Classification Notes

Finding Order in Diversity

 

Key Vocabulary

Watch for the following terms and keep a list of their definitions in your notebook as you read through the notes:

 

Taxonomy

Taxonomy is the science of classification. Scientists classify organisms and assign each one a universally accepted name.

Ex:  Mountain lion, puma, cougar, and panther are all common names for the same organism. It would be confusing for scientists to communicate across the world about an organism only using common names.

image of mountain lion

Image source

There are many tree frogs but only one with the scientific name Agalychnis callidryas.

image of red-eyed tree frog

Image source

There was a need for scientists to try to standardize common names.  Scientists first used physical characteristics of an organism attached to the common name to try to be more descriptive, but the names were too long and scientists differed in the ways that they described physical characteristics.

Binomial nomenclature is the two name identification system that was developed in the 1700's, a Swedish botanist named Carolus Linnaeus.

The genus and species are always written in italics.

The genus name is capitalized and the species name is lower cased.

The genus name is a noun and the species is an adjective that describes the noun and a trait of the organism

Ex: The American black bear (Ursus americanus) the polar bear (Ursus maritimus) and the Alaskan brown bear (Ursus arctos), are three different species. They have similar features and are recognizable as bears, but they do not interbreed with each other so they are classified in the same genus, but as three separate species.

image of 3 species of bears--polar bear, grizzly bear, black bear

Image sources

  1. Kingdom-broadest and most inclusive level that includes a group of related phyla
  2. Phylum-a group of related classes
  3. Class-a group of related orders
  4. Order-a group of related families
  5. Family-a group of related genera
  6. Genus-a group of related species
  7. Species-smallest and least inclusive level that names one particular type of organism

Linnean rank

 image of cowry shell

 image of triceratops

 image of human

kingdom

Animalia

Animalia

Animalia

phylum

Molusca

Chordata

Chordata

class

Gastropoda

Reptilia

Mammalia

order

Mesogastropoda

Ornithischia (Predentata)

Primates

family

Cypraeidae

Ceratopsidae

Hominidae

genus

Cyraea

Triceratops

Homo

species

tigris

horridus

sapiens

 Hierarchy of organisms, ending in the scientific (genus and species) names of humans (Homo sapiens) and two related species.

Image sources

Modern Evolutionary Classification

Aristotle grouped organisms based on where they lived (land, sea, air); some scientists grouped organisms based on structural features (all organisms with wings went in one group, everything with fins in one class), but there were obvious problems with classifying organisms in this way

Scientists determine an organisms evolutionary history by looking at gene sequence similarities in its DNA and RNA as well as looking at its physical characteristics.

Example: Here is a gene sequence of 3 similar organisms. Which two are more closely related?

#1

#2

#3

A-T

A-T

A-T

T-A

T-A

T-A

G-C

G-C

G-C

C-G

C-G

C-G

C-G

C-G

C-G

G-C

T-A

G-C

A-T

A-T

A-T

T-A

C-G

T-A

T-A

G-C

T-A

A-T

C-G

G-C

(Organisms #1 and #3 are most closely related because they have the least number of differences between them)

Scientists then construct a diagram called a cladogram (a tree-like diagram) that shows the evolutionary relationships among organisms

example of a cladogram

Image source

Cladogram showing the evolutionary relationships of reptiles.

 

Similarities in RNA and DNA

 

Molecular Clocks

 

Kingdoms and Domains

As scientists learned more about an organism's physical characteristics, DNA sequences and evolutionary history, the kingdom system was modified.

In the 1700s, scientists only recognized two kingdoms: plants and animals.

By the 1800s, scientists grouped all living things into one of three kingdoms: plants, animals and protista

By the 1950s, scientists grouped all living things into one of five kingdoms: plants, animals, protista, fungi and monera (bacteria)

By the 1990s, scientists grouped all living things into one of six kingdoms: plants, animals, protista, fungi, eubacteria (modern bacteria) and archaebacteria (ancient bacteria)

Scientists today have also developed one other taxon that is larger and more inclusive than the kingdom, the domain.

 

There are 3 domains of life:  Bacteria, Archaea, and Eukarya. These domains encompass the six kingdoms.

Description of the 3 domains of life  

 



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