INTRODUCTION TO CLADISTICS - Fog.ccsf.edu

Biology 101b
Dr. Bibit Traut
INTRODUCTION TO CLADISTICS
The evolutionary history of a species or group of related species, is based on common
ancestries. Systematics is the study of this diversity of species and attempts to
understand evolutionary relationships. The reconstruction of these evolutionary
relationships using diagrams is known as phylogenetics. In viewing these diagrams,
the tips of the branches represent the most recent products of evolution along each
branch, and each branch point highlights where speciation occurs. Phylogenetic history
can be inferred from certain morphological and molecular homologies of living
organisms. A homologous character reflects a similarity resulting from common
ancestry, and is therefore used in phylogenetics. Whereas an analogous character is a
similarity due to convergent evolution, rather than shared ancestry, and can lead to
misconceptions in constructing a phylogeny.
The most current preferred method of phylogenetic analysis is Cladistics. This method
compares characters (feature possesed by a taxon) among organisms and scores the
character states. This scored data is usually then entered into a computer and the
simplest solution is found that relates shared characters of taxa into an evolutionary
classification known as a cladogram. A clade within a cladogram is defined as a group
of species that includes an ancestral species and all of its descendants and is defined
by its evolutionary novelties. Part of the construction of clades is to sort between
shared ancestral and shared derived charaters. An ancestral (“primitive”) character is
a homologous character shared beyond the clade (e.g. examine mammal clade--
backbone). A derived character is an evolutionary novelty unique to a particular clade
(e.g. hair in mammals). The method used to differentiate between shared derived and
shared primitive characteristics is known as outgroup comparison. The outgroup is a
group of species closely related to the group being studied (“ingroup”)based on the
assumption that homologies present in both outgroup & ingroup are ancestral
characters pre-dating divergence of both groups from a common ancestor. Ultimately
decisions in cladogram construction seek the “simplest” solution. This is done by using
the principle of maximum parsimony (fewest evolutionary events to have occurred in
the form of shared derived characters) and maximum likelihood (reflects the most
likely sequence of evolutionary events, based on certain rules of DNA). Clades can be
nested within larger clades, but not all groupings or organisms qualify as clades.
Student Activity: The Cladisticules
(The Cladisticule activity has been provide by the Cyberinfrastructure for Phylogenetic
Research Project—CIPRES; funded by National Science Foundation)
Carefully examine the Cladisticules. Work with at least one other student to fill in the
data matrix. Think about which characters would be most likely to develop first when
you begin to organize the data matrix into a cladogram. Before we construct a
cladogram, we will discuss how to turn the data matrix into a venn diagram.
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Biology 101b
Dr. Bibit Traut
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Biology 101b
Dr. Bibit Traut
Anatomy of a Cladisticule
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Phylogenetic Analysis of the Cladisticules – Data Matrix
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head fused to thorax
yes (0) no
(1)
feet two
toes (0)
three toes
(1)
numebr of legs
four (0) six
(1)
antennae absent
(0) present (1)
horns absent
(0)
present (1)
thorax
white (0)
hour glass (1)
abdomen
white (0)
black (1)
Joe (OG)
April
Mike
Tanya
Bobby
Jason
Jerry
Jane
Assignment:
1. For cladisticules: Fill in matrix, use this to draw Venn diagram, and create a cladogram.
2. Complete cladistics activity for plants.
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