Reading Comprehension

PT118 · S2 · P3 · Q19 Lichen-forming Fungi DNA

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A lichen consists of a fungus living in symbiosis (i.e., a mutually beneficial relationship) with an alga.

Topic

The evolutionary origins and classification of lichen-forming fungi, and how new DNA studies are reshaping our understanding of fungal evolution.

Framework

Old / New

Main Point

New DNA research has clarified that lichen-forming fungi are not outliers with mysterious origins, but are closely related to already-known branches of the fungus family tree, reshaping previous assumptions about both their identity and evolutionary patterns. (Most Valuable Sentence: - end of P1)

P1: Introducing Lichen Origins & New Research

We learn that lichens are a partnership between a fungus and an alga, and that their evolutionary background has been mysterious until now. Recent DNA studies have found that lichen-forming fungi are not evolutionary outliers, but are actually closely related to other familiar fungi, which explains why some lichens visually resemble mushrooms.

P2: Historical Challenges and New Technologies

The author explains why fungi, and especially lichen-forming fungi, have been difficult to classify: they're hard to separate from their symbiotic partners and lack clear physical features. In the past, this led to lichens being considered a separate group with unknown origins. However, new genetic tools have allowed researchers to analyze their DNA and place them on five known branches of the fungal family tree, and the number is likely to grow with more research.

P3: Evolutionary Implications of the New Findings

The new research challenges the old assumption that parasitic organisms naturally evolve to become more benign and mutually beneficial over time. The updated fungus family tree shows that both harmful and helpful fungi have appeared throughout evolutionary history, suggesting fungi can switch between parasitism and mutualism rather than follow a one-way evolutionary path.

19.

Which one of the following, if true, most weakens the author's criticism of the assumption that parasitic interactions generally evolve toward symbiosis?

Evolutionary theorists now postulate that
Opposite
We're trying to defend the original assumption that parasitic interactions evolve away from parasitism and toward mutualism (benignity). This answer is saying the opposite. (The author's criticism would weaken this claim, because she would point to the fact that benign and harmful fungus is found both early and late in the evolutionary tree, but this claim doesn't weaken the author's criticism.)
12% picked this
The evolutionary tree of fungi
Too Weak
This almost gives us a way to say, "Hey, author -- you can't use what's going on with the fungus family tree to dispute this generalization about parasitic relationships because the fungus family tree is an outlier; it's an unrepresentative sample." But we can't get that it's an outlier / unrepresentative sample simply from "somewhat more complex" than other parasitic or symbiotic organisms.
5% picked this
The DNA of fungi involved
No Impact
This doesn't have anything to do with the author's criticism (which is about benign and harmful fungi showing up both early and late in the evolutionary timelines of fungi). The fact that the DNA is harder to isolate in one form than the other is a technical hurdle for research scientists. It isn't any sort of commentary on how parasitic fungi evolve.
8% picked this
The placement of lichen-forming fungi
Strengthens
This reinforces the author's notion that, "Now that we have a cleaned up picture of the fungus family tree (now that the timelines of certain parasitic fungi aren't being masked by being placed in a separate group), we can see that parasitic fungi sometimes evolved much later than symbiotic ones". The assumption the author is fighting is that fungi evolve in this direction: parasitic --------> symbiotic / benign When we see parasitic stuff evolving later than symbiotic stuff, it suggests that the assumption is wrong (and that the author is right).
19% picked this
Correct
Branches of the fungus family
Why this is right
This allows us to defend the original assumption that parasitic interactions evolve over time to greater benignity and symbiosis. The author is like, "But wait .... you can find harmful fungi late in the timeline, meaning that some fungi evolved from more benign forms into more parasitic forms". This answer responds, "Yes, but those were evolutionary dead-ends. Just as we expected, it's not a viable long-term strategy to be a harmful parasite. That's why whenever you see a branch of the fungus family tree that 'experimented' with going from being more benign/symbiotic to being more harmful/parasitic, that branch dies out shortly thereafter." Evolution is spurred by mutations, which allows nature to "try out" different survival strategies. This answer is saying that even though you can see the fungus family tree "try out" parasitism early on and later on, it's not a viable road to go down. So maybe in the short term we might see fungus evolve toward more parasitism, since those branches die out shortly thereafter, it still shows that over time we're going to be left with more and more benign interactions, and fewer and fewer parasitic ones.
Skill tested: Weaken · how this choice captures the passage's function is the move to repeat next time.
55% picked this

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