: 6

7 connecting with your child IDENTIFICATION OF ORGANISMS Creating a Dichotomous Key for Identifying Plants To help your child learn more about the identifi cation of organisms, work together to create a dichotomous key that can be used to identify various plants. For this activity, search for images of different plants on the Internet or in old magazines. Try to fi nd at least six different plants with varying characteristics (such as a pine tree, a palm tree, a strawberry plant, etc.), and print out pictures of each plant. If you are not able to print the images, have your child draw sketches of each plant. Write down at least fi ve distinguishing characteristics for each plant. Characteristics to consider include: leaf shape, leaf vein pattern, presence or absence of a trunk, habitat (land or water; jungle or desert; etc.), and any other defi ning characteristics you would use when describing the plant to another person. If possible, write down the binomial classifi cation (genus and species name) of each plant as well. If you are not able to find the binomial classification, write down the common name of each plant. Next, create a list of yes-no questions that address the characteristics identified in at least one plant. Using index cards or construction paper cut into squares, write the name (genus and species or common name) and a description of a single species on each card. Next, write the yes-no questions on another set of index cards, one question per card. It might be useful to use a different color pen for the questions. Using the yes-no questions as a guide, start sorting the piles into a branching tree. Be sure that each branch only has two options for answers. You may need to rearrange your questions until you arrive at a dichotomous key in which the questions become increasingly restrictive and the plants make up the bottom row. Once you are satisfied with your key, draw it on a piece of paper. As you perform this activity, you may wish to discuss the following questions with your child: Why are only inherited traits used to classify the plants? Look at the final dichotomous key. Did you end up grouping any plants together? If so, do these plants have any differences? How are dichotomous keys helpful to scientists when they discover what they think might be a new species? 7