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How Do Scientists use Cladograms and Phylogentic Trees to Study Past Relationships?
Essential Question: How do organisms continue to adapt to their environment?
Focus Question:
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How do scientists use cladograms and phylogenetic trees to study past relationships
What characteristics help scientists classify organisms?
What is a cladogram?
What is a phonetics?

Preview:
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In the past, biologists would group organisms based solely on their physical appearance
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Cladistics: is a branch of biology that determines the evolutionary relationships between living things
based on derived (resulting) similarity
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It is based on PHYLOGENY, which is the study of evolutionary relationships
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A major task of systematics is to determine the ancestral relationships among
known species (both living and extinct)
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The most commonly used methods to infer phylogenies include cladistics and
phonetics
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Why do you think they are?

Traditional Systematics
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Scientists have traditionally used similarities in appearance and structure to group organisms
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Some groups look similar but turn out to be distantly related
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Phylogenetic

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A phylogenetic tree or evolutionary tree is a branching diagram or "tree" showing the inferred
evolutionary relationships among various biological species or other entities—their phylogeny—based
upon similarities and differences in their physical or genetic characteristics
Scientists who study systematics are interested in phylogeny, or the ancestral relationships between
species
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Through the process of convergent evolution, similarities may evolve in groups that are not closely
related
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Similarities that arise through convergent evolution are called analogous characters
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This family tree, or phylogenetic tree, represents a hypothesis of the relationships between several
groups
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Cladistics is a method that compares shared characteristics
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A cladogram is a phylogenetic tree that is drawn in a specific way
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Biologists compare many kinds of evidence and apply logic carefully in order to infer phylogenies
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Morphological Evidence
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Morphology refers to the physical structure or anatomy of organisms
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Scientists must look carefully at similar traits, to avoid using analogous characters for classification
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Organisms that share ancestral genes often show similarities during the process of development
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Molecular Evidence
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Scientists can now use genetic information to infer phylogenies
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Some mutations may be passed on to all species that have a common ancestor
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o First, the sequence of DNA bases in a gene (or of amino acids in a protein) is determined for
several species
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Evidence of Order and Time
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Cladistics can determine only the relative order of divergence, or branching, in a phylogenetic tree
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o For example, using cladistics, scientists have identified lancelets as the closest relative of
vertebrates
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o Scientists can measure the genetic differences between taxa and estimate time of divergence
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o You must use these 5 features to draw out a cladogram
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o However, this approach has been problematic
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Cladistics analysis is used to select the most likely phylogeny among a given set of organisms
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