Classification of Living Things

20-3

Chapter 20

Classification of Classification of Living ThingsLiving Things 2

Cladistic SystematicsCladistic Systematics

Now that we know how to read phylogenetic Now that we know how to read phylogenetic trees….how are they made?trees….how are they made?

Based on shared anatomical characteristicsBased on shared anatomical characteristics

Cladogram:Cladogram: a branching diagram that shows the a branching diagram that shows the relationship among species in regard to their relationship among species in regard to their shared derived characteristicsshared derived characteristics

A cladogram is a special type of phylogenetic treeA cladogram is a special type of phylogenetic tree

Classification of Classification of Living ThingsLiving Things 3

Cladistic SystematicsCladistic Systematics

Cladogram:Cladogram: Traces evolutionary history of the Traces evolutionary history of the group under studygroup under study

Uses shared derived characters to:Uses shared derived characters to:

Classify organisms, andClassify organisms, and

Arrange taxa into a cladogramArrange taxa into a cladogram

A A cladeclade is an evolutionary branch that includes: is an evolutionary branch that includes: A common ancestor, together withA common ancestor, together with

All its descendent speciesAll its descendent species

Constructing a CladogramWhen constructing a When constructing a cladogram, the first cladogram, the first step is to make a step is to make a chart with the chart with the characteristics to be characteristics to be compared.compared.

Notice in the chart on Notice in the chart on the right all of the the right all of the organisms have a organisms have a notochord, but not all notochord, but not all of them are of them are vertebrates.vertebrates.

4

Constructing a CladogramOnly the newt, snake Only the newt, snake and lizard have lungs and lizard have lungs and a 3-chambered and a 3-chambered heart.heart.

Snakes and lizards Snakes and lizards have internalization have internalization fertilization and fertilization and amniotic membranes amniotic membranes in their eggs.in their eggs.

Reptiles (snakes and Reptiles (snakes and lizards) are a lizards) are a cladeclade

Clade:Clade: an evolutionary an evolutionary branch that includes a branch that includes a common ancestor common ancestor together with all its together with all its descendant speciesdescendant species

5

Constructing a CladogramThe newt (an The newt (an amphibian) and the eel amphibian) and the eel (fish) are all (fish) are all evolutionary evolutionary descendants of reptilesdescendants of reptiles

The next step is to draw The next step is to draw a cladogram showing a cladogram showing these relationshipsthese relationships

The goal is to have the The goal is to have the simplest drawing simplest drawing possiblepossible

What is the deal then What is the deal then with bony limbs and with bony limbs and long cylindrical body?long cylindrical body?

6

Constructing a Cladogramparsimony: parsimony: the fewest number of assumptions is the most logical. results in the simplest cladogram possible

7

Classification of Classification of Living ThingsLiving Things 8

ParsimonyParsimony

Cladists are always guided by the principle of Cladists are always guided by the principle of parsimonyparsimony

The arrangement requiring the fewest The arrangement requiring the fewest assumptions is preferredassumptions is preferred

This would:This would:

Leave the fewest number of shared derived Leave the fewest number of shared derived characters unexplainedcharacters unexplained

Minimize the number of assumed evolutionary Minimize the number of assumed evolutionary changeschanges

The reliability of a cladogram is dependent on The reliability of a cladogram is dependent on the knowledge and skill of the investigatorthe knowledge and skill of the investigator

Alternate, Simplified Cladograms

a. X, Y and Z share the same characteristics.b. & c. b. and c. are unlikely examples as the shared

characteristics in Y and Z would have had to evolve independently

9

10Cladistic Versus Traditional View ofReptilian Phylogeny

Classification of Classification of Living ThingsLiving Things 11

Phenetic SystematicsPhenetic Systematics

Assumes it will never be possible to construct Assumes it will never be possible to construct a truly phylogenetic classification systema truly phylogenetic classification system

Species are classified according to the total Species are classified according to the total number of shared similaritiesnumber of shared similarities

Disregards assumed phylogenetic Disregards assumed phylogenetic considerationsconsiderations

Ignores issues of convergent or parallel Ignores issues of convergent or parallel evolutionevolution

Classification of Classification of Living ThingsLiving Things 12

Traditional SystematicsTraditional Systematics

Mainly uses anatomical dataMainly uses anatomical data

Classify organisms using assumed phylogeny Classify organisms using assumed phylogeny with emphasis on phenotypewith emphasis on phenotype

Stress both common ancestry and degree of Stress both common ancestry and degree of structural difference among divergent groupsstructural difference among divergent groups

Construct phylogenetic trees by applying Construct phylogenetic trees by applying evolutionary principles to categoriesevolutionary principles to categories

Not strict in making sure all taxa are Not strict in making sure all taxa are monophyleticmonophyletic