The processes of evolution

Actually, the best explanation shows swim bladders developed from lungs, not the other way around. Lungs adapted from pharyngeal structures originally functioning in filter feeding (which in turn developed from gill pouches) in Ostracoderms (bottom feeders — by the Devonian they were extinct). Remember that there doesn’t have to be anything fancy here — just a moist membrane that allows gas diffusion from an area of high concentration to an area of low concentration. Some modern amphibians, for example, breathe through the skin or swallow air and use just such gut pouches as "lungs". There are also a number of modern fish — especially in fresh water subject to periodic low oxygen content (but also some salt water fish like many of the Gobiidae) that gulp air, then hold the bubble in their mouths for diffusion. Obviously, a thin epithelium in the mouth is worthwhile at this stage — thinner epithelium means easier cross-membrane diffusion. Development of small pharangeal pouches came next. Look at the anatomy of either Polypterus or Polydon species — each have small pharangeal pouches attached to their gut surrounded by capillaries — just like modern lungs, except these are still attached to the gut. Fossils discovered of the early fresh water teleosts (Cheirolepis, for example) show skeletal adaptations similar to the modern fish mentioned.There are enough modern analogs of freshwater fish that rely on air when their ponds/streams/mudholes/intertidal pools dry up or turn anoxic showing the full range of adaptations (from mouth bubbles to lungs) that this is one of the easier transitions to visualize. With paleoecology showing a long Devonian drought, it makes sense that critters who could use some air would be better adapted - and survive - over the ones who couldn't.Hope that answers your question.

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Originally posted by mark24:
Quetzal,
Interesting, do you have a cite? I came across this once before but dismissed it as a teachers thought experiment, & investigated no further.

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Mark

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Hey Mark.I think it's pretty much concensus these days. Here's one cite: Mallatt, J. (1984). Early vertebrate evolution: pharyngeal structure and the origin of gnathostomes. Journal of Zoology, 204, 169-183. I don't have the original paper, but my "marginal notes" say this discusses the evolution of pharyngeal pouches from gill structures. Caroll talks about the lung -> swim bladder adaptation in "Vertebrate Paleontology" (1988). Most comparative anatomy/zoology courses that talk about fish evolution are saying it these days, afaik.

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All the examples shown here are of fish that already possesses the capability to live outside water. Their breathing system is already adapted for it. I am aware of their existance. However, evolution dictates that this capability is not automatic, but that it must have been develop. Therefore, I wish to concentrate on fishes that does not have this capability. Lets take a fish that cannot survive outside water, and cannot use its fins to move around outside water. Acording to evolution, this is what the precursors of land animals must have been like. The question is, how do these fishes get the capability to move on land? Remember, the genetic code to create lungs does not exist yet, so this process cannot be compared with the live cicle of an amphibian, whos genetic code already contain the information to form lungs.

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That's not entirely accurate. If you'll re-read my post, you'll note that by the mid-Devonian, early lungs had already developed in fish. It's the ones whose fossils are found in fresh water (rather, formerly fresh water) that were the ancestors of most of the modern fish, as well as amphibians, and all terrestrial vertebrates (basically every vertebrate except the Chondrichthyes (sharks, skates, rays, etc). Almost no amphibian has lungs. Many of them have lung-like structures attached to their gut - as do many fish (swim bladders are derived from lungs).Beyond that, are you asking for information on the development of the tetrapod limb (necessary for locomotion), or lungs? It appears you're changing the question.

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I do not agree with the arguement that it was food that lured them out to land. When a whale beaches itself, only animals that can move on land will come out from the water to feed on it. You do not see fishes (such as sharks) that cannot live above water, struggling out to join the feast. If this was the case, evolution is feasable: The fish that can crawl out better, is more likely to survive and pass on their genes. But this is crearly not the case. They simply don't come out. Similarly, the lush plants of the tropics do not lure fish that can not move on land. Science assume that things worked in the past as they work today. Therefore, we can savely descard the theory that propose that fish were "lured by the advantages of walking on land". This advantage, however great it might be, cannot change the genetic makeup of animals, because genetics is chemistry, and habitat advantiges is not.

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Actually, sharks are one of the few fish not descended from the Actinopterygians of the Devonian. They really CAN'T use air because their ancestors (the Chondrichthyes) never had lungs. This is also why sharks don't have swim bladders like most modern fish.As far as the selection pressures that led to the terrestrial radiation of the tetrapods, well, the jury is still out. We have quite compelling evidence that it happened, but not quite so much as to the "why". There are two major hypotheses. Romer, among others, postulated that the shallow seas and lakes of the Devonian supercontinent were subject to repeated and severe droughts - like southern Africa today. With water supplies being uncertain, natural selection would favor those organisms with a greater or lesser capability to use air directly. Watching a blenny - a salt-water goby - sitting on a stone gulping air is an amazing example of this. Blennys have no lungs - their pharyngeal pouches have evolved into swim bladders like most other teleosts - so they hold a bubble in their mouths and absorb oxygen through the epithelium. They can sit there for about ten minutes before having to dive back in their pools. As an aside, the lungfish and mudskippers mentioned are NOT the likely ancestors of tetrapods, although the Dipnoi have been around since the Devonian. They are illustrative of the lifestyle, but not necessarily in the line of descent.Others have postulated that tetrapods evolved from shallow sea/coastal dwellers. Being able to sit on the bottom and snag passing arthropods was a great way to make a living. This is a very similar to modern Ambystomatidae (salamanders with both lungs and, at least in the neotenic versions, external gills), among others. The ones who were able to move further up the beach, or stay out of the water longer, had better chances to get food and hence reproduced more. Remember, this represents a LOOOONG slow, millions-of-years process. It wasn't just some Icthyostega that woke up one morning and decided to take a stroll. In any event, this idea is not as unlikely as it might seem: the one thing the study of biodiversity shows us more than anything else is that life will radiate to fill any available niche given half a chance. And it's often caused by some small adaptation that allows organisms to exploit new ecosystems.

My pleasure, amigo. One of the great things about hanging out on this board is that I literally learn something new every day.

Hanno,Basically that's correct, although I would say that there was one surviving lineage that led up the long eons to the diversity of life on the planet. However, I'm not sure that saying that a single cell was the last common ancestor is very informative. The biggest problem we have once you get to the bacterial stage (which lasted about 2 billion years) of the development of life, is that the lineages start getting very, hmm, promiscuous. A whole lot of gene swapping between wildly different organisms (such as between archaea and bacteria) and a lot of serial endosymbiotic events took place. Meaning that the "root of the tree of life" more resembles a tangled mangrove than a single root. I'm not sure you can even say (at least when you get back around 3.8-4 gya) that we're even talking "DNA lifeforms" were the last common ancestor. What the theory DOES say is that, once DNA was adopted as RNA's baggage carrier, everything else in history was based on it.If you're asking whether there was a particular DNA or RNA "code" that was the last common ancestor of all life, I think the answer is probably "no". There were a bunch of them - but all based on the same kind of nucleic acids - which is why we say "all life on Earth is related".

Hanno: Mark has it right on the money. It's pretty weird when you get to the prokaryotes. Basically, lateral gene transfer is pure larmarckism: inheritance of acquired traits. It would be sort of like a monkey and a bird mating, producing a primate with wings in one generation. Really, though, they're just exchanging a few bits of DNA. Prokaryotes don't have a nucleus, so their DNA is just hanging around inside their cell membrane. It's quite fascinating, IMO.

Great links, Itz. Thanks [quetzal runs around immediately downloading the articles].

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Originally posted by Hanno:
So, if you get down to that level, there are no distinct species, right? Well, that just blew my arguement to bits. I thought the theory says that all live came from a single organism. I was going to argue genetic regresion, but seems like that one won't work so well anymore. Oh, well....

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Well, you can separate bacteria into clonal lineages that are roughly equivalent to "species", based on actual genetic makeup, but we're getting a bit more technical here than your question required. For our purposes, there is no "hard and fast", reproductively isolated species like we mean when we talk about metazoan taxa.

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I thought of something else that is bothering me. Evolusion in an animals livestyle. Take that fish that swims upstream to mate where it was born. What's it called? Macerral? I think that's it, though I don't think that's the spelling. Anyway. Here you have a fish that mated in the oceans just like any other fish. Now it has to evolve the instinct to swim all the way back up a river. I don't think this can be caused by natural selection: The fish that just mate in the ocean where it is has a much better chance to pass on its genes than the fish that desides: "Mmmm I wonder what mating is like way up there". Chances are, he would not succeed. And even if it did, there must be at least two that was sucessful in their attempt before they can breed. The species as a whole needs to perfect this skill to swim all the way up a river, and it is more likely that none of those very early pioneers survived to pass on their genes.

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Actually, it isn't that hard (sorry, again - eventually you WILL ask a question I haven't already researched somewhere ). The question really departs a bit from evolution, and lands in the area of behavioral ecology. One thing to realize is that although the pathways are different, the selection pressures that lead to the evolution of migration in fish are really no different from those that lead to migration in birds, sea turtles, etc. IOW, food supply, predation pressure, climate, etc. The specifics will vary depending on whatever organism you’re dealing with. Moreover, not all members of a particular fish species — or even population — will necessarily migrate. I don’t know about mackerel, but this is certainly true of many species of salmon, for instance (see jack salmon, which grow to full size in their native river while the rest of their population does full migration). Anyway, the idea is that there is selection pressure related to higher productivity in oceans than in rivers, hence the adaptive advantage of migration. It’s interesting to note that the reverse is true in the tropics, where rivers have higher productivity — some marine species grow to adulthood in the rivers, but breed in either saline estuary or even the open ocean. (search for anadromy and catadromy for examples). We can still see this in action — coho salmon were introduced into New Zealand, and soon diversified (primarily due to the genetic plasticity of this species) into both anadromous and catadromous populations — with some being extremely lazy and adapting to a purely lake- or river-dwelling lifestyle - all in only ten years. Given the number of salmon populations that are known to thrive even cut off from the ocean, this adaptability doesn't really surprise me.Here are a couple of articles: Evolution of Diadromy in Fishes and Evolution of Life History and Migration in Fish