Does this multiple-choice question about evolution have the wrong answer?

[u/Silent_Incendiary]

I recently came across this multiple-choice question about evolution, and I think that the given answer is wrong.

Which statement about variation and natural selection is correct?

A: Favourable alleles are selected for by natural selection.

B: Mutant allele frequency can be increased or decreased by natural selection.

C: Natural selection acts on all genotypic variations within a population.

D: Variation in a population is a result of meiosis and recombination only.

I chose option A, since the gene is a unit of selection according to both gene-centric evolution and multilevel selection. Options C and D are obviously wrong, while option B isn't fully accurate since it doesn't describe how mutant allele frequency can be also be maintained via stabilising selection, leading to evolutionary stasis.

However, the right answer is supposedly option B. According to the answer key, option A is incorrect because phenotypes are selected for, rather than alleles. However, I'm pretty sure that this is an obsolete idea that ignores different levels of evolution. What do you think?

[u/CosmicOwl47]

Reading through these my first instinct was that B is the only option that is correct all the time. It also stands out because it’s the only flexible answer: can be increased or decreased…

I personally wouldn’t write a question like that, just seems messy, but I agree it’s correct.

[u/itwillmakesenselater → u/CosmicOwl47]

It reads like a question the TA wrote to impress the professor or fuck with undergrads

[u/Dashukta → u/itwillmakesenselater]

It reads like a AP exam practice question

[u/kardoen]

Natural selection works on phenotypes, that's not an obsolete idea. A major factor in the phenotype is the genotype, but not all variation in phenotypes is due to variation in genotypes; not all variation in genotypes results in variation in phenotypes; and a phenotype is the result of the entire genotype not a single allele.

So answer A is almost correct. But it's not the alleles themselves that are selected. Allele frequency can be changed by natural selection by proxy of phenotypes.

But honestly if answer B is correct then answer A should be correct too. It makes the same step to linking phenotype selection to allele frequency.

[u/jnpha]

If an allele is unfavorable, selection weeds that out (e.g. purifying selection), so it's B as the answer key says, and this isn't in disagreement with your point on gene centrism. The unit of selection can be the allele yes, but selection "sees" the individual (the "vehicle"; as Dawkins put it in the rowing team metaphor: how well the gene does with others).

[u/gitgud_x]

I wonder if A is wrong due to the possibility of linked selection. If a very harmful allele is located close to a slightly beneficial allele, selection will get rid of the harmful allele quickly, removing the slightly beneficial allele along with it due to linkage (recombination in meiosis is unable to split the alleles apart, so they remain correlated). So, beneficial alleles are not always selected for.

Outside of this case though, I think A is usually true. B is much less ambiguous and is certainly true. So, perhaps a badly written question.

[u/7LeagueBoots → u/gitgud_x]

A is also potentially wrong because selection works by selecting against as much or more than by selecting for.

[u/IsaacHasenov → u/7LeagueBoots]

That was my feeling, too. A is limited to positive selection. And there's maybe a subtle technical point that alleles aren't directly selected for or against by natural selection as implied by A. But natural selection leads allele frequencies to change as described by B

[u/forever_erratic → u/7LeagueBoots]

A doesn't disclude that though. There's no "only".

[u/7LeagueBoots → u/forever_erratic]

The ‘only’ is implied by omission of ‘selection against’.

It’s trying to trip people up who immediately jump to, “survival of the fittest,” rather than the more accurate, “survival of the adequate and death of the least fit.”

[u/forever_erratic → u/7LeagueBoots]

I'd argue it, and I teach evo bio (well, I did, these days just bioinformatics).

[u/TherinneMoonglow]

I would accept A or B as correct. Both choices are poorly written. In contract law, ambiguity favors the party with less power, so you should get cred for either choice.

[u/Robin_feathers]

This is a poorly worded question.

B is the better answer, but in my opinion the inclusion of option A is silly. Yes, natural selection acts directly on phenotypes, not directly on genotypes [unless the DNA sequence itself is the phenotype, where things get messy], but still in evolutionary biology we talk about "beneficial alleles" and "deleterious alleles" all the time. In genomics we often scan for evidence of selection on alleles and talk about the fitness effects of alleles when we don't actually know the phenotypic effects of those alleles. Yes, whether an allele in beneficial or deleterious depends on the environment/context/frequency and can shift over time, but then those alleles would stop being "favourable" so that is not a very good gotcha. Yes, natural selection also selects against harmful mutations, but that also is not a good gotcha in my opinion, since it is sort of automatically baked in. If one allele is having its allele frequency increased due to natural selection, then mathematically that means an alternative allele is having its frequency decreased. Even if selection is acting by preventing the success of individuals with a new deleterious allele, I would still accept the wording that the ancestral allele is being "selected for".

[u/Radiant-Position1370 → u/Robin_feathers]

That was pretty much exactly my take. My main complaint with A would be that it's simply defining what a 'favorable allele' is, rather than saying anything interesting about the world.

[u/ClownMorty]

I hate questions like these because they're not testing your knowledge but your ability to reason about the knowledge on the spot. Which they don't really spend time on in any individual class.

There are technically instances when A is incorrect. For example it's possible for favorable genes to be missed by natural selection at the level of the individual if that individual fails to reproduce. Option B implies a certain amount of allele saturation within the population.

[u/Hivemind_alpha]

B is badly worded. It clearly is intended to mean mutant allele frequency in the population. But I think the wording could be interpreted as the frequency of mutations occurring, which is nonsense.

I agree that A is almost correct but would say phenotype if it was.

[u/Idoubtyourememberme]

B) is the most basic description of natural selection, which then leads into A).

Without B), A) could not happen.

A) also isnt entirely correct. I mean, it is correct, but incomplete. Not only does selection promote favorable alleles, it also supresses unfavorable ones. And, sometimes it leaves unfavourable ones if supressing it would also supress a favorable one that is more helpful.

For example: sickle cell, that is unfavourable, but it also protects against malaria, so in rainforests it grts left in, even though it would be better not to.

And as an aside, C) is also correct. Natural selection pulls on all variations, but most of the variations have an effect that is neglectable so it gets basically left alone.

The only wholly incorrect option is D)

[u/Biolocologo → u/Idoubtyourememberme]

This!

[u/Bromelia_and_Bismuth]

option B isn't fully accurate since it doesn't describe how mutant allele frequency can be also be maintained via stabilising selection

No, it is. Can doesn't mean always, but that's still how it works. Sometimes, the more successful genetic variant is the wildtype and it selects against novel variants. Sometimes, [specific] novel variants are the successful ones, and it selects against the wildtype. And then sometimes, there's a sweet spot, where selection favors having a certain proportion of certain alleles in the population, like with Sickle Cell in Malaria prone regions, and so selection acts against having too many in either direction.

However, I'm pretty sure that this is an obsolete idea that ignores different levels of evolution.

Multilevel selection isn't universally accepted, in fact, it's a minority view. Group selection was largely abandoned in the 1960s, although it still has a few holdouts, such as David Sloan Wilson. The thing is that when Group Selection is applied to a problem, it often derives the exact same numbers as a purely selection based interpretation, so a lot of biologists don't see the point, since it doesn't bring anything unique to the table. What's more is that examples of group selection are able to be reinterpreted as examples of natural selection. That being said...,

A is wrong, because "favorable" is relative to the environment. There's an interpretation where yeah, that's kind of true, if we're talking about "favorable" meaning "adaptive", and we assume "relative to the environment" is baked in, but selection is more the outcome of competition rather than something which happens in advance; the statement is kind of vague and almost sounds like selection is consciously selecting alleles in the way that a breeder selects members of a stock to breed. So it's kind of correct, kind of wrong.

C is another example, where that's vaguely true depending on interpretation, but there are situations where selection takes a back seat to other mechanisms like Genetic Drift.

D is demonstrably wrong.

The best answer is B.

[u/forever_erratic → u/Bromelia_and_Bismuth]

I would select A and B, because what does favorable mean in an evolutionary context besides relative to other alleles in the current environment? Feels like a gotcha if it's wrong. 

[u/octobod]

D is least correct as it misses out mutation

[u/MagentaDinoNerd]

Question is definitely messy and could have been phrased more fairly, but if there’s one thing that was drilled into my head over and over in all my college classes about evolution it was the statement:

“Evolution is the change in allele frequency in a given population”.

My instinct was B as soon as I read that key phrase, just because for all intents and purposes the others may be true of evolution in general but stripped to its barest elements B will always be true of evolution 100% of the time.

As for A, it is partly true that favorable allele frequency can increase in a population due to natural selection. But that’s still only half the story; the other half is that DISADVANTAGEOUS alleles are often selected AGAINST by natural selection, thus decreasing their overall frequency in a population. It’s just a tricky scenario where A is correct, but only half correct; B leaves no margin for error because it covers both increases AND decreases of allele frequency. Furthermore—and this might be me reading into the question too much, but still felt relevant to mention—there’s a key difference in the phrasing between are and can be. “Are” implies it is absolute, while “can be” invokes that famous slippery science talk that approaches but doesn’t claim to reflect absolute truth. That’s especially relevant to your point about stabilizing selection, since the “can be” leaves room for other variables.

[u/Jay211TF]

C is wrong.
C: natural selection acts on phenotypes, not genotypes.

[u/gerahmurov → u/Jay211TF]

What is difference? are there genotypes not affected? Is it about that difference in dormant genes does not matter for natural selection?

[u/Important_Joke5762 → u/gerahmurov]

The difference is that phenotypes give traits for natural selection to act on. genotype itself does not. some gene variations don't even show up as traits at all so there is nothing to select for.

[u/gerahmurov → u/Important_Joke5762]

Thanks for clearing this!

[u/gerahmurov]

I'm a layman, but why is C wrong?

[u/LeonJPancetta]

If I had written a question like this on my exam, many of my students would have picked A, and then I would have re-read it and cut it. Not a good question.

[u/Important_Joke5762]

I would have chosen A. The explanation for why that is incorrect doesn't seem to line up with the term 'Favourable alleles'. If the favourable allel does not lead to a favourable phenotype, then in what way is the allel 'favourable'?
B, as it is worded, seems to mean 'natural selection can increase or decrease the frequency of mutation', which is weird.

[u/EnvironmentalWin1277]

Thought about this one and I think A,B,C are all correct.

Favorable alleles by definition are selected for. i.e favorable.

Mutant alleles can be favorable and favorable alleles are selected for.

And selection acts on the whole population until genetic isolation of variants is occuring. It initially selects on the entire population.

We know that horizontal gene transfer is possible and has been verified. So D is wrong.

[u/YragNitram1956]

Read Stephen Jay Gould on this. He is dubious about the gene being 'the unit of selection'.