All in the Family

Excerpted and Adapted from the “All in the Family” web activity at http://www.pbs.org/wgbh/evolution/change/family

One of Darwin’s most revolutionary ideas was that all living things on Earth are related, connected to each other like branches on a giant “tree of life.”

How are all living things related? At the root of the tree of life lies a single-celled organism, the 3.8 billion-year-old ancestor that gave rise to all subsequent life forms. All living things are descendents of that ancestor. That includes you, your cat, your potted plant, the fruit you ate for lunch, the last mosquito that bit you, and the cotton that’s woven into your shirt.

What evidence supports the conclusion that all living things are evolutionarily descended from the same common ancestor?

On the surface, it may not look like all living things are related. But looks can be deceiving.

Close relatives don’t always look alike. Consider the elephant and the hyrax pictured to the right. The elephant is

a huge hulk with tusks and a trunk, while the hyrax is a small, furry, rodent-like animal. They seem as different as can be, but hyrax is one of the elephant’s closest relatives. This example illustrates that external appearance often is not a good guide to evolutionary relatedness. Rather, similarities in internal anatomy, development and DNA and protein molecules provide better evidence concerning evolutionary relationships.

To complete the remaining sections of this activity, you will need to log on to http://www.pbs.org/wgbh/evolution/change/family

Tree 1: Wolf, Shark, and Dolphin

1.  Consider the wolf, shark, and dolphin. Select the two that you think are most closely related. The computer will then draw a phylogeny, i.e. an evolutionary tree, for these three animals. What evidence did you use to make your decision?

2.  Scientists use many lines of evidence to determine the relationships among different animals. Compare the anatomies of the wolf, shark, and dolphin for reliable evidence about which two are the closest relatives and fill out the chart below.

Limbs / Jaws / Skulls
Which two animals have the most similar limbs/jaws/skulls? / 1.
2. / 1.
2. / 1.
2.
What is similar about their limbs/jaws/skulls?
What is different about the third animal’s limb/jaw/skull?

3.  Now, if you were wrong before, use the data you collected in the table above and adjust the phylogeny.

4.  In biological terms, a homology is a shared character that is similar due to shared ancestry. An analogy is a shared character that is similar despite different evolutionary origins. Using the pictures of the animals and the evidence you collected in the table above, describe one analogy and one homology in this phylogeny.

All living things are related to each other like the branches on one big “tree of life.” Since everything is related, you can draw trees to show the evolutionary relationships among any group of organisms. The closest relatives are put in the “top” branches because their evolutionary paths split from a common ancestor more recently. More distant relatives are put on lower branches that split of the ancestral line much earlier.

How can you know which group branched off first? Primitive traits existed in the ancestor of the whole tree. More recently evolved groups keep some primitive traits but have evolved others into new, derived traits. To figure out which group branched off most recently, you need to know which group shares the most derived, or newer, traits. The older group will have more primitive traits.

Dolphins, wolves, and sharks are all distantly related to lampreys, primitive fishes that look a lot like sucker-mouthed eels. Many of the characteristics of the lamprey have stayed primitive, but the rest of these animals have evolved into newer (more derived) forms.

5.  Compare the lamprey’s skull, jaw, and limbs to the three other animals. Which animal most resembles the lamprey’s anatomy? What evidence did you use to make that decision?

Tree 2: Bear, Raccoon, and Opossum

1.  Consider the pictures of the bear, raccoon, and opossum. Select the two that you think are most closely related and create a phylogeny. What evidence did you use to make that decision?

2.  Compare the bear, raccoon, and opossum for reliable evidence about which two are the closest relatives and fill out the chart below.

Pouch / Placenta / Maturity at Birth / DNA Sequence
Which two animals have the most similar pouch/placenta/maturity at birth/DNA sequence? / 1.
2. / 1.
2. / 1.
2. / 1.
2.
What is similar about their pouch/placenta/maturity at birth/DNA sequence?
What is different about the third animal’s pouch/placenta/maturity at birth/DNA sequence?

3.  Now, if you were wrong before, use the data you collected in the table above and adjust the phylogeny.

4.  Using the pictures of the animals and the evidence you collected in the table above, describe one analogy and one homology in this phylogeny?

5.  Is the placenta a primitive or derived trait? How do you know?

Tree 3: Human, Tunicate, and Sponge

1.  Consider the human, tunicate, and sponge. Select the two that you think are most closely related and create a phylogeny. What evidence did you use to make that decision?

2.  Compare the human, tunicate, and sponge for reliable evidence about which two are the closest relatives and fill out the chart below.

Brain / Notochord / Gill Arches / DNA Sequence
Which two animals have the most similar brain/notochord/gill arches/DNA sequence? / 1.
2. / 1.
2. / 1.
2. / 1.
2.
What is similar about their brain/notochord/gill arches/DNA sequence?
What is different about the third animal’s brain/notochord/gill arches/DNA sequence?

3.  Now, if you were wrong before, use the data you collected in the table above and adjust the phylogeny.

4.  Using the pictures of the animals and the evidence you collected in the table above, describe one analogy and one homology in this phylogeny?

5.  Name two derived traits found in this phylogeny.

6.  Given the above three phylogenies, what can you say about why scientists use many different lines of evidence to reconstruct evolutionary histories?