Q: Can you please explain question 4 on midterm 2? How do humans share common ancestry with dinosaurs then? Is it just a question of the fact that dinosaurs also were amniotes? Or that they are all vertebrates?
How do we know how far back "common ancestry" means?
Then can humans claim common ancestry with nearly every animal on our cladograms? Because they are all "animals"?
A: YES! We all share a common ancestry, and we can take it back as far as the Last Universal Common Ancestor (LUCA) that all living organisms on earth are thought to share.
Monday, June 9, 2008
Why are "fish" paraphyletic?
Q: Can you please explain question 3 on midterm 2? It asks why fish are a paraphyletic group.
The answer is because they don't include tetrapods, but I don't understand why tetrapods have anything to do with this.
A: To answer this lets look at why fish are not a monophyletic group. Such a group contains an ancestor and ALL of it's decendents. A monophyletic group of "fish" has to include all creatures that descended from fish (and therefore from the common ancestor of all fish). Tetrapods emerged from one type of fish. So if we don't include them in the clade of "fish", then fish are defined as a paraphyletic group (includes an ancestor but not all of its decendents).
The answer is because they don't include tetrapods, but I don't understand why tetrapods have anything to do with this.
A: To answer this lets look at why fish are not a monophyletic group. Such a group contains an ancestor and ALL of it's decendents. A monophyletic group of "fish" has to include all creatures that descended from fish (and therefore from the common ancestor of all fish). Tetrapods emerged from one type of fish. So if we don't include them in the clade of "fish", then fish are defined as a paraphyletic group (includes an ancestor but not all of its decendents).
Sunday, June 8, 2008
Crushed Egg Shells
Q: for the evidence for parental care at nest sites, I don't understand why crushed eggshells is an indicator of parental care, couldn't the eggs be crushed by other critters (since they were laid on the ground) or by natural disasters?
A: good point, but I guess if everything else about the nest is intact (and there are baby dinosaurs in the nest) then it's assumed to be the baby dinosaurs who crushed the egg shells. It shows that these babies were not just hatched when the disaster (whatever it was) quickly buried them.
A: good point, but I guess if everything else about the nest is intact (and there are baby dinosaurs in the nest) then it's assumed to be the baby dinosaurs who crushed the egg shells. It shows that these babies were not just hatched when the disaster (whatever it was) quickly buried them.
Saturday, June 7, 2008
Who is the closest relative?
Q: ...So closest related is same as asking who shares the more recent common ancestor, but which way do you go on the cladogram-up or down? For example, on lecture 5, slide 75, I don't understand why its the duck when the snail is closer to the fish? And wouldn't the snail be the most recent ancestor?
A: This is a common source of confusion and it's partly because there is some merit to your interpretation, especially when we start saying that certain fish (lungfish) are more closely related to cows than they are to other fish. But, the way cladistics defines relatedness has the advantage of being a clearly defined consistent definition. And it is, as you said, whoever shares the more recent common ancestor. In slide 75, lecture 5, note the time arrow. Any common ancestor higher up on the chart is a more recent common ancestor. To find the common ancestor you go down the cladogram (back in time), and never back up, from each of the taxa until you find a common point. For the fish and the snail that is the bottom of the chart (the first divergence = the oldest divergence = the oldest common ancestor). For the fish and the duck it is the node above, so the fish and the duck share a more recent common ancestor.
Q: what about "sister groups?" Is that the same as a common ancestor?
A: Yes, it's much the same thing. Sister groups share a common ancestor. It's the same as asking "who is the taxon's closest relative. Or, with whom does it share its most recent common ancestor.
I've posted an additional handout related to all this: branchingdiagrams.pdf.
A: This is a common source of confusion and it's partly because there is some merit to your interpretation, especially when we start saying that certain fish (lungfish) are more closely related to cows than they are to other fish. But, the way cladistics defines relatedness has the advantage of being a clearly defined consistent definition. And it is, as you said, whoever shares the more recent common ancestor. In slide 75, lecture 5, note the time arrow. Any common ancestor higher up on the chart is a more recent common ancestor. To find the common ancestor you go down the cladogram (back in time), and never back up, from each of the taxa until you find a common point. For the fish and the snail that is the bottom of the chart (the first divergence = the oldest divergence = the oldest common ancestor). For the fish and the duck it is the node above, so the fish and the duck share a more recent common ancestor.
Q: what about "sister groups?" Is that the same as a common ancestor?
A: Yes, it's much the same thing. Sister groups share a common ancestor. It's the same as asking "who is the taxon's closest relative. Or, with whom does it share its most recent common ancestor.
I've posted an additional handout related to all this: branchingdiagrams.pdf.
midterm1 question Layers A, B & C
Q: I was wondering if you could explain question 29 on midterm 1 to me.
#29. You're on a dig in Montana and find a fossil in a layer that's on top of a 125 Ma volcanic layer. In Mongolia (very far away) you find three sedimentary layers (A(bottom) ,B, and C(top)), each containing a different type of fossil. The fossil in layer B is the same species as the one you found on the dig in Montana. Which of the below statements is true? It might help you to draw a picture.
I. The fossil in layer A is older than that in B
II. All three fossils are younger than 125 Ma
III. Fossil A is older than 125 Ma
IV. Fossils B and C are younger than 125 Ma
a. I. and III.
b. I and IV
c. III. and IV.
d. I. and II.
e. noodly appendage!
The answer is (b) but if you could explain why "III. Fossil A is older than 125 Ma" is wrong. Since B and C are younger 125 Ma and layer A is underneath the volcanic layer, then doesn't that make it older?
A: We know that the layer that has the B-type fossil is less than 125 million years old from the Montana site. But we don't know how much less... it could be 5 million years old from what we're told. Additionally, we know that layer A is older than layer B, but we don't know how much older. So all we can say is that fossil A is older than fossil B, but that could still be much younger than 125 million years.
Review materials for the final?
Q: will you be posting practice questions and/or review material for the final?
A: I will be posting some extra review materials by late saturday.
A: I will be posting some extra review materials by late saturday.
Curving of exam scores
Q: You said you added five percent to the midterm scores because of the class average being lower than last year (70). Will you update the scores on the my UCLA grades anytime soon, because they are still the same without the curve, or just wait and incorporate the new scores into our final grades? And finally, will you be curving the final as well if the average is below last years?
A: I'm not actually going to add 5 percent to the midterm scores, nor to any scores. Instead the distribution of grades will be such that the average score will be approximately a B- (whatever percentage that is). Please see the distribution in the syllabus to get an idea of how the spread of grades will look. Since the average for that distribution was 75% you can approximate what grade you got for each midterm by adding 5% to your own midterm score and see where that falls on the distribution.
A: I'm not actually going to add 5 percent to the midterm scores, nor to any scores. Instead the distribution of grades will be such that the average score will be approximately a B- (whatever percentage that is). Please see the distribution in the syllabus to get an idea of how the spread of grades will look. Since the average for that distribution was 75% you can approximate what grade you got for each midterm by adding 5% to your own midterm score and see where that falls on the distribution.
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