37.4
Phylogeny
Phylogeny is concerned with the evolutionary diversification of organisms or groups of organisms. A group of organisms with a name is called a taxon (…
Phylogeny describes the ancestral relationships between organisms. The connections often take the form of a tree called the Phylogenetic tree, with tips, branches, nodes —points where branches meet, and roots.
The phylogenetic tree represents evolutionary relationships inferred from morphological or molecular data.
The tips of the tree generally represent extant or living taxa.
The branches represent evolutionary changes between ancestors and descendants, such as a change in the DNA sequence or the evolution of a characteristic, like feathers.
Species that share an immediate common ancestor —sister taxa, share nodes, like reptiles and birds. These are each other's closest relatives.
Rodents and humans share a common ancestor. But it would be wrong to assume that humans evolved from rodents or the other way around.
The node, in this example, represents an extinct common ancestor that was not like a rodent or a human.
The root of the tree represents the most recent common ancestor of all organisms in the tree, such as Metaspriggina in this case.
View the full transcript and gain access to JoVE Core videos
Q1: What are the main components of a phylogenetic tree?
A phylogenetic tree contains tips representing living organisms, branches showing evolutionary changes like DNA sequence shifts or trait evolution, nodes marking where branches meet and representing common ancestors, and a root representing the most recent common ancestor of all organisms in the tree.
Q2: How do sister taxa relate to common ancestors?
Sister taxa are species that share an immediate common ancestor, represented by a shared node on the phylogenetic tree. They are each other's closest relatives. For example, reptiles and birds are sister taxa that diverged from a single common ancestor, making them more closely related to each other than to other organisms.
Q3: Why is it incorrect to say humans evolved from rodents?
Humans and rodents share a common ancestor, but the node representing this ancestor depicts an extinct organism that was neither a rodent nor a human. The phylogenetic tree shows that both lineages evolved separately from this shared ancestor, not that one species descended directly from the other.
Q4: What does branch length represent in a scaled phylogenetic tree?
In a scaled phylogenetic tree, branch length can represent either the amount of time elapsed since taxa diverged from a common ancestor or the number of evolutionary changes that occurred. In unscaled trees, branch length has no specific meaning and serves only to show relationships between organisms.
Q5: What is a basal taxon in phylogenetic classification?
A basal taxon is a lineage that evolved early from the root of the phylogenetic tree and remained unbranched. It represents an organism or group that diverged early in evolutionary history and did not subsequently split into multiple descendant lineages.
Q6: How do phylogenetic trees show evolutionary relationships between different taxa?
Phylogenetic trees are branching diagrams where each taxon appears at a branch tip, and branches represent relationships between organisms. Closely related taxa share a larger portion of their genome and group together on the tree. All taxa connecting to a specific node share a recent common ancestor, revealing their evolutionary history.
Q7: What data sources are used to construct phylogenetic trees?
Phylogenetic trees represent evolutionary relationships inferred from morphological data, such as physical characteristics like feathers, or molecular data, such as DNA sequences. These sources of evidence help scientists determine how closely organisms are related and reconstruct their shared evolutionary history.