Phylogeny, Common Ancestry & Extinction
- Read a phylogenetic tree or cladogram to infer evolutionary relationships
- Use shared derived characters to identify clades and common ancestors
- Explain how shared cellular features support common ancestry and how extinction reshapes biodiversity
Trees that map descent
A phylogenetic tree (or cladogram) is a branching diagram that hypothesizes how species are related through common ancestry. The tips are the species being compared; each node (branch point) represents a common ancestor from which the lineages above it diverged. Two species are more closely related when they share a more recent common ancestor — that is, when their branches join lower and more recently on the tree. A clade is a common ancestor together with all of its descendants; a valid clade can be "clipped off" the tree in a single cut.
Shared derived characters build the branches
Trees are built from shared derived characters — evolutionary novelties that appeared in a common ancestor and were passed to all its descendants. A character shared by a group but absent in more distant relatives marks a branch point: for example, a backbone unites all vertebrates, while four limbs unite a smaller clade within them. Characters are often placed on the tree with an outgroup, a more distantly related species that lacks the derived traits and roots the comparison. Reading a cladogram, each mark along a lineage is a new shared derived trait that defines the clade above it.
Common ancestry and extinction
The deepest evidence for common ancestry is that all living things share the same core machinery: DNA and RNA as genetic material, a nearly universal genetic code, ribosomes that build proteins, ATP as energy currency, and common metabolic pathways. Such deep similarities are best explained by descent from a single common ancestor. Over the long run, extinction — the permanent loss of a species — is the normal fate of most lineages; over 99% of all species that ever lived are extinct. Mass extinctions periodically erase large fractions of biodiversity, but they also open ecological niches, triggering bursts of diversification (adaptive radiation) in the survivors — the rise of mammals after the extinction of non-avian dinosaurs is the classic case.
On a cladogram, a lamprey (no jaws) branches off first, then a shark, a salamander, and a lizard. Jaws mark the branch point after the lamprey, and four limbs mark the branch point after the shark. Are a salamander and a lizard more closely related to each other than either is to a shark? Justify using the shared derived characters.
- 1.Locate the shared derived characters: the salamander and lizard both have four limbs, a trait that arose in an ancestor after the shark lineage split off.
- 2.Because that four-limb ancestor is more recent than the jawed ancestor shared with the shark, the salamander and lizard share a more recent common ancestor with each other than with the shark.
- 3.More recent shared ancestry means closer relationship, so salamander and lizard form a clade that excludes the shark.
On a phylogenetic tree, what does a node (branch point) represent?
Judge relatedness by the most recent shared node, never by how physically similar two species look or how close their tips sit on the page. You can rotate branches around a node without changing the relationships — the branching order and shared ancestors are what matter, not the left-to-right arrangement of the tips.
All living organisms use DNA as genetic material, share nearly the same genetic code, and build proteins with ribosomes. What does this shared molecular machinery most strongly support?
For cladogram questions, translate "most closely related" into "shares the most recent common ancestor," and read shared derived characters from the branch points. When asked to justify relatedness on free response, cite the specific shared derived character and the node it defines.
Answer the 2 checkpoints as you read.
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