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Phylogenetic Trees
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 branche…
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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Q1: What are the main components of a phylogenetic tree?
Phylogenetic trees contain four key structural elements: tips represent species or taxa, branches trace evolutionary change such as DNA sequence modifications or trait evolution, nodes mark where branches meet and represent common ancestors, and the root represents the most recent common ancestor of all organisms shown. These components work together to display evolutionary relationships.
Q2: How do sister taxa and clades differ in phylogenetic trees?
Sister taxa are two groups sharing a recent common ancestor, while a clade is a monophyletic group that includes a common ancestor and all its descendants. For example, all birds form a bird clade. Sister taxa represent a specific relationship between two lineages, whereas clades encompass entire evolutionary lineages with all their descendants included.
Q3: What is a paraphyletic group and how does it differ from a polyphyletic group?
A paraphyletic group includes a common ancestor but excludes some descendants. Reptiles are paraphyletic when birds are excluded, despite birds evolving from reptile ancestors. A polyphyletic group includes organisms whose most recent common ancestor is not in the group, like the outdated classification Insectivora, which grouped unrelated insect-eating mammals from different ancestral lineages.
Q4: How does maximum parsimony help build phylogenetic trees?
Maximum parsimony favors the tree requiring the fewest evolutionary changes. For example, whales and elk share more anatomical features than whales and salmon, even though whales and salmon both live in water. Grouping whales with elk requires fewer evolutionary changes, making it the most parsimonious tree and the most likely accurate representation of evolutionary relationships.
Q5: What does branch length represent in a phylogenetic tree?
Branch length can depict time or the relative amount of evolutionary change among organisms, such as the number of amino acid changes in DNA sequences. However, if no legend accompanies the tree, branch length is arbitrary and should not be interpreted. Always check for a legend to understand what branch length represents in any phylogenetic tree.
Q6: Why would a scientist use an outgroup when constructing a phylogenetic tree?
An outgroup is an organism not closely related to the organisms being arranged on the tree. Scientists include an outgroup to root the tree by establishing the most recent common ancestor of all organisms of interest. Without an outgroup, the tree remains unrooted, resembling a snowflake pattern rather than a traditional tree structure.
Q7: How does maximum likelihood differ from maximum parsimony in phylogenetic analysis?
Maximum likelihood considers that some genetic changes occur more frequently than others, estimating the tree most likely to have produced observed DNA sequences. Maximum parsimony simply favors the tree with fewest evolutionary changes. Maximum likelihood accounts for the probability of specific nucleotide substitutions, making it more sophisticated for analyzing molecular data.