Objectives

1. Explain why biologists have

taxonomic systems,

2.Describe the components of a species’

scientific name.

3. Describe the structure of the modern

Linnaean system of classification.

4. Be able to use types of taxonomic

methods to classify organisms.

18.1 The Need for Systems

About 1.7 million species have been

named and described by scientists.

Scientists think that millions more are

undiscovered.

The practice of naming and classifying

organisms is called taxonomy.

The Need Continued

Biologists use taxonomic systems to

consistently name, categorize

organisms, and their knowledge of

organisms.

Taxonomic systems do not use common

names, which may be confusing

because they are different in different

places

Scientific Nomenclature

Various naming systems were invented

in the early days of European biology

Some used long, descriptive Latin

phrases called polynomials.

The general term for any one of these

categories is a taxon (plural, taxa)

Scientific Nomenclature

A simpler and more consistent system

was developed by Swedish biologist

Carl Linnaeus in the 1750s

Which has been universally adopted

Linnaeus introduced a two-word naming

system called binomial nomenclature.

Naming Rules

Linnaeus’ basic approach has been

universally adopted.

The unique, two-part name for a species

is now called a scientific name.

Rules continued

No 2 species can have the same

scientific name.

All scientific names are made up of two

Latin or Latin-like terms

first term: genus names

○ All the members of a genus share the same

genus name

second term: species identifier, and is often

descriptive.

Nomenclature Continued

For example, the scientific name Apis

mellifera belongs to the European

honeybee.

Mellifera derives from Latin word for honey

Genus is written first and is capitalized

followed by species identifier which is

lowercase

Both terms are italicized

Linnaean System

Organisms are grouped at successive levels of the hierarchy based on similarities in their form and structure.

The 8 levels of modern classification are domain, kingdom, phylum, class, order, family, genus, species. The domain category was invented after

Linnaeus’ time and recognizes the most basic differences among cell types

There are 6 kingdoms that fit within the three domains

Linnaean System Continued

A phylum is a subgroup within a kingdom

A class is a subgroup within a phylum

Order is a subgroup within a class

Family is a subgroup within an order

Genus a subgroup within a family

Each genus is made up of species with uniquely shared traits, such that the species are thought to be closely related

18.2 Objectives

Discuss the type of problems that arise when scientists try to group organisms by apparent similarities.

Explain how cladistics is used to construct evolutionary relationships.

Discuss evidence scientists use to analyze cladistics relationships.

Traditional Systematics Systematics: the branch of biology that deals with

classification and nomenclature; taxonomy.

Scientists have traditionally used similarities in appearance and structure to group organisms. However, this approach has been problematic.

Some groups look similar but turn out to be distantly related and vice versa.

Birds were seen as a separate, modern group that was not related to any reptile group.

Fossil evidence has convinced scientists that birds evolved from one of the many lineages of dinosaurs.

What is Phylogenetics?

Phylogenetics - is the branch of life

science concerned with the analysis of

molecular sequencing data to study

evolutionary relationships among groups

of organisms.

Phylogenetics

Scientists who study systematics are interested in phylogeny, or the ancestral relationships between species.

Grouping organisms by similarity is often assumed to reflect phylogeny, but inferring phylogeny is complex in practice.

Reconstructing a species’ phylogeny is like trying to draw a huge family tree over millions of generations.

Phylogenetics Continued

Not all similar characteristics are

inherited from a common ancestor.

Consider the wings of an insect and the

wings of a bird.

Both enable flight, but the structures of

the two wings differ.

Fossil evidence also shows that insects

with wings existed long before birds

appeared.

Phylogenetics

Through the process of convergent evolution, similarities may evolve in groups that are not closely related.

Similar features may evolve because the groups have adopted similar habitats or lifestyles.

Similarities that arise through convergent evolution are called analogous characters.

Phylogenetics

Fossil evidence now shows that birds are considered part of the “family tree” of dinosaurs.

This family tree, or phylogenetic tree, represents a hypothesis of the relationships between several groups

Leaf type Petal Color Dot color Thorns present Number of petals

A

B

C

B C

A

B

Data matrix

Cladistics

Cladistics a phylogenetic classification system that uses shared derived characters and ancestry as the sole criterion for grouping taxa.

Cladistics is an objective method that unites systematics with phylogenetics.

Cladistic analysis is used to select the most likely phylogeny among a given set of organisms.

Cladistics continued

Cladistics focuses on finding characters that are shared between different groups because of shared ancestry.

A shared character is defined as ancestral if it is thought to have evolved in a common ancestor of both groups. Seed production is a shared ancestral character

among those groups all living conifers and flowering plants, and some prehistoric plants.

A derived character is one that evolved in one group but not the other.

Cladistics continued

Cladistics infers relatedness by identifying shared derived and ancestral characters among groups, while avoiding analogous characters.

Scientists construct a cladogram to show relationships between groups.

A cladogram is a phylogenetic tree that is drawn in a specific way. All groups that arise from one point on a cladogram

belong to a clade.

A clade is a set of groups that are related by descent from a single ancestral lineage

Cladistics continued

Each clade is usually

compared with an

outgroup, or group

that lacks some of the

shared characteristics.

Inferring Evolutionary Relatedness

1.Morphological Evidence

Morphology refers to the physical structure or anatomy of organisms.

Scientists must look carefully at similar traits, to avoid using analogous characters for classification.

An important part of morphology in multicellular species is the pattern of development from embryo to adult.

Inferring Relatedness

2. Molecular Evidence

Scientists can now use genetic information to infer phylogenies.

Recall that as genes are passed on from generation to generation, mutations occur.

Some mutations may be passed on to all species that have a common ancestor.

Inferring relatedness

3. Evidence of Order and Time

Cladistics can determine only the

relative order of divergence, or

branching, in a phylogenetic tree.

The fossil record can often be used to

infer the actual time when a group

may have begun to “branch off.”

Dichotomous Keys

What is it?

Identification key that contains major characteristics of groups of organisms

What is it used for?

To identify an unknown organism

How does it work?

Key contains list of contrasting descriptions, you identify which character your unknown organism has and eventually you ID your unknown.