Taxonomy & PhylogenyIntroductionClassificationPhylogenyCladogramsQuiz

In this tutorial, you will learn:

The traditional classification scheme of Linnaeus.

Two theories of taxonomy: Traditional evolutionary taxonomy Cladistics

How to read a cladogram

Credits:Figures and images by N. Wheat unless otherwise noted.Image of Linnaeus from WikipediaFunded by Title V-STEM grant P031S090007.

Introduction The practice of categorizing organisms

according to similar features goes back to Aristotle.

The goal of Taxonomy today is to produce a formal system for naming and classifying species to illustrate their evolutionary relationships.

Classification vs. Systematization

In classification, the taxonomist asks whether the species being classified contains the defining feature of a certain taxonomic grouping. Focus is on features.

Classification vs. Systematization

In systematization, the taxonomist asks whether the characteristics of a species support the hypothesis that it descends from the most recent common ancestor of the taxonomic group. Focus is on evolutionary origin of those

features.

Linnaeus and Classification

In the 18th century, Carolus Linnaeus designed the hierarchical classification system still in use today. Kingdom Phylum Class Order Family Genus Species

Taxa

Taxa (singular = taxon) are the major groups of organisms. Each rank can be subdivided into

additional levels of taxa. Superclass, suborder, etc.

Binomial Nomenclature

Binomial nomenclature is the system Linnaeus developed for naming species. The two-part scientific name includes the

genus and species Names are latinized and italicized, only

the genus is capatilized. Sitta carolinensis

Phylogeny

The goal of systematics is to determine the phylogeny – the evolutionary history – of a species or group of related species.

Phylogeny

Phylogenies are inferred by identifying organismal features, characters, that vary among species.

These characters can be: Morphological Chromosomal Molecular Behavioral or ecological

Homology

Homologous characters are shared characters that result from common ancestry.

Homoplasy

Homoplasies are shared characters that are not a result of common ancestry, but of independent evolution of similar characters (they are not homologous). Can result from convergent evolution.

Convergent Evolution

Convergent evolution occurs when natural selection, working under similar environmental pressures, produces similar (analogous) adaptations in organisms from different evolutionary lineages.

Phylogeny

When trying to determine evolutionary relationships (inferring a phylogeny), we only want to consider homologous characters. Homoplasies can create errors.

Shared Primitive and Shared Derived Characteristics

Focusing on homologous structures, we need to determine when that character arose. Newer characters tell us more! Primitive (older) vs. derived (newer)

characters

Shared Primitive and Shared Derived Characteristics

A shared primitive character is a homologous structure that is older than the branching of a particular clade from other members of that clade. It is shared by more than just the taxon

we are trying to define. Example – mammals all have a backbone,

but so do other vertebrates.

Shared Primitive and Shared Derived Characteristics

A shared derived character is a new evolutionary feature, unique to a particular group. Example - all mammals have hair, and no

other animals have hair.

These are the features that are most useful for determining evolutionary relationships!

Ancestral Character States

The ancestral character state is the form of the character that was present in the common ancestor of the group.

Variations of the character that arose later are called derived character states.

Polarity

Polarity (which version of the trait is ancestral) is determined by using outgroup comparison. An outgroup is closely related, but not

part of the group being examined (the ingroup).

Polarity

An ancestral character is one that is found in both the study group and the outgroup.

Derived character groups are those found in the study groups but not the outgroups.

Clades

Clades are groups that share derived characters and form a subset within a larger group. A clade is a unit of common evolutionary

descent.

Synapomorphy

A synapomorphy is a derived character that is shared by all the members of the clade.

Using synapomorphies to define clades will result in a nested hierarchy of clades.

Symplesiomorphy

Ancestral character states for a taxon are called plesiomorphic.

Symplesiomorphies are shared ancestral characters. Symplesiomorphies do not provide useful

information for forming a nested series of clades.

Cladogram

The nested hierarchy of clades can be shown as a cladogram that is based on synapomorphies.

Monophyletic

A valid clade is monophyletic, it consists of the ancestor species and all its descendants.

Paraphyletic

A paraphyletic clade consists of an ancestral species and some, but not all, of the descendants.

Polyphyletic

A polyphyletic clade includes many species that lack a common ancestor.

Cladistics

Cladistics, also called phylogenetic systematics, is a taxonomic theory that is based on cladograms. All taxa must be monophyletic!

Evolutionary Taxonomy Traditional

Evolutionary taxonomy is based on common descent and the amount of evolutionary change to rank higher taxa. Sometimes this type

of classification includes paraphyletic groupings.

Cladistics

Since all groupings must be monophyletic in cladistics, the paraphyletic arrangement of ape families doesn’t work.

Humans, chimpanzees, gorillas, and orangutans are now all included together in one monophyletic family - Hominidae.

Sister Groups

A sister group is a pair of taxa that are most closely related to each other.

Humans are most closely related to chimpanzees, so humans & chimpanzees form a sister group.

Gorillas form a sister group to the clade containing humans and chimpanzees.

Cladistics vs. Evolutionary Taxonomy

The important difference between these two theories of taxonomy is that traditional evolutionary taxonomy sometimes accepts paraphyletic clades, while cladistics does not. Both accept monophyletic clades. Both reject polyphyletic clades.

Who developed the classification system including the binomial scientific name in use today?

Darwin Mendel Linnaeus Aristotle

Question 1

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Question 1

Which of the following are considered to be taxa?

Phylum Class Family Species All of the above

Question 2

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Question 2

Which example of binomial nomenclature is formatted

correctly? Homo sapiens homo sapiens Homo Sapiens Homo Sapiens It doesn ’

t matter, they are all correct

Question 3

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Question 3

Which of these terms is associated with characters that share

common ancestry. Homology Convergent evolution Homoplasy Analogy All of the above

Question 4

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Question 4

When defining a clade, what type of characters should we use?

Homoplasies Shared primitive characters

= symplesiomorphies Shared derived characters =

synapomorphies All of the above

Question 5

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Question 5

The branching diagram that illustrates the nesting hierarchy of

clades is called a Family tree Cladogram Phylogenetic diagram Taxonomic web

Question 6

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Question 6

A monophyletic clade is one that contains

An ancestor and all of its descendants.

An ancestor and some of its descendants.

Several species that lack a common ancestor.

One species.

Question 7

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Question 7

A polyphyletic clade is one that contains

An ancestor and all of its descendants.

An ancestor and some of its descendants.

Several species that lack a common ancestor.

One species.

Question 8

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Question 8

A paraphyletic clade is one that contains

An ancestor and all of its descendants.

An ancestor and some of its descendants.

Several species that lack a common ancestor.

One species.

Question 9

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Question 9

This cladogram illustrates a ________________ clade.

Monophyletic Paraphyletic Polyphyletic None of the above

Question 10

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Question 10

Which of the following statements about the difference between cladistics and traditional

evolutionary taxonomy is false?

Both accept monophyletic clades Both accept paraphyletic clades Cladistics rejects paraphyletic clades,

while evolutionary taxonomy sometimes accepts them.

Both reject polyphyletic clades.

Question 11

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Question 11

Which of the following cladograms incorrectly illustrates sister

groups?

Question 12

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Question 12