1.14 Why are organisms classified into groups ?

AIM: Why are organisms classified into groups?

Warm – up: Complete Exercise 3 in Lab

Necessary TermsTaxonomy is the classification of organisms. Most

biologists feel that classification should be based upon phylogeny.

Phylogeny is the ancestry of organisms- how organisms are related by evolution

Systematics is the study of phylogeny.

Cladistics is a means of understanding the phylogeny of organisms.

Phylogeny and Biochemistry

• Phylogeny of many groups has been studied by comparing structures of proteins or other biochemicals (DNA, RNA)

What about mutations?

Molecular Clocks• Estimate time of

divergence by comparing numbers of neutral mutations in DNA, which tend to accumulate in the DNA of a lineage at a fairly constant rate.

• 70,000 years ago, Euro – Japanese split

• 140,000 African – Euro\Japanese split

• 5 mya – human – chimps split

Biochemistry and Common Ancestry

• All use DNA (or RNA) as genetic material• All use same universal genetic code• All use same 20 amino acids in building

proteins• All use left, not right handed amino acids)

What does this information tell us?

Cladistics and Taxonomy

Classification in Biology

Classification in BiologyArranging living organisms into groups



WHY BOTHER?


WHY BOTHER?1. Species Identification: Easier to find out to which

species an organism belong to if everything is organized




2. Predictive Value: If several members of a group have a particular characteristic, another species in this group may also have this characteristic




2. Predictive Value: If several members of a group have a particular characteristic, another species in this group may also have this characteristic

3. Evolutionary Links: Species in the same group probably share characterstics because they have evolved from a common ancestor

Hierarchicalclassification

Carolus LinnaeusBinomial nomenclature

Kingdom Phylum Class Order Family Genus Species

Seven basic categories of biological classification

More categories are added to recognize similarities among groups of taxa within these levels, for example superfamilies, etc.

Kingdom

Kingdom Phylum

Kingdom Phylum Class

Kingdom Phylum Class Order

Kingdom Phylum Class Order Family

Kingdom Phylum Class Order Family Genus

Kingdom Phylum Class Order Family Genus Species

Classification of Humans

Kingdom (Animalia)


Kingdom (Animalia)

Phylum (Chordata)


Kingdom (Animalia)

Phylum (Chordata)

Class (Mammalia)


Kingdom (Animalia)

Phylum (Chordata)

Class (Mammalia)

Order (Primates)


Kingdom (Animalia)

Phylum (Chordata)

Class (Mammalia)

Order (Primates)

Family (Hominidae)


Kingdom (Animalia)

Phylum (Chordata)

Class (Mammalia)

Order (Primates)

Family (Hominidae)

Genus (Homo)


Kingdom (Animalia)

Phylum (Chordata)

Class (Mammalia)

Order (Primates)

Family (Hominidae)

Genus (Homo)

Species (Homo sapiens)


Artificial vs. Natural Classification

• Natural Classification: matches evolutionary origins of the species in the group

• Artificial classification: Help with species identification but have no other value…

ex. Insects, birds, bats are put in the same group. WHY?

Convergent evolution and analogous structures: Not all similarity represents common ancestry!

The Ocotillo (SW U.S.A) and Allauidia (Madagascar) are not closely related. Resemblance due to independent adaptations to similar environmental pressures.

Organisms with homologous structures should be classified in

the same group because they have a common ancestry, even if

they look different!

Homology vs. Analogy

• Homology refers to having a trait in common because it has been inherited from a common ancestor. – CLADOGRAMS BASED ON HOMOLOGIES

• Species can also evolve the same traits independently. Independent evolution of the same characteristic is called convergent evolution and those traits are called analagous.

Cladograms and Classification Classification of many groups has been re-examined using

cladograms. In many cases, cladograms have confirmed existing classifications. Not surprising since both traditional classification and cladistics attempt to reflect phylogeny. Cladograms can be difficult to reconcile with traditional classifications, because the nodes can occur at any point. It can therefore seem rather arbitrary how the hierarchy of taxa is fitted to the clades. In some cases, cladistics suggests radically different phylogenies. The strength of cladistics is that the comparisons between organisms are objective, based on molecular differences. The weakness is that molecular differences are analyzed on the basis of probabilities. Occasionally, improbable events occur makine the analysis wrong. Therefore, cladistics should not be treated as infallible. However, it can stimulate a reinterpretation of the data on which traditional classifications have been based.

Cladogram: A phylogenetic diagram that classifies organisms according to shared


Clade: Group of organisms that evolved from a common ancestor (the ancestor plus all its descendants)


Clade: Group of organisms that evolved from a common ancestor (the ancestor plus all its descendants)

Cladistics: method of classification of living organisms based on construction and analysis of cladograms

Connection between classification

Constructing a Cladogram


Ancestral (=homologous) trait: A trait shared due to common ancestry


Ancestral (=homologous) trait: A trait shared due to common ancestry

Derived trait: A trait that differs from a common ancestor

Cats are more similar to dogs than they are to frogs, because they share a more recent common ancestor with dogs

An outgroup is used to decide which characteristics are ancestral

Ingroup Outgroup

Procedure 1: Classifying Doodlebirds

?

Why are legs, beak and crest not useful in the analysis?

(outgroup)

Web toes, Three tails

TAXA Petal no. Leaf Seed shape

A 5 compound flat

B 4 compound round

C 4 simple round

D 5 compound round

Outgroup 4 simple round

A B C D

A 1 (compound) 0 2 (compound, 5)

B 0 1 (compound)

C 0

D

Technology

1.14 Why are organisms classified into groups ?