# Flatfish and evolution **URL:** \ **Category:** Factual Questions\ **Created:** [November 21, 2012, 2:58pm UTC](https://boards.straightdope.com/t/flatfish-and-evolution/641720 "2012-11-21T14:58:28Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![iiandyiiii](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/iiandyiiii/32/7924_2.png) [@iiandyiiii](https://boards.straightdope.com/u/iiandyiiii)\ **Post date:** [November 21, 2012, 2:58pm UTC](https://boards.straightdope.com/t/flatfish-and-evolution/641720/1 "2012-11-21T14:58:28Z")
So flatfish, like flounder and halibut, hatch with their eyes on either side of their head, like other fish, and then one eye migrates to the other side as they grow, due to their bottom-dwelling lifestyle. And I think I read that intermediate forms have been found in the fossil record- one eye had moved partially to the “top” of the head in these fossils or something. So I don’t doubt that this did indeed evolve. But how did it evolve? Presumably, ancient flatfish had both eyes on the same side of their head- and perhaps lived at least partially on the bottom. So one eye was always useless. I can understand that having both eyes on the new “top” of the fish is very helpful. But how is the intermediate form adaptive? At some point, the right eye (for a particular species ancestor) would have moved just a little ways up towards the top of the head, then a little farther, then around the top, etc. (over many many generations). How is having one eye a little farther up on the new “bottom” any better than the original form? ---
**Author:** ![iiandyiiii](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/iiandyiiii/32/7924_2.png) [@iiandyiiii](https://boards.straightdope.com/u/iiandyiiii)\ **Post date:** [November 21, 2012, 2:59pm UTC](https://boards.straightdope.com/t/flatfish-and-evolution/641720/2 "2012-11-21T14:59:31Z")
Mods- this should go in General Questions- my mistake. ---
**Author:** ![njtt](https://avatars.discourse-cdn.com/v4/letter/n/ecd19e/32.png) [@njtt](https://boards.straightdope.com/u/njtt)\ **Post date:** [November 21, 2012, 3:06pm UTC](https://boards.straightdope.com/t/flatfish-and-evolution/641720/3 "2012-11-21T15:06:36Z")
I do not see why there has to be an intermediate form in this case, not in the sense of the eye migrating just part of the way round the head, anyway. Why could there not be a mutation affecting skull development that caused one eye to move right round in one generation (which then proved advantageous)? Evolution does not have to go in gradual stages in the sense of something moving its position a little bit at a time. If you need to change lots of independent features to get from A to B, then you will need to pass through intermediate stages where only some of those features have changed, but, here, you are really only changing one feature. The fact that you are changing it quite a lot, quantitatively, is beside the point. ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [November 21, 2012, 3:09pm UTC](https://boards.straightdope.com/t/flatfish-and-evolution/641720/4 "2012-11-21T15:09:05Z")
Like most of evolution it happened very slowly. First the fish has to evolve the ‘flat’ characteristic, lying on it’s side\*. Over time fish that have one eye a little farther up get a survival advantage. This couples with other changes to the fishes body. It all just happens very slowly over a very long time. There’s constant variation and mutation in animals that can result in these changes when there’s a survival advantage to any of them. \*As an example. The eye could have moved first and the ‘flatness’ following to take advantage of it. ---
**Author:** ![Nava](https://avatars.discourse-cdn.com/v4/letter/n/da6949/32.png) [@Nava](https://boards.straightdope.com/u/Nava)\ **Post date:** [November 21, 2012, 3:10pm UTC](https://boards.straightdope.com/t/flatfish-and-evolution/641720/5 "2012-11-21T15:10:28Z")
Yeah, it’s perfectly possible that those fossils aren’t adult forms. A current-day fossilized “teenaged” flatfish would have one eye in its usual spot and the other one halfway through its rotating; one from X years ago would as well. ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [November 21, 2012, 3:12pm UTC](https://boards.straightdope.com/t/flatfish-and-evolution/641720/6 "2012-11-21T15:12:53Z")
> [@njtt](#): > > I do not see why there has to be an intermediate form in this case, not in the sense of the eye migrating just part of the way round the head, anyway. Why could there not be a mutation affecting skull development that caused one eye to move right round in one generation (which then proved advantageous)? Evolution does not have to go in gradual stages in the sense of something moving its position a little bit at a time. > > If you need to change lots of independent features to get from A to B, then you will need to pass through intermediate stages where only some of those features have changed, but, here, you are really only changing one feature. The fact that you are changing it quite a lot, quantitatively, is beside the point. In this case he mentions the intermediate forms which imply some slow evolution. However it’s possible the intermediate forms are independent, and the result of a common mutation that produced different fish. I think there may have been a variety of mutations contributing to the ‘flat’ orientation of these fish. Changes to balance to lie flat, changes in skin coloration, other changes in the head. But you’re right, without examining much more of the fossil record and the physiology of the whole fish, you can’t tell if it’s a gradual or sudden change. ---
**Author:** ![iiandyiiii](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/iiandyiiii/32/7924_2.png) [@iiandyiiii](https://boards.straightdope.com/u/iiandyiiii)\ **Post date:** [November 21, 2012, 3:23pm UTC](https://boards.straightdope.com/t/flatfish-and-evolution/641720/7 "2012-11-21T15:23:25Z")
I think the sudden vs gradual question has been answered by the fossil record- [it was gradual](http://en.wikipedia.org/wiki/Flatfish#Evolution) (at least that’s the consensus right now, I think). But I’m curious how this came about- how was the new eye position in these intermediate forms a beneficial adaptation? ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [November 21, 2012, 3:31pm UTC](https://boards.straightdope.com/t/flatfish-and-evolution/641720/8 "2012-11-21T15:31:03Z")
It depends on when the eye changes occurred relative to the other changes. Before the fish lose their bladders entirely they may have just had a tilted orientation, or only spent part of the time lying flat. So the intermediate positions would have been useful during the transition. Also, what’s not clear to me is whether there are other changes to the head. Is it just the eye that moves? Do they end up with both sets of gills on top of the head? What changes occur in the fish’s skull to accomodate this? It could just be a secondary adaptation resulting from other changes that in the end had a benefit. ---
**Author:** ![njtt](https://avatars.discourse-cdn.com/v4/letter/n/ecd19e/32.png) [@njtt](https://boards.straightdope.com/u/njtt)\ **Post date:** [November 21, 2012, 3:42pm UTC](https://boards.straightdope.com/t/flatfish-and-evolution/641720/9 "2012-11-21T15:42:55Z")
> [@TriPolar](#): > > In this case he mentions the intermediate forms which imply some slow evolution. However it’s possible the intermediate forms are independent, and the result of a common mutation that produced different fish. I think there may have been a variety of mutations contributing to the ‘flat’ orientation of these fish. Changes to balance to lie flat, changes in skin coloration, other changes in the head. But you’re right, without examining much more of the fossil record and the physiology of the whole fish, you can’t tell if it’s a gradual or sudden change. Well in his OP he “thought he read” that there were intermediate forms, which is hardly evidence that there were. Furthermore I am not talking about the whole process of flatfish evolution, which I am sure involved changes to many features of the ancestor species, and thus would indeed have involved intermediate forms. I am talking just about the change the OP asked about, the change in position of the eye, a single feature. There is absolutely no reason, in what we understand about genetics and developmental biology, why we should expect such a single-feature change to be gradual. In fact, I would say that it is _much_ more likely that it would not be. Indeed, if people are claiming to have found intermediate fossil forms where the eye has moved only a a little way around, then I am skeptical of those claims. Maybe they only _think_ they are intermediate forms on the way to flatfish because _they_ have fallen into the trap of thinking that even single-feature changes must be gradual. ---
**Author:** ![TriPolar](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/tripolar/32/3008_2.png) [@TriPolar](https://boards.straightdope.com/u/TriPolar)\ **Post date:** [November 21, 2012, 3:53pm UTC](https://boards.straightdope.com/t/flatfish-and-evolution/641720/10 "2012-11-21T15:53:39Z")
I don’t have anything to justify this with, but I’m under the impression that the bigger the change the more likely it is to be gradual. How to define a big change can be difficult though. Having the skin color change on a large animal can be a very sudden big change, but I’m thinking more in terms of the total number of adaptations required. An eye is not an independent feature. There would be changes to the skull, the nerves, the skin, maybe even the orientation of the brain. So a closer examination would reveal just how many changes were required to end up with this result. The more changes there are, the more likely the changes is to be gradual. ---
**Author:** ![HoneyBadgerDC](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/honeybadgerdc/32/1033_2.png) [@HoneyBadgerDC](https://boards.straightdope.com/u/HoneyBadgerDC)\ **Post date:** [November 21, 2012, 3:57pm UTC](https://boards.straightdope.com/t/flatfish-and-evolution/641720/11 "2012-11-21T15:57:53Z")
A halibut can lay upward of a million eggs, so we have millions of fish laying millions of eggs. Chances of mutations are great. Maybe 1 in a hundred billion mutants survive and have a sister or brother rmutant close enough by to reinforce the successful mutation. A gene seldom carries just one identifying characteristic. I tend to go with the mutant theory and then slow evolution in the refinements. ---
**Author:** ![iiandyiiii](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/iiandyiiii/32/7924_2.png) [@iiandyiiii](https://boards.straightdope.com/u/iiandyiiii)\ **Post date:** [November 21, 2012, 4:12pm UTC](https://boards.straightdope.com/t/flatfish-and-evolution/641720/12 "2012-11-21T16:12:22Z")
> [@njtt](#): > > Well in his OP he “thought he read” that there were intermediate forms, which is hardly evidence that there were. Furthermore I am not talking about the whole process of flatfish evolution, which I am sure involved changes to many features of the ancestor species, and thus would indeed have involved intermediate forms. I am talking just about the change the OP asked about, the change in position of the eye, a single feature. > > There is absolutely no reason, in what we understand about genetics and developmental biology, why we should expect such a single-feature change to be gradual. In fact, I would say that it is _much_ more likely that it would not be. Indeed, if people are claiming to have found intermediate fossil forms where the eye has moved only a a little way around, then I am skeptical of those claims. Maybe they only _think_ they are intermediate forms on the way to flatfish because _they_ have fallen into the trap of thinking that even single-feature changes must be gradual. The link I posted earlier (from Wikipedia) seems to indicate that the consensus is that it was a gradual change. ---
**Author:** ![panache45](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/panache45/32/64_2.png) [@panache45](https://boards.straightdope.com/u/panache45)\ **Post date:** [November 21, 2012, 5:36pm UTC](https://boards.straightdope.com/t/flatfish-and-evolution/641720/13 "2012-11-21T17:36:34Z")
Is it possible that, rather than the eye movement being an adaptation to the fish lying flat, the eye movement came first, as a spontaneous mutation . . . and then the flatness evolved as an adaptation of the new eye placement? ---
**Author:** ![Really\_Not\_All\_That\_Bright](https://avatars.discourse-cdn.com/v4/letter/r/e8c25b/32.png) [@Really\_Not\_All\_That\_Bright](https://boards.straightdope.com/u/Really_Not_All_That_Bright)\ **Post date:** [November 21, 2012, 5:40pm UTC](https://boards.straightdope.com/t/flatfish-and-evolution/641720/14 "2012-11-21T17:40:05Z")
It’s possible, but unlikely. The eye-on-the-ground thing would have had to be advantageous or at least non-harmful itself or it would have been selected out before the eyes-on-side mutation occurred. ---
**Author:** ![RaftPeople](https://avatars.discourse-cdn.com/v4/letter/r/6f9a4e/32.png) [@RaftPeople](https://boards.straightdope.com/u/RaftPeople)\ **Post date:** [November 21, 2012, 5:54pm UTC](https://boards.straightdope.com/t/flatfish-and-evolution/641720/15 "2012-11-21T17:54:01Z")
> [@njtt](#): > > There is absolutely no reason, in what we understand about genetics and developmental biology, why we should expect such a single-feature change to be gradual. In fact, I would say that it is _much_ more likely that it would not be. Indeed, if people are claiming to have found intermediate fossil forms where the eye has moved only a a little way around, then I am skeptical of those claims. Maybe they only _think_ they are intermediate forms on the way to flatfish because _they_ have fallen into the trap of thinking that even single-feature changes must be gradual. I’m surprised by this comment, but I’m not a biologist. These are my thoughts: 1 - The target is not known, therefore a one-shot change to optimal is highly unlikely 2 - Any random movement that is an improvement for any reason will be propagated 3 - The shape of the fish probably (I’m not familiar, just guessing here) didn’t change all at once and the eye movement probably went with the shape change somewhat 4 - The placement of physical features is based on a complex process of chemical changes over time (right?) and to just suddenly alter that process so the eye moves way over there seems like it would be less likely than smaller movements. ---
**Author:** ![dracoi](https://avatars.discourse-cdn.com/v4/letter/d/90db22/32.png) [@dracoi](https://boards.straightdope.com/u/dracoi)\ **Post date:** [November 21, 2012, 5:57pm UTC](https://boards.straightdope.com/t/flatfish-and-evolution/641720/16 "2012-11-21T17:57:52Z")
The adaptive element here is the flatness more than the eyes. In other words, a one-eyed flat halibut does better in surviving to reproduce than a two-eyed upright halibut. A two-eyed flat halibut does even better. So… assuming that interpretations of the fossil evidence is correct… that’s just what we had: an initial advantage was granted simply by going flat, and this advantage outweighed the effective loss of an eye. (Though I suppose an eye on the bottom has its own advantages in some ways…) Many fish are already pretty flat, even those that swim around in open water, so this first step is pretty straightforward and mostly behavioral. Once flat, with a less-useful eye, survival was selectively preferred for those flat fish that made the eye marginally more useful by shifting it around until it arrived at the top. Why is is more useful even when it’s still underneath? This is speculation, but I’ll bet we can find a behavioral explanation. If you have one eye on top and below and you’re scared, it might make sense to tip up on your side (briefly) so that you get a better look around. You’re giving up some camouflage for a better look. If your eye moves closer to your edge, then you don’t have to tip up so much. So you get the better look without giving up camouflage. Once the eye moves far enough, you no longer have to tip yourself at all to put it to use. ---
**Author:** ![Really\_Not\_All\_That\_Bright](https://avatars.discourse-cdn.com/v4/letter/r/e8c25b/32.png) [@Really\_Not\_All\_That\_Bright](https://boards.straightdope.com/u/Really_Not_All_That_Bright)\ **Post date:** [November 21, 2012, 6:00pm UTC](https://boards.straightdope.com/t/flatfish-and-evolution/641720/17 "2012-11-21T18:00:38Z")
It just occurred to me that even normal fish don’t have stereoscopic vision, so it probably wouldn’t affect the fish at all to lose the use of one eye while it’s on the seafloor. ---
**Author:** ![Darwins\_Finch](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/darwins_finch/32/20304_2.png) [@Darwins\_Finch](https://boards.straightdope.com/u/Darwins_Finch)\ **Post date:** [November 21, 2012, 6:05pm UTC](https://boards.straightdope.com/t/flatfish-and-evolution/641720/18 "2012-11-21T18:05:44Z")
First, the “lower” eye migrates to the “top” of the head as a consequence of skull reshaping during development in extant flatfishes. The larval forms start off symmetrical, and the asymmetry arises during development. Second, we have intermediate forms in the fossil record (e.g., _Amphistium_, _Heteronectes_) showing a gradual movement of the “lower” eye in adult forms. So, that would imply that there was a gradual shift in developmental timing which resulted in the current asymmetry of adult flatfish. Unfortunately, what we don’t know is what triggered the change in developmental mechanisms. Since it was, indeed, a gradual change, there must have been some sort of benefit to a partially-migrated (relative to extant forms) eye, but this is likely a problem that will require more study before anything approaching a definitive answer can be found. ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [November 21, 2012, 6:05pm UTC](https://boards.straightdope.com/t/flatfish-and-evolution/641720/19 "2012-11-21T18:05:46Z")
It is worth noting that the eye migrates to the other side _during the lifetime of each individual fish_ - they start out looking like ordinary little fish - free-swimming , then their eye migrates when they are young, and they adopt a bottom-dwelling habit. I mention this only because it emphasises, I think, that this is more complex than a single generation mutation that just gave rise to lopsided heads. ---
**Author:** ![RedSwinglineOne](https://avatars.discourse-cdn.com/v4/letter/r/a9adbd/32.png) [@RedSwinglineOne](https://boards.straightdope.com/u/RedSwinglineOne)\ **Post date:** [November 21, 2012, 6:20pm UTC](https://boards.straightdope.com/t/flatfish-and-evolution/641720/20 "2012-11-21T18:20:14Z")
> [@dracoi](#): > > If you have one eye on top and below and you’re scared, it might make sense to tip up on your side (briefly) so that you get a better look around. You’re giving up some camouflage for a better look. If your eye moves closer to your edge, then you don’t have to tip up so much. So you get the better look without giving up camouflage. Once the eye moves far enough, you no longer have to tip yourself at all to put it to use. This matches my understanding of things. The eye movement actually involves lots of skull movement as well [(PZ speaks)](http://scienceblogs.com/pharyngula/2006/07/31/why-are-flounder-funny-looking/) so I can imagine an early form with a slightly curved skull that allowed the lower eye some vision without completely raising the head or the rest of the body. [Next page](https://boards.straightdope.com/t/flatfish-and-evolution/641720.md?page=2)