Interpret phylogenetic trees and cladograms that show evolutionary relationships based on shared characteristics and molecular evidence (Ohio's Learning Standards for Science, Biology, B.E.2).

What this topic is asking

Ohio standard B.E.2 expands biological classification to molecular evidence, and the model curriculum expects students to interpret phylogenetic trees and cladograms built from shared characteristics and DNA data. The Ohio Biology EOC turns this into items where you read a diagram to judge which species are most or least related, or use a character table to reason about grouping. The crosscutting idea is patterns: a branching diagram is a model of the pattern of common ancestry. This builds on classification and taxonomy and on the molecular evidence from the evidence for evolution.

What a phylogenetic tree shows

A phylogeny is the evolutionary history of a group of organisms, and a phylogenetic tree (or cladogram) is a branching diagram that represents it. The parts:

• Tips (the ends of the branches) are the species (or other groups) being compared.
• Nodes (the points where branches split) represent common ancestors: the point from which two lineages diverged.
• Branches trace lines of descent through time, usually with the base (root) as the oldest ancestor and the tips as the present.

The diagram is a model: it summarizes a hypothesis about how the species are related, based on evidence.

Reading relatedness

The most tested skill is judging how closely related two species are from the diagram. The rule is simple:

The more recently two species share a common ancestor (the more recent the node where their branches meet), the more closely related they are.

So to compare two species, trace each branch back until they join at a node: that node is their most recent common ancestor. Two species that join at a recent node (near the tips) are closely related; two species that join only at a deep node (near the base) are distantly related. A species that branches off first, at the base, is the least related to all the others.

Building a cladogram from characters

Cladograms are constructed from shared characteristics. The logic:

• A characteristic (a backbone, four limbs, hair, a particular DNA sequence) that is shared by a group of species is evidence they inherited it from a common ancestor that first had the trait.
• That shared trait groups those species together on one branch, and the branch point marks the ancestor in which the trait arose.
• Traits shared by more species define the larger, older groups (deeper branch points); traits shared by fewer species define smaller, more recent groups (branch points nearer the tips).

For example, in a character table, "has a backbone" might be shared by all the vertebrates (a deep branch point), while "has hair" is shared only by the mammals (a more recent branch point that splits them off from the others).

Reading the table: each new trait is gained by a smaller, more recent group. The lamprey (only a backbone) branches off first; the mouse (which has all four traits, including hair) is set apart most recently.

Molecular and structural evidence together

Modern cladograms use both structural characters and molecular data (DNA and protein sequences). More similar sequences mean a more recent common ancestor, so molecular comparisons place species on the tree the same way shared structures do. When structural and molecular evidence agree, the tree is well supported, and molecular data can settle cases where appearances are misleading (for example, where convergent evolution made unrelated species look alike).

Try this

Q1. On a cladogram, what does a node (branch point) represent? [1]

• Cue. A common ancestor from which the branches descend (the point where two lineages diverged).

Q2. Two species' branches meet at a node near the tips of a cladogram; a third species branches off at the base. State which two are most closely related. [1]

• Cue. The two whose branches meet near the tips (the more recent common ancestor).