系統樹

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.

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.

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 A polyphyletic group includes organisms whose most recent common ancestor is not in the group, like the outdated classification Insectivora, which grouped unrelated insect-eating mammals from different ancestral lineages.

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.

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.

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.

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.