Transcript Phenetics vs. Cladistics

Phenetics vs. Cladistics
Phenetics
 The study of relationships among a group of
organisms on the basis of the degree of
similarity between them, be that similarity
molecular, phenotypic, or anatomical.
 A tree-like network expressing phenetic
relationships is called a phenogram or a
phylogentic tree
Phenetics
 Considers as many characteristics as
possible to classify organisms into groups
based on overall similarity
 Does not necessarily attempt to reconstruct
evolutionary relationships, just to produce
groups that can be named
Cladistics
 The study of the pathways of evolution.
 Cladists are interested in the branching
sequence.
 A ladder-like network that expresses such
ancestor-descendant relationships is called a
cladogram.
Cladistics
 Considers only a restricted set of
characteristics of the organism that is being
classified
 Organisms are assigned to a group because
they share unique derived characteristics not
found in other organisms
 Seeks to determine the order in which
evolutionary lines diverged or branched
Cladistics vs. Phenetics
 Linnaean classification is essentially phenetic, that is,
it groups organisms with similar characteristics
together into a hierarchial series of levels or grades
vertebrates
fish
amphibia
reptiles
mammals
birds
Cladistics vs. Phenetics
 Modern tendancy is to order by descent (cladistic order). The
previous group is not a cladistic arrangement since amphibia
and reptiles are both descendents of fish and mammals and
birds are both descendents of reptiles.
 Evolution: change in gene frequency from
generation to generation
 Phylogeny: evolutionary history of the
development of a taxon
 Taxon: name designating an organism or group
of organisms. (A taxon is assigned a rank and
can be placed at a particular level in a systematic
hierarchy reflecting evolutionary relationships. )
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Hierarchy: ranking or ordering
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Cladistics: system of arranging taxa by analysis of
derived characteristics
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Derived characters: unique characteristics which
evolved from a common ancestor
Amniote Egg
 Having an amnion (innermost of the
extraembryonic membranes) forming a fluid
filled sac around the embryo
 Contains nutrients for developing embryo
 Found in reptiles, birds and mammals
 Clade: a taxon or group consisting of a single
species and all of its descendents
 Cladogram: diagram of evolutionary
relationships
Phylogenetic Trees and
Cladograms
 Phylogenetic patterns generated from
branching processes may be represented in
at least two different ways: phylogenetic trees
and cladograms.
 We can think of both cladograms and trees as
phylogenetic maps. Both express hypotheses
about a phylogeny — a pattern of evolution.
Phylogentic trees
Cladograms
 Cladograms are reconstructed by comparing the distribution of
characters among species.
 Characters are inherited attributes of organisms;
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Morphological
Genetic
Developmental
Behavioral
Physiological
Biochemical
and so forth.
 Outer covering, number of legs, or gene sequence, for example,
are characters of organisms that vary. The variation can be
characterized by counting or measuring or describing it.
Cladograms
Cladogram based on
amino acid sequences
What this cladogram suggests is that the lineage leading to
vertebrates has been dominated by the evolution of characters
that make an animal stronger, faster, and smarter.
Variation in same cladogram
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These are the same cladogram, just drawn a bit differently. In this cladogram,
for example, a bird and crocodile are sister taxa, relative to a snake or lizard, or
even more distantly related vertebrates. That information about branching
pattern is useful in itself, but branching pattern alone does not really tell us much
about the group or the sorts of evolutionary changes that occurred in it. Plus,
one has to assume that there is some good reason why this cladogram is drawn,
and not some other alternative cladogram, for example, one that puts birds and
mammals together. After all, both birds and mammals are endothermic (i.e.,
"warm blooded"), and crocodiles, lizards, and snakes are not.
Phylogenetic tree vs. cladogram
 Branching diagrams that
 Branching diagram that
illustrates both branching
patterns and time; branch
lengths have meaning in the
sense that longer branches
imply longer periods of time.
 General hypothesis of
evolutionary relationships
illustrate patterns of
phylogenetic relationships in
a nested hierarchy. (Time is
included in cladograms only
in a relative sense, in the
internested structure of the
cladogram itself.)
 Shows specific
evolutionary connections
of derived characters