2.3
Phylogenetic Trees
Phylogenetic trees show the evolutionary relationships among organisms. These relationships are shown in a branching diagram with tips, branches, nodes, and a root.
The tips generally represent species, also called taxa. The branches trace evolutionary change, such as changes in DNA sequences or the evolution of new traits like feathers.
When two groups share a recent common ancestor, they are called sister taxa. The point where branches meet is called a node and represents a common ancestor. The root of the tree represents the common ancestor shared by all taxa shown in the tree.
Phylogenetic trees show the evolutionary relationships among organisms based on common ancestry.
A group that includes a common ancestor and all of its descendants is called a clade, or a monophyletic group. For example, all birds belong to the bird clade.
A paraphyletic group includes a common ancestor but not all of its descendants. For example, reptiles are often considered paraphyletic when birds are excluded, even though birds evolved from reptile ancestors.
A polyphyletic group includes organisms whose most recent common ancestor is not included in the group. For example, older classifications grouped certain insect-eating mammals as “Insectivora,” even though these species evolved separately from different ancestral lineages.
Scientists build phylogenetic trees by comparing traits among organisms. These traits may include physical characteristics or DNA sequences.
Two common methods for building trees are maximum parsimony and maximum likelihood.
Maximum parsimony favors the tree that requires the fewest evolutionary changes.
For example, consider elk, salmon, and whales. Because salmon and whales both live in water, they might appear closely related. However, their anatomy shows that whales share more features with mammals like elk. Grouping whales with elk requires fewer evolutionary changes, which fits the principle of maximum parsimony.
Another method is maximum likelihood. It considers that some genetic changes happen more often than others. This method estimates the tree most likely to have produced the observed DNA sequences.
For example, when comparing DNA sequences, scientists may consider that certain nucleotide substitutions happen more frequently than others.
Computer programs analyze these data to infer phylogenetic trees that best explain evolutionary relationships.
Phylogenetic trees come in many forms. It matters in which sequence the organisms are arranged from the bottom to the top of the tree, but the branches can rotate at their nodes without altering the information. The lines connecting individual nodes can be straight, angled, or even curved.
The length of the branches can depict time or the relative amount of change among organisms. For instance, the branch length might indicate the number of amino acid changes in the sequence that underlies the phylogenetic tree. The exact meaning must be shown clearly on a legend accompanying the phylogenetic tree. If such a legend is not present, the branch length is arbitrary, and the reader should not infer any information.
Trees may or may not have a root. The tree is unrooted if the most recent common ancestor of all organisms of interest is unknown. In this case, the depiction of the phylogenetic relationships resembles a snowflake, not a tree. The scientist can root the tree by including an outgroup into the analysis. An outgroup is an organism that is not closely related to any of the organisms that the scientist wishes to arrange on the tree.
Phylogenetic trees show the evolutionary relationships among organisms. These relationships are shown in a branching diagram with tips, branches, nodes, and a root.
The tips generally represent species, also called taxa. The branches trace evolutionary change, such as changes in DNA sequences or the evolution of new traits like feathers.
When two groups share a recent common ancestor, they are called sister taxa. The point where branches meet is called a node and represents a common ancestor. The root of the tree represents the common ancestor shared by all taxa shown in the tree.
Phylogenetic trees show the evolutionary relationships among organisms based on common ancestry.
A group that includes a common ancestor and all of its descendants is called a clade, or a monophyletic group. For example, all birds belong to the bird clade.
A paraphyletic group includes a common ancestor but not all of its descendants. For example, reptiles are often considered paraphyletic when birds are excluded, even though birds evolved from reptile ancestors.
A polyphyletic group includes organisms whose most recent common ancestor is not included in the group. For example, older classifications grouped certain insect-eating mammals as “Insectivora,” even though these species evolved separately from different ancestral lineages.
Scientists build phylogenetic trees by comparing traits among organisms. These traits may include physical characteristics or DNA sequences.
Two common methods for building trees are maximum parsimony and maximum likelihood.
Maximum parsimony favors the tree that requires the fewest evolutionary changes.
For example, consider elk, salmon, and whales. Because salmon and whales both live in water, they might appear closely related. However, their anatomy shows that whales share more features with mammals like elk. Grouping whales with elk requires fewer evolutionary changes, which fits the principle of maximum parsimony.
Another method is maximum likelihood. It considers that some genetic changes happen more often than others. This method estimates the tree most likely to have produced the observed DNA sequences.
For example, when comparing DNA sequences, scientists may consider that certain nucleotide substitutions happen more frequently than others.
Computer programs analyze these data to infer phylogenetic trees that best explain evolutionary relationships.
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2.3 : Phylogenetic Trees
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