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. 0.001 · Each lineage adds changes for 30 million years, so divide 0.06 by twice the age, 60, to get 0.001 per million years. 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