--- title: "Building a Phylogeny: Homology, Shared Derived Characters and Parsimony" description: "Phylogenetic trees are hypotheses about common ancestry. Shared derived character states can support groups, while convergence can mislead. Parsimony compares candidate trees by the minimum character " canonical: https://lightmysky.com/learn/science/building-a-phylogeny-homology-shared-derived-characters-and-parsimony-mt_GFmiVZ8g-F source: https://lightmysky.com/learn/science/building-a-phylogeny-homology-shared-derived-characters-and-parsimony-mt_GFmiVZ8g-F.md retrieved: 2026-11-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # Building a Phylogeny: Homology, Shared Derived Characters and Parsimony Phylogenetic trees are hypotheses about common ancestry. Shared derived character states can support groups, while convergence can mislead. Parsimony compares candidate trees by the minimum character changes they require. The lowest score does not guarantee the true evolutionary history. Subject: Science · Area: Genetics & Evolution · Ages 19 to 21 Page: https://lightmysky.com/learn/science/building-a-phylogeny-homology-shared-derived-characters-and-parsimony-mt_GFmiVZ8g-F ## Ready when they can - Separates homology from convergence using a stated character and its distribution. - Counts the changes a candidate tree requires and compares two trees on that count. - Explains why an ancestral state retained by every sampled member does not distinguish branching relationships within the studied group. ## Lesson: Comparing phylogenetic trees Speciation introduced lineage splitting. A phylogenetic tree is a hypothesis about relationships among lineages. Homologous features come from common ancestry, such as corresponding bones in a whale flipper and a human arm. Bird and insect wings evolved separately as flight structures. Similar function alone does not establish close relationship. Compare several characters. A character is a feature; its state might be present or absent. An ancestral state was present in the ancestor being considered. A derived state differs from it and can be a gain or a loss. Shared derived states can support a subgroup. A state retained throughout the studied group does not resolve its internal branching. An outgroup lies outside the studied group and helps infer ancestral states; it is not the group's direct ancestor. *(drawing: The example below explains these scores. Bars compare minimum change counts; they do not depict branching relationships.)* **Example.** The most recent common ancestor of L, M and N has state 0. L and M have state 1; N has state 0. Sister taxa share an immediate common ancestor. If L and M are sisters, one change on their shared ancestral branch explains both state-1 tips. If L and N are sisters, no single changed branch reaches L and M without N. At least two changes are needed, such as separate gains in L and M. A clade contains an ancestor and all its descendants. Omitting a descendant makes the proposed group incomplete. A shared derived state is evidence for a clade, not proof: convergence or later losses can obscure its history. Hair supports mammals as a group. Backbones are ancestral within mammals and occur in other vertebrates, so they do not identify mammals alone. Under equal-weight parsimony, gains and losses each count as changes. Find each character's minimum count on a candidate tree, then add the counts across characters. Prefer the lowest total. Equal totals leave a tie under this rule. Repeated changes can mislead, so the preferred tree remains a hypothesis. **Recap.** Compare inherited character states, identify complete clades, and total gains and losses. Parsimony ranks candidate trees without proving their history. *Lesson, questions and flashcards written for this dataset.* ## Practice 8 questions on this page, each with its working shown. ## Needs first - [Speciation: Isolation, Hybrid Zones and Where the Boundary Blurs](https://lightmysky.com/learn/science/speciation-isolation-hybrid-zones-and-where-the-boundary-blurs-mt_ngZxpFlwo1) ## Optional connections - [Taxonomy, the three domains and what a species name encodes](https://lightmysky.com/learn/science/taxonomy-the-three-domains-and-what-a-species-name-encodes-mt_12ucjuHtHE) ## Opens up - [Animal body plans: symmetry, cavities and the deep splits](https://lightmysky.com/learn/science/animal-body-plans-symmetry-cavities-and-the-deep-splits-mt_hEC72cvFQF) - [Distance and Likelihood Trees, and What Branch Support Means](https://lightmysky.com/learn/science/distance-and-likelihood-trees-and-what-branch-support-means-mt_mEO9PUEdh6)