Chapter 17A. Table of Contents. Section 1 Categories of Biological Classification. Section 2 How Biologists Classify Organisms

Transcription

1 Classification of Organisms Table of Contents Section 1 Categories of Biological Classification

2 Section 1 Categories of Biological Classification Classification

3 Section 1 Categories of Biological Classification Taxonomy The science of naming and classifying organisms is called taxonomy. Until the mid-1700s, biologists named a particular type of organism by adding descriptive phrases to the name of the genus. This system was polynomial and was cumbersome and awkward.

4 Section 1 Categories of Biological Classification Taxonomy, continued A Simpler System A simpler system for naming organisms was developed by the Swedish biologist Carl Linnaeus. Linnaeus s two-word system for naming organisms is called binomial nomenclature. Over the past 250 years since Linnaeus first used two-part binomial species names, his approach has been universally adopted.

5 Section 1 Categories of Biological Classification Species Name

6 Section 1 Categories of Biological Classification Taxonomy, continued Scientific Name The unique two-part name for a species is now referred to as its scientific name. The first word is the genus to which the organism belongs. A genus is a taxonomic category containing similar species. The second word in a scientific name identifies one particular kind of organism within the genus, called a species. A species is the basic biological unit in the Linnaean system of classification.

7 Section 1 Categories of Biological Classification Classifying Organisms Linnaeus worked out a broad system of classification for plants and animals in which an organism s form and structure are the basis for arranging specimens in a collection. The genera and species that he described were later organized into a ranked system of groups that increase in inclusiveness.

8 Section 1 Categories of Biological Classification Classifying Organisms, continued Similar genera are grouped into a family. Similar families are combined into an order. Orders with common properties are united in a class. Classes with similar characteristics are assigned to a phylum. Similar phyla are collected into a kingdom. Similar kingdoms are grouped into domains.

9 Section 1 Categories of Biological Classification Biological Hierarchy of Classification

10 Section 1 Categories of Biological Classification Linnaeus s Levels of Classification

11 Section 1 Categories of Biological Classification Classification Hierarchy of Organisms

12 Section 1 Categories of Biological Classification Classifying Organisms, continued Classification of the Honeybee Each level of classification is based on characteristics shared by all the organisms it contains. The honeybee s scientific name, Apis mellifera, indicates that it belongs to the genus Apis, which is classified in the family Apidae. All members of the family Apidae are bees that live either alone or in hives, as does Apis mellifera.

13 Section 1 Categories of Biological Classification Classification of a Bee

14 What Is a Species? In 1942, the biologist Ernst Mayr of Harvard University proposed a biologically based definition of species, which is called the biological species concept. Mayr defined a biological species as a group of natural populations that are interbreeding or that could interbreed, and that are reproductively isolated from other such groups. Sometimes individuals of different species interbreed and produce offspring called hybrids.

15 Species

16 What Is a Species?, continued Evaluating the Biological Species Concept The biological species concept works well for most members of the kingdom Animalia, in which strong barriers to hybridization usually exist. But the biological species concept fails to describe species that reproduce asexually, such as all species of bacteria and some species of protists, fungi, plants, and even some animals. In practice, modern biologists recognize species by studying an organism s features.

17 What Is a Species?, continued Number of Species The number of species in the world is much greater than the number described. Only about 1.5 million species have been described to date. Scientists estimate that 5 million to 10 million more species may live in the tropics alone.

18 Evolutionary History Classification based on similarities should reflect an organism s phylogeny, that is, its evolutionary history. Through the process called convergent evolution, similarities evolve in organisms not closely related to one another, often because the organisms live in similar habitats. Similarities that arise through convergent evolution are called analogous characters.

19 Phylogeny

20 Phylogenic Diagram of Mammals

21 Comparing Convergent and Divergent Evolution

22 Analogous Features

23 Evolutionary History, continued Cladistics Cladistics (the evolutionary history) is a method of analysis that reconstructs phylogenies by inferring relationships based on shared characters. With respect to two different groups, a character is defined as an ancestral character if it evolved in a common ancestor of both groups. A derived character evolved in an ancestor of one group but not of the other.

24 Cladistics

25 Evolutionary History, continued Cladistics Cladistics is based on the principle that shared derived characters provide evidence that two groups are relatively closely related, and the degree of difference is not considered. A biologist using cladistics constructs a branching diagram called a cladogram, which shows the evolutionary relationships among groups of organisms. Organisms that share derived characters, are grouped together on the cladogram.

26 Cladograms

27 Cladogram: Mammals, Reptiles, and Birds

28 Cladogram: Major Groups of Plants

29 Evolutionary History, continued Considering Characters The great strength of cladistics is objectivity. If a computer is fed the same set of data repeatedly, it will make exactly the same cladogram every time. The disadvantage of cladistics is that the degree of difference between organisms is not considered. Cladistic analysis does not take into account variations in the strength of a character, such as the size or location of a fin or the effectiveness of a lung. Each character is treated equally.

30 Evolutionary History, continued Evolutionary Systematics In evolutionary systematics, taxonomists give varying degrees of importance to characters and thus produce a subjective analysis of evolutionary relationships. In this type of analysis, evolutionary relationships are displayed in a branching diagram called a phylogenic tree. Evolutionary systematics involves the full observational power of the biologist, along with any biases he or she may have.

31 Evolutionary Systematics and Cladistic Taxonomy

32 Systematics