Hi. So I am having problems with my biology hw, can you help explain it? Thank you very much.
(I personally don't know how this tree works.)
The phylogenetic tree shown below indicates the evolutionary relationships for a hypothetical group of modern organisms, labeled 1–5, and their ancestors, labeled A–E.
1. Which two modern organisms are likely to be most closely related?
2. What was the most recent common ancestor of organisms 1 and 5?￼￼
No problem. I am happy to help.
Phylogenetic trees show the descent and common ancestry for a set of organisms. The do NOT organize based on which creature is "better." There is no judgement in that regard. If you think of the tree as having an x-y axis, the "y" axis (up/down) is the one that you care about in this tree. As you move up the tree, you are moving toward more recent organisms. As you move down the tree, those are the less recent organisms.
Anytime a branch splits off, that location is called a "node." Nodes indicate a branching of species. So in your tree, there are 5 different species shown, and 5 different nodes that indicate when a species differentiated itself enough from the main branch.
A node also indicates a common ancestor. For example, species 3 and 4 have a common ancestor "A." But species 2 does not share that common ancestor because it branched off before (lower) than point "A." Species 2, 3, and 4 do share a common ancestry of "B," "C," "D," and "E" though because all four of those nodes occur below species 2, 3, and 4.
Your first question asks "Which two modern organisms are likely to be most closely related?" The two most closely related organisms are 3 and 4. They share a single node. All of the other species exist by themselves at separate nodes.
Question 2: "What was the most recent common ancestor of organisms 1 and 5?￼￼" The most recent common ancestor for those two organisms is ancestor "D." Species 5 branches before point "C" so that can't be it. Species 1 is a descendant of "C" and "D," and species 5 is a direct descendant of "D." They most recently share ancestor "D."
Derived vs. ancestral character
A derived characteristic is a characteristic that shows up in a species as a result of evolutionary change and/or mutation. It isn't necessarily linked to any ancestor. An ancestral characteristic is a characteristic that is being passed on from ancestor to ancestor.
For example: All birds have wings. No matter which species you choose, that characteristic is shared among the ancestors of birds. Wings are ancestral characteristics to birds. Bats also have wings, but they are not birds. They are mammals. Bats are the only mammal with wings, so there is no common ancestor with wings for bats to be related to. Their wings are derived characteristics.