系統樹

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.