Is Metamorphosis Irreducibly Complex?

It is not my intention to argue against the TOE nor is it to argue for ID.I was hoping someone could help me understand how metamorphosis could evolve. It’s complexity seems irreducible. But, it happened in both insects and amphibians, so there’s gotta be a way.I read about reducing the complexity of the eye, it made a lot of sense and I learned. I can’t seem to come up with a way of reducing the complexity of metamorphosis though. Perhaps someone on here can help me. To make it easier lets talk about one species, I’ll pick butterfly.Were there caterpillars that metamorphosed into some kind of primitive butterfly?
Or, were there butterflies that came from a different caterpillar?
What kind of transitional stage could there be between a species that does not metamorph and one that does?Here is the definition of metamorphosis: Thanks in advance for those who help me.

Thread moved here from the Proposed New Topics forum.

It may take quite a few posts to do this one justice, but let's see if we can get started with a short one. I will build on it later.There are four life stages in holometabolous insects (those with complete metamorphosis like your butterfly)Egg
Larva
Pupa
AdultFor background, let's consider briefly what each stage accomplishes from a life history perspective.Egg - development only
Larva - feeding and development
Pupa - development only
Adult - feeding and reproductionThe most derived stages can, in terms of basic life history, be inferred to be 'egg' and 'pupa' - they are hihgly evolved developmental stages that do not accumulate any nutrients and do not function in repoduction. They have no biological significance without a larval or adult stage to feed, grow and reproduce.It remains to be determined which is more ancestral: the larval life stage or the adult.Since the adult is invariably the most complex morphology, it is likely the most 'derived' ("highly evolved") stage. Reproductive specialization in the adult life stage is also a derived state, because many insects can reproduce successfully without complete metamorphosis to an adult form. These lineages lack a true pupal stage. They are said to be 'hemimetabolous' (incomplete metamorphosis).I can add more, if you like, after other responses, but if you are going to start a whole bunch of new threads in the same evening, beware you are assuming the responsibility of following up on them all

I'm having trouble making sense of this reply... Since metamorphosis occurs during the pupa stage, maybe we should talk about that one, unless the others are neccessary for the discussion. please do Well, I only started 2, and the first one was started on 5/19(a week ago). I work evenings and tonight was slow, so I had time to post on this site. I was just going to propose this topic, since I hadn't seen anything of the other one, but then when I got on I saw that the other had been promoted. So, I replied to the other one and went ahead with my original intention of proposing this topic.

OK. When I say 'derived' I mean secondarily evolved.The egg is not really an 'essential' life stage like the larval stage - it was likely secondarily evolved specifically for advantages it conferred in reproduction. We know that some insects can still reproduce without ovipary, and some species are capable of both ovipary and vivipary 9e.g. aphids). But eggs are really good for surviving periods when there is nothing to eat or conditions are hostile because they can lie dormant for extended periods and also contain all necessary reources for development to the next stage. Therefore, they likely evolved before the pupal stage.The pupal stage is the other 'purely developmental' stage. I suspect it was the last of the four life stages to evolve, simply because we can find insects that have all of the other three life stages, but no combinations of three life stages exist that include the pupa.However, it is truly the most amazing of the four.
The whole larval body reverts to a sort of soup that is then re-organized to form an entire new body plan. It is also costly in various ways. Pupae are very vulnerable to predation, being for the most part without defences, and as much as 30% of body weight can be lost during pupation as there is no intake of food, but expensive developmental processes continue. So what is the selective advantage of such a dramatic restructuring of the entire body plan?The most evident one is that it enables a transition to an adult form that is completely different from the larva, as opposed to one that is simply a larger version of the immature. One important consequence is the partitioning of functions between the two life stages - larvae remain simple and become specialized to feed and grow without the costs of developing adult features such as wings and gonads - their relative success in feeding provides all the resources for pupation and determines the size of the adult.Adults are specialized for reproduction, and usually for dispersal as well. Since many insects have life cycles far shorter than one year, this complex life history could have had great advantages for adaptation to seasonality in the environment. Eggs are produced at one time of the year, larval stage occurs when the greatest concentration of food is available, etc. Complete metamorphosis has been adopted by most of the highly successful insect orders (Lepidoptera, Coleoptera, Hymenoptera, etc.) but there are still a number of important orders in whichc it has never evolved e.g.Orthoptera (grasshoppers) and Hemiptera (true bugs), so other insects have found solutions to these problems without adopting metamorphosis.So the order of life history stage development in the ancestral lineage giving rise to higher insect orders is most likely:1. larva
2. adult
3. egg
4. pupaalthough it is also conceivable that the egg stage came before an adult stage. Sometimes developmental pathways can become truncated and larval forms can attain maturity, something known as neotony.It is also possible that evolution of complete metamorphosis was not a single event in one lineage, but could have evolved more than once independently in several lineages.So metamorphosis is by no means 'irreducibly complex'. Its complexity can be reduced by considering the distribution of various life stage combinations present in extant insect orders, and by considering specific life stage functions in life history.This message has been edited by EZscience, 05-27-2005 06:10 AM

So now let's focus on the pupal stage and how it might have evolved.Recall that in insects there are various larval 'stadia' or 'instars', each defined by a 'molt' in which the cuticle is shed to permit growth. The fact the number of larval stadia is highly variable among insect orders (as few as 2 or 3 to as many as 12-15) is evidence that the number of larval molts was quite variable in ancestral lineages.Now suppose for a minute that, as a result of some mutation in the developmental cascade, the penultimate molt (the one before the final molt to the adult form) resulted in a quiescent, non-feeding stage. I have already mentioned below the potential costs of this type of change, but the benefit of the final instar becoming devoted entirely to development (no longer needing to feedi or grow like a larva) would be the opportunity for more dramtic re-organization of the body plan to improve the success of the adult function, which is reproduction. So the pupal stage can simply be thought of as a modification and of the final larval instar that results in its specialization for developmental functions, at the expense of feeding and growth. It would have to occur under circumstances where all the necessary resources for pupal development could be reliably obtained by earlier instars and stored for later use during the non-feeding stage.There is aditional evidence to suggest this is an entirely credible scenario. Consider Arachnids that are only distantly related to insects with in the Phylum Arthropoda and have no true metamorphosis. Nevertheless, some species of mites (Acari) still pass through various immatural nymphal stages, some of which can be sessile, non-feeding stages that are specialized for either development or dispersal. Not a pupal stage, but functionally analagous.

That makes a lot of sense. I'd never thought of it that way before. Thanks. I didn't really know much about insects and your posts have taught me a lot. Insects are truly amazing, simple yet complex. Metamophosis definately seems like a difficult evolution. I bet they failed this one a lot before they got it right

That's almost invariably the case when it comes to any significant evolutionary advance. We simply don't see any descendents of the 'failed prototypes' so we are inclined to underestimate both their number and their significance. Focusing solely on survivors can easily delude the observer into believing a 'directed' process is involved.

I wouldn't be so sure about that. It is possible that the earliest holometabolous insects had a very long period of metamorphosis with a variety of intermediate transitional stages. These transitional stages would have been perfected over a long period of evolution. The origin of true metamorphosis (of the caterpillar to butterfly variety) would then result simply from a speeding up of the process, and combining of various sequential stages into a single transition. The metamorphic process might have been perfected as a slow set of morphological changes, then sped up to the rapid miraculous process we see today.I understand that this kind of slow metamorphosis can be found in many marine invertebrates, I'll see if I can hunt down a nice example for you.Mick

An example would be cool.Could you also specify a little those relative words, very long and long, that you used?

The question of this topic seems to be answered in that the complexity of metamorphosis has been reduced. I was sure that it could be but couldn’t figure it out on my own. Thanks for the help.Now, it seems we have some conflicting hypotheses on how metamorphosis evolved. Would you guys care to further discuss, not the ‘if’ but, the ‘how’ metamorphosis evolved?

It is my guess that there are still a lot of failures of metamorphosis.The test is: How many catapillers fail to make it through to butterfiles? Are there mutations which screw the process? Is this similar to the failure rate of human pregnancies?

That's very insightful Ned.
As a matter of fact, as one who rears many different insect species from different life stages, I can tell you that every single molt is a critical stage for survival, whether it is a molt to the next larval instar, the molt from larva to pupa, or the molt from pupa to adult. There many things that can go wrong. Most often, it is failure of the next stage to free itself of its old skin - that can mean strangulation. The pupal stage can be very sensitive to temperature. Unless the proper range of T is experienced, there can be deformities in the adult. I notice this a lot with beetles. I suspect it is because different enzymes active at different times in development have different thermal optima for complete functionality. Constant temperature regimes can be fatal. Among the parasitic Hymenoptera, many of which attack hosts insects that vary in suitability, it always amazes me how many individuals can die as fully formed adults without successful pupal emergence.This message has been edited by EZscience, 05-31-2005 10:16 PM

I am wondering why you specifically picked metamorphosis to be irreducibly complex? Wouldn't virtually all other development also fall into the same category?How about the chicken and the egg? How about mammalian embryos and the placenta? Larval transitions of Cnidaria?I guess the problem I have is in understanding what actually falls into the category of irreducibly complex? Is there an accepted definition of the term that has been show to actually be applicable to a specific example in nature?