Reading the Family Tree in DNA

Science · Animals of the World · ages 13-14

Name ______________________ Date ____________

To build a phylogenetic tree, scientists mainly compare which of these between species?
- Fossil sizes
- DNA sequences
- Number of offspring
Species A has the DNA letters A G T C A. Species B has A C T T A at the same spots. Line them up and count: at how many positions do the letters differ? Type the number.
Answer: ______________
If two species have many DNA differences, they probably shared a common ancestor more recently than two species with few differences.
Circle one: True False
All snakes, from tiny garden snakes to huge pythons, share a stretchy jaw that one long-ago ancestor had, and their lizard cousins do not have it. Why would a cladistics scientist use this jaw trait to group snakes together?
- Because the jaw helps snakes catch their food
- Because the jaw makes all snakes look alike
- Because the whole snake family inherited the trait from one common ancestor
Which of these is most likely to pick up DNA directly from an unrelated species?
- A bird
- A crocodile
- A bacterium
Horizontal gene transfer means DNA passes from a parent to its offspring.
Circle one: True False
The molecular clock estimates how long ago two species shared a common ancestor by measuring what?
- How many species live in the same habitat
- How many DNA differences have built up between them
- How large the two species grow
Crocodiles and birds both descend from an ancient group that lizards split away from much earlier. What is that group called?
- Amphibians
- Archosaurs
- Mammals
Zara finds two fish that look almost identical and says they must be close relatives. What would a scientist using cladistics check first?
- Whether they were caught in the same lake
- Whether their DNA shows shared derived characters from a recent common ancestor
- Whether their fins and scales look the same
Species A and B differ at 2 DNA letters. Species A and C differ at 8 letters. Using the molecular clock, which pair shared a common ancestor more recently?
- A and B, because fewer differences means less time has passed
- A and C, because more differences means less time has passed
- Both pairs split at exactly the same time

Answer key

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

Reading the Family Tree in DNA W1-mt_fqAkSv3cUE-s1

DNA sequences · Lining up DNA sequences shows which species share the most recent common ancestor.
2 · Lining up the letters shows 2 positions that differ: position 2 (G and C) and position 4 (C and T).
False · It works the other way: more differences usually means more time has passed since the common ancestor.
Because the whole snake family inherited the trait from one common ancestor · A trait that appeared in one common ancestor and marks the whole group is exactly what cladistics groups by. Usefulness and looks do not decide groupings.
A bacterium · Horizontal gene transfer happens mostly in bacteria. Birds and crocodiles inherit DNA only from their parents.
False · Parent to offspring is ordinary inheritance. Horizontal gene transfer is DNA moving sideways between unrelated species.
How many DNA differences have built up between them · More accumulated DNA differences usually mean more time has passed since the split.
Archosaurs · The ancient group is the archosaurs. Crocodiles and birds are both living branches of it.
Whether their DNA shows shared derived characters from a recent common ancestor · Cladistics trusts shared derived characters found in DNA, not looks. Looking alike is not enough on its own.
A and B, because fewer differences means less time has passed · Fewer DNA differences means less time since the split, so A and B shared an ancestor more recently.