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For a homework assignment I received the following question:

Which statement best explains the evolution of fins in whales and fish?

a. The common ancestor of whales and fish possessed genes for fins.

b. Whales and fish possess the same mutations in their genomes.

c. Fins evolved in whales and fish because they both use them to swim in water.

d. Fins evolved in whales and fish because of different mutations that occurred in their genomes.

e. Fins evolved in whales and fish because their most recent common ancestor swam in water.

My answer is (c). Of the given options, it seems most accurate. I feel that the language is unclear as many organisms who do not have fins swim in water. That can be a logic against the option. But, the given answer is (d). That seems too bizarre as even humans and fishes have different mutations in their genomes. (c) seems more appropriate as it can exemplify convergent evolution.

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    $\begingroup$ FWIW, I agree that C is a better answer, because it demonstrates that you understand the concept of convergent evolution, and "different organisms have different mutations" is a pretty trivial observation. It could be the answer to virtually any question you could ask about different organisms evolving differently. $\endgroup$ – swbarnes2 Mar 1 '17 at 19:36
  • $\begingroup$ It sounds like "d" isn't saying that having different mutations caused fins to evolve, but it's making a contrast to "b" - both whales and fish fins evolved via mutations that occurred in their genomes, but they weren't the same mutations in both lineages. Which is accurate. Is there a single answer or is this a "select all that apply" situation ? $\endgroup$ – Oosaka Mar 1 '17 at 20:42
  • $\begingroup$ I'm curious, have you asked your instructor about this question? Did they explain the reasoning for the correct answer and their reasoning why another answer is incorrect? $\endgroup$ – Bryan Krause Mar 1 '17 at 22:40
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Short answer
d) is definitely correct.

Background
The crucial element is that whales returned from land to the sea and re-evolved fins.

  • a) is incorrect, as the common ancestor may not have had fins. In fact, it is thought it was a sea squirt, a sedentary species without fins, that was the most recent ancestor of fish.
  • b) is incorrect, as they may share some mutations but overall they have very different mutations and very different genomes altogether, being fish and mammals;
  • c) is incorrect as they have to develop fins first before they can use them;
  • d) is correct, as the emphasis in the answer is on

Fins evolved in whales and fish because of different mutations that occurred in their genomes.

Fins in whales have developed from arms and feet. Hence there are different mutations going on, as fish don't have arms and feet.

  • e) The common ancestor from fish and whales may have been in the water, but may not have had fins.
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    $\begingroup$ I disagree that "c" is incorrect. First, by the logic of "they have to develop fins before they can use them" nothing would ever evolve. Both whales and fish started out swimming with things that weren't fins (arms/legs for whales, some kind of appendage for fish), so selection pressure developed for swimming ability and those whose limbs were better for swimming did better, presumably swam more, leading eventually to the evolution of fins. "c" might be meant to be incorrect because it sounds Lamarckian as written, but the fact is, whales and fish swimming is integral to why they have fins. $\endgroup$ – Oosaka Mar 1 '17 at 20:32
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    $\begingroup$ Also for e): I'd say whether the common ancestor of fish and whales had fins is irrelevant - "e" is wrong because that common ancestor being in water isn't the cause of whales having fins. After all, it's our common ancestor too and we don't have fins. Whales have fins because their mammalian ancestors became aquatic, not because the first vertebrates lived in water. $\endgroup$ – Oosaka Mar 1 '17 at 20:39
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    $\begingroup$ actually all tetrapods have the genes for fins, limbs form as fins first then apoptosis kills the cells between the individual digits turning a flipper/fin thing into a hand or foot with individual toes. That's why thalidomide did what it did it intercepts the pathways for apoptosis. $\endgroup$ – John Mar 1 '17 at 21:05
  • $\begingroup$ @Rozenn Keribin: Also ichthyosaurs. And FTM penguins started out swimming with wings. So our original fish ancestor develops fins, which evolve into legs as Tiktaallik and friends move onto the land. Ichthyosaurs and cetaceans go back to the sea and the legs re-evolve into flukes. (Note intermediate cases like seals & otters.) And penguin ancestors evolve their front legs into wings, and then into flippers... $\endgroup$ – jamesqf Mar 2 '17 at 4:37
  • $\begingroup$ Your answer is almost correct.. I do not like how you say "whales returned from land to the sea and re-evolved fins." Or how you said "Fins in whales have developed from arms and feet.".. These statements are basically the equivalent of saying that humans transformed from chimps. That is not how evolution works.. $\endgroup$ – Bob Mar 8 '17 at 1:57
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I agree with you that the question is ambiguous, and also that the most sensible answer would be C. However, one could make a more or less reasonable argument in favor of several other answers, too.

a. The common ancestor of whales and fish possessed genes for fins.

Technically, this statement is true. At least some of the fins of whales and fish are even distantly homologous, even though the lineages leading to tetrapods (including whales) and to ray-finned fishes diverged quite early in their evolutionary history.

(Specifically, the flippers of whales are modified tetrapod forelimbs, which are homologous to the pectoral fins of ray-finned fishes. Whether one should also consider the tails fins of whales and fish to be homologous is a bit more debatable: the actual fin structures are very different, and presumably share few if any developmental pathways, but the tail and the spinal column which they attach to and are powered by is clearly a shared feature of both groups.)

However, while shared evolutionary history may explain why fish and whales have similarly placed flippers / pectoral fins, and why they both have a flexible spine that supports undulatory swimming with the help of a tail fin, it does not explain the convergent evolution of whales' forelimbs (which were formerly adapted for walking on land) into fin-like flippers. So in that sense, while perhaps partially correct, this answer is also quite incomplete.

b. Whales and fish possess the same mutations in their genomes.

It's hard to even tell what this statement means. Insofar as it's a restatement of the fact that whales and fish share some of their evolutionary history (up to the divergence of the bone-finned and ray-finned fishes), it's no more or less correct than statement A above.

However, I suspect that the intended meaning of this statement is that whales and (ray-finned) fish would've separately evolved the same "fin-making" mutations after their lineages diverges, which is both clearly incorrect and also, even prima facie, statistically very unlikely. So I would rule this one out.

c. Fins evolved in whales and fish because they both use them to swim in water.

This statement is written using teleological language, which some may consider misleading, as it could be misinterpreted as implying that evolution was a directed process: it could be interpreted as saying that whales needed to be able to swim, so they chose to evolve fins (or that some guiding consciousness granted them fins). This is, of course, incorrect — or, at least, we have no scientific evidence of there being any such conscious force guiding evolution, or of any organism (with the arguable exception of humans) being able to deliberately direct their own evolution.

However, it's quite possible to interpret this statement in a way that makes it perfectly correct: as both whales and fish move around by swimming, possessing fins (or something similar to fins) is an advantageous trait for them, and has thus been favored by natural selection. If pressed, I would thus choose this one as the most correct answer out of the given options.

In particular, note that the argument made by AliceD that "they have to develop fins first before they can use them" is not really true: the ancestors of both whales and fish were swimming long before they evolved fins, and thus the selection pressure in favor of having fins (or fin-like structures) was already present before the fins themselves evolved.

In particular, the first aquatic ancestors of whales are believed to have swum much like modern-day otters and seals, using their flipper-like limbs as makeshift fins. While there's no discrete transition point where one could definitely say that whale limbs turned into proper fins, it is clear that the fins, and their direct evolutionary predecessors, were being actively used for swimming, and that this fact directly drove their evolution into the actual fins that modern whales have.

d. Fins evolved in whales and fish because of different mutations that occurred in their genomes.

While technically correct, this statement is so vague that it's all but meaningless. All evolution is driven by mutations, and those mutations, being basically random, are rarely if ever the same in any two lineages.

Certainly, it would be silly to claim that "different mutations occurring in their genomes" is an explanation of why whales and fish evolved fins: humans also constantly have different mutations occurring in our genomes, but we clearly don't seem to be evolving fins. Neither are pigs or bumblebees or sunflowers, even though all of those species (and, of course, all others too) are also constantly experiencing different mutations in their genomes.

Really, this answer reminds me of the story about a person in a car who gets lost and stops to ask a passerby where they are; the passerby, after a moment's thought, replies "you're in your car." While technically correct, that answer is absolutely useless — and so is this one, too.

e. Fins evolved in whales and fish because their most recent common ancestor swam in water.

Again, this answer could be interpreted as basically a restatement of answer A, and thus as being partially correct. However, it completely ignores the convergent selection pressure described in answer C, which is responsible for the more recent evolution of whale flippers into something resembling fish fins more than, say, human hands or the feet of hippopotamuses (currently believed to be the closest living relatives of whales and other cetaceans). Thus, I would not pick this answer, for the same reason that I wouldn't pick answer A.

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    $\begingroup$ Nice one +1. Great answer $\endgroup$ – AliceD Mar 1 '17 at 20:58
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    $\begingroup$ While technically correct, [d] is so vague that it's all but meaningless. All evolution is driven by mutations, and those mutations, being basically random, are rarely if ever the same in any two lineages. Depending on the education level that the course was pitched at, the mere recognition that whales and fish have different lineages, and have not inherited fins (in their present form) from their common ancestor but rather evolved them separately, might actually be non-trivial hence credit-worthy. Many High School students would even go so far wrong as to think that whales are fish! $\endgroup$ – Silverfish Mar 1 '17 at 22:08
  • $\begingroup$ [Having said that, my first instinct on reading the question was the same as yours. The theme of the question suggested the examiner wanted to know if students understood the idea of convergent evolution, and (c) at least seemed to describe this, albeit in a verbally slightly clumsy way. I just felt that (d) is not entirely without substance, depending on the background the students are coming from!] $\endgroup$ – Silverfish Mar 1 '17 at 22:16

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