Question about evolution - intermediate evolutionary traits

  1. Hi, I have question about what the current state of evolutionary biology has to say about how complex biological structures form. I'll just illustrate my question with an example, and I apologize in advance for using the incorrect terminology - I have very little biology training.

    For me it's easy to understand how a trait like long necks in giraffes would be selected for and evolve. If having a long neck is advantageous, then whatever animals happen to have slightly longer necks, can collect slightly more leaves and will be slightly better off, and they are more likely to pass their long-neck genes on to their offspring. This is a gradual and continuous process, which occurs in small steps, and each small step helps the individual animal survive in it's environment and reproduce more effectively. This is the general process of evolution, and seems to be perfectly intuitive.

    Now, what I don't find as intuitive is how biological structures, such as the wing of a bird for example, are able to evolve. The reason being, I don't see how the intermediate steps from say a normal walking leg to a wing help the animal to survive. To me, a half evolved wing - something much less functional than a normal leg, and much less functional than a normal wing - seems like a very unlikely thing to be selected for. What possible path of small success steps could you take toward evolving a wing, that represents a continuous advantage to the animal?

    I'm using the example of a wing, but I'm sure there are far better ones. Thanks.
  2. jcsd
  3. Borek

    Staff: Mentor

  4. This question is kind of in the same ball park as the creationists complaint of why we don't see more transitional forms in the fossil record. The answer is that transitional forms mark ephemeral epochs relative to stable forms, which last a long time. The probability of a creature being fossilized is vanishingly small, so it is little surprise that we mostly see the stable forms in the fossil record. Even so, there are plenty of transitional forms available to make a convincing argument that evolution is a fact.

    As far as the selective adaptiveness of transitional forms, the same argument applies. There are plenty of animals alive today with vestigial attributes that are not adaptive, such as birds with wings that can't fly for instance. What typically happens is that selection will vigorously act on the intermediate form if it is a crucial appendage, such as a limb, as in your example, until a stable form is reached. In the meantime, these creatures just lumber around like salamanders. As long as they're able to reproduce, they can keep the germ line going until one of their decendents hit on an adaptive, stable form. They don't necessarily need to be pretty during that transitional stage.
  5. As John Maynard Smith points out, among the more common answers to this kind of question is that the early steps down the road to the final form were for purposes other than the apparent final function. The example Maynard Smith cites is of the bird feather, now seemingly so superbly adapted to the purposes of flight, but originally developed for warmth. Again, that is a hopeless over simplification of the actual process but is a good demonstration of the point.
  6. The wing example is actually a pretty example related to your question.

    Say, you have a bipedal dinosaur that independently evolved feathers as a body coating. Feathers primary purpose is to insulate the creature & protect it from the elements (won't get as cold drying out after late fall rainstorm, etc). This is probably why they appeared in small, chicken sized, dinosaurs. Small bodied animals have more difficulty in maintaining its core body temp than larger ones.

    So, you have this small feather covered dino walking around doing it's thing and, say, a larger predator comes along to make it lunch. Of course, it is going to flee for its life! At one point in time the fleeing feathered dino may have waved its feathered arms while in flight and perhaps that gave it a bit of an extra boost to successfully escape. Those genes get passed along. Then some generations later, a fleeing feathered dino (waving its arms) may have caught a little bit of air doing this (it need only be propelled an extra foot or two) and this lead to increased escape success.

    From there, feathered dinos who could 'get a little air' when fleeing would then be favored because they more successfully escape. As successive generations evolved to get better at using 'air' to aid in getting away from threats, eventually came the ability to glide distances and then powered flight. This survival feature was so important for the dinos who could capitalize on this that other body features evolved to enhance this ability (large chest of high energy white muscle, thin hollow bones, etc) too... and became birds as we know them.

    So this is really the same mechanism that leads to giraffes having long legs, etc. In the instance of flight, things are so evolved as to make the specific evolutionary process unclear. E.g. the limbs that constitute wings are so evolved along as to make them pretty useless for anything other than flying. However, if you closely look a bird's plucked wing, you can still see the digits of it's dino ancestor's hands/paws.
  7. D H

    Staff: Mentor

    This is not how flight evolved. It didn't just suddenly happen.

    Since there were a large number of dinosaurs with "half evolved wings" they certainly must have conferred significant value. Flight might have developed as a bottom-up phenomenon (not-quite flight that enabled running animals to escape from predators or pounce on prey), a top-down phenomenon (e.g., flying squirrels can't fly but they can glide), or something else. Paleontologists aren't sure. Just because they aren't sure doesn't mean it didn't happen. It just means they don't know. What they do know is that it took quite some time, with all kinds of feathered dinosaurs benefiting from "half-evolved wings".
  8. That was a very nice story, Leeboy. However, at best, it was a distant relative to his question. If this were a GRE test, they would have given you a zero for a very nice answer to a question that wasn't asked. The OP's question is how does a transitional species get by with non-adaptive attributes between it's stable forms.
  9. Not necessarily, that's like saying the wings of an ostrich or chicken confer some significant value that outweighs the cumbersomeness of those appendages. The key factor is that, as long as it doesn't confer some significant detriment to the animal, it has the chance to stick around long enough as a vestigial structure until it morphs into some useful trait. Take another flightless bird, the penguin. It's wings are useless for flight, but if you've ever seen "March of the penguins," you can see that these just might be on their way to evolving to be useful swimming appendages.
  11. D H

    Staff: Mentor

    Might be? Have you watched films of penguins underwater? They are flying. It's just that they're flying through the water rather than flying through the air. The very different nature of water versus air means that their wings look quite different from the typical bird's wings, which are adapted for flying through the air.
  12. It's not necessarily right to assume everything that evolved had or has an adaptive purpose, or was "selected for".

    Gould and Lewontin coined... or should I say, adapted from architecture, the term Spandrel to describe inadvertent by-products of something that is selected for. An example in humans would be our chins, we're the only primates to have such strong chins, you can spend forever trying to establish an adaptive purpose for evolving a chin (and many people have) but the fact is we adapted a hominid face, shortened muzzle and this thing invariably pops out that we call a chin.

    Evolution is not really about optimising every trait and it's not necessarily that efficient and a lot of stuff just gets 'carried along' and if at some point this becomes useful then great and if not, as long as it doesn't have a massively negative impact on the survival of a species it will continue. Was it Andre Lwoff who said something like "Evolution is not an inventor it is a tinkerer", you don't decide to grow a leg because it will be useful, you have to 'build upon' or tinker with pre existing structures.
  13. That appears to be a case of evolution taking a feature useful for one thing (flying) and successfully adapting it into an alternative use (swimming). Penguins indeed look like they are flying when underwater and they are very accomplished swimmers too. This adaptation makes sense when one considers that air and water are fluids and wings are evolved to propel the animal through the fluid it is within.
  14. On the specific issue of flying it is worth remembering the point that it is a transition that has been made independently on multiple occasions. Not just birds and bats but others too – there were flying reptiles, and as referred to by DH, flying squirrels do appear to be in the middle of that process right now. So, contrary to how it might seem, the evidence suggests that it is not actually all that difficult a transition to make. Indubitably, as will all these things, it takes time, but given enough time, it really shouldn’t be that surprising that it has been achieved.
  15. Vestigial organs aren't really relevant to my question though. That's an example of something which evolved for some purpose, and then later began to lose its original purpose. My question is about how such a complicated structure came about in the first place through a series of very small steps, each of which seem quite unlikely. I also don't buy the hopping around to avoid predators argument, I think that's just too simplistic and I don't see how that alone is going to guide the development of all the adaptations needed for true flight.

    I never claimed it didn't happen or I was doubtful it happened, it obviously did given that birds and bats exist today. My question was how does evolutionary biology explain it, to which you seem to be responding "no one knows". Also, how does one distinguish between "half-evolved wings" in the fossil record, and vestigial organs like DiracPool mentions?

    Right, this I agree with, but the chin is an example of a relatively simple structure which doesn't really have much functionality. Your life would be no different with or without a chin. A complicated structure like a wing, which has many components and requires many different adoptions, seems just too complicated to have just happened, or to be a mere by-product of some other set of adaptations.

    Again, I don't say I doubt it happened or that an explanation exists, I'm simply asking for what that explanation is, and if it doesn't exist yet, then that's fair.
  16. Borek

    Staff: Mentor

    Please read the book I linked to, it was written to specifically address this particular question.
  17. Ok - I was hoping there might be a short answer, but if I have time I'll check it out.
  18. Ryan_m_b

    Staff: Mentor

    Like many features the evolution of wings, and therefore flight, came from pre-existing structures that had different purposes and were incrementally mutated and selected for over very long periods. This is known as exaptation: the adaptation of a trait from one benefit to another. Take the feathers of a wing, there are fossilised remains of feathered dinosaurs that had no capacity to fly. There are various hypotheses regarding the beneficial nature of these feathers such as heat regulation, camouflage and decoration to attract a mate. Given the wide range of uses for feathers on birds currently many of these could be true.

    Flight possibly evolved from feathered dinosaurs that were able to use their limbs to boost their running speed. This is an obviously beneficial trait when evading predators or catching prey and so over time lighter dinosaurs with more feathers and longer arms could have been selected for. This would have led to species capable of leaping and even gliding small distances (like "flying" squirrels) before eventually giving rise to flight.

    Here's a short video from a prominent popular life science author that explains some of these points further

    Last edited by a moderator: Sep 25, 2014
  19. D H

    Staff: Mentor

    Those very small steps were not unlikely. Pile up those small steps over time and eventually evolution develops radically new structure over time.

    Lots of people have hypotheses. The video Ryan just posted references one of those, the "Wing-Assisted Incline Running" hypothesis. Models of how flight evolved can be cast into one of two broad themes, bottom-up and top-down. Bottom-up models say that flight evolved from activities that occurred on the ground, top-down from activities above the ground. There are a number of variations on these two main theme. Wing-assisted incline running is a specialization of the bottom-up model of the evolution of flight. Pouncing on prey from an elevation is a specialization of the top-down model. Which is right? There's no clear-cut winner, and the proponents of one hypothesis spend just as much time attacking other hypotheses as they do defending their own.

    The right answer, right now, is that nobody knows for sure. There's nothing wrong in science with answering "we don't know" to a question.
  20. Borek

    Staff: Mentor

    Actually questions that we don't know answers to are what makes the science work :smile:
  21. On the contrary, vestigial organs/appendages are principally important. Where I think you are getting hung up here is in the concept of long tracts of time. Evolution works gradually through morphing the body and its systems to new environments and challenges over periods of time that are inconceivable to us. What if I told you that, at some time in the future, our legs were to be transformed into appendages whereby the combination of our knee and foot were to serve as catapult like structures whereby we could bounce around and maneuver with an efficiency and speed 3 times our present set up? A future "evolutionist" may post a question to a thread, saying, evolution is a farce because nature never would have selected that "intermediary" trainwreck of a biology those humans possessed in the 21st century, whereby the leg was strait?! What an inefficient, slow, and cumbersome setup, not to mention the issues of back pain and osteoporosis it exacerbated.
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