Using Trees: Ancestral States, Dates and Comparative Tests · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Putting family trees to work

Science · Genetics & Evolution · ages 20-21
Name ______________________   Date ____________

  1. You rotate some branches of a tree left to right for a neater figure. What changes?

    • The branching order reverses throughout the tree
    • The root moves to the youngest tip
    • Nothing about relationships changes at all

  2. Every member of one subgroup shares a trait. What is the likely ancestral state?

    • The common ancestor of the subgroup had it
    • The ancestor lacked it and each species gained it separately
    • Nothing can be said until every fossil is found

  3. What does a half life measurement let geologists date?

    • The speed of evolution along a branch
    • The time since a mineral formed, from parent set against daughter
    • The number of species in an ancient community

  4. Two lineages differ by 0.06 substitutions per site, and a fossil anchors their split at 30 million years ago. What is the rate per million years?

    Answer: ______________

  5. Ten species share a trait from one ancestor, but a study counts ten independent events. What is the honest count?

    • One, since a single ancient origin explains all ten
    • Ten, since each species was measured separately
    • Zero, since shared traits carry no information

  6. A trait is scattered across distant branches of the tree. How do you reconstruct its history?

    • Assign it to the root without further thought
    • Count each carrier as a fully independent origin
    • Weigh gains against losses across the whole tree

  7. If no fossil of a lineage was ever found, the lineage never existed.

    Circle one:   True   False

  8. A study claims a trait correlation across 12 species proves adaptation, but the tree shows only two independent origins. How strong is the evidence?

    • As strong as 12 independent events
    • As strong as two events, so far weaker than claimed
    • Stronger than claimed, since trees add certainty
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Answer key

For grown-ups. Fold this page away before handing over the rest.

Putting family trees to work W1-mt_lEYWID9Ffs-s1

  1. Nothing about relationships changes at all · Rotation is cosmetic. Only the branching order carries meaning, and rotation preserves it.
  2. The common ancestor of the subgroup had it · One shared inheritance beats many separate gains. Parsimony hands the trait to the common ancestor.
  3. The time since a mineral formed, from parent set against daughter · Fixed decay pace turns an isotope ratio into a clock. Parent against daughter gives time sealed.
  4. 0.002 · Divide genetic distance by the anchored age. That rate then dates the rest of the tree.
  5. One, since a single ancient origin explains all ten · Shared inheritance is one event wearing ten faces. Counting species instead of origins inflates the evidence.
  6. Weigh gains against losses across the whole tree · Scattered patterns need full tree accounting. Gains versus losses decides the cheapest history.
  7. False · Absence is weak evidence because preservation is patchy. Gaps are expected even for real lineages.
  8. As strong as two events, so far weaker than claimed · Correlation evidence equals independent origins, not species counted. Two origins is a hint, not proof.
Worksheet · LightMySky