7.9 Phylogeny
Keywords
| English Term | 中文翻译 | Definition & Explanation |
|---|---|---|
| Phylogeny | 系统发生学 | The evolutionary history of a species or group of related species. |
| Cladogram | 分支图 | A branching diagram showing the cladistic relationship between a number of species, highlighting sequence of branching. |
| Phylogenetic Tree | 系统发生树 | A branching diagram that represents a hypothesis about the evolutionary history of a group of organisms, often calibrated to show time. |
| Node | 节点 (分叉点) | A branch point on a phylogenetic tree that represents the most recent common ancestor of any two descending groups. |
| Out-group | 外群 | A lineage that falls outside the clade being studied but is closely related; used as a baseline for comparison. |
| Shared Derived Character | 共有衍征 | An evolutionary novelty that is unique to a particular clade (e.g., hair in mammals). |
1. Visualizing Evolutionary Relationships
Evolutionary biologists use diagrams to map out how species are related to one another. Both cladograms and phylogenetic trees show the evolutionary relationships among lineages, but there is a slight distinction: * Cladograms primarily show the relative branching order of lineages (who is more closely related to whom). * Phylogenetic Trees often have branch lengths that represent the amount of genetic change over time, which can be calibrated by fossil records or a "molecular clock."
Reading the Tree: Nodes
Both diagrams can be used to illustrate when speciation occurred. When a line splits into two branches, the split is called a node. A node represents the most recent common ancestor (MRCA) of any two groups or lineages connected to it. The more recently two species share a node, the more closely related they are.
2. Constructing the Tree
How do scientists know where to put species on the tree? They use traits that are either gained or lost during evolution: * Shared characters: Traits present in more than one lineage. * Shared, derived characters: An evolutionary novelty unique to a specific clade. These indicate a highly specific common ancestry and are the most informative features for constructing trees. (e.g., All mammals have hair. Hair is a shared derived character that separates mammals from reptiles). * The Out-group: When building a tree, scientists select an out-group. This is the lineage that is least closely related to the remainder of the organisms being studied. It serves as a reference point to determine which traits are ancestral and which are newly derived.
Morphological vs. Molecular Data
Historically, phylogenetic trees were constructed purely from morphological similarities (bone structures of living or fossil species). Today, they are constructed heavily using DNA and protein sequence similarities.
- Molecular data typically provide much more accurate and reliable evidence than morphological traits, because it bypasses the confusion caused by convergent evolution (analogous structures).
- Because they represent hypotheses, phylogenetic trees and cladograms are constantly being revised whenever new fossil or DNA evidence is discovered.