blike Posted December 12, 2002 Posted December 12, 2002 Its about right time to stir up some controversy Alright, given that convergent evolution happened, what mechanisms put restraints on convergence? I understand that different environments play a role; I don't need fins. Likewise, a fish wouldn't have much use for my legs. I'm asking what puts constraits on organisms that live in the same environment as one another. Someone mentioned whales (previously land mammals) developing fins, much like fish. What kept the whales from developing gills, eliminating the need to surface? Why do we have larger brains? I'm sure all of us would agree that larger brains is advantageous; why doesn't my dog have a larger brain?
fafalone Posted December 12, 2002 Posted December 12, 2002 Time. It takes millions of years to evolve major features; it's still going.
blike Posted December 12, 2002 Author Posted December 12, 2002 Reasonable; but what about organisms that have been around a hideously long time. Horseshoe crabs, they dont' have fins, correct? Why not? They surely would be faster; both for catching prey and escaping danger..
fafalone Posted December 12, 2002 Posted December 12, 2002 Competition; for its particular niche, it is the top organism. If it started developping gills, it would be outcompeted.
Radical Edward Posted December 13, 2002 Posted December 13, 2002 evolution doesn't have an aim in mind, so given any realistic situation, something like a crab wouldn't have any drive to push it out of it's niche and into one that involved it having fins. especially since there are already creatures with well evolved fins out there that would make out promordial fisabs extinct.
aman Posted December 13, 2002 Posted December 13, 2002 Evolution seems to be driven a lot by mathematics, from the geometry of a conch shell to even the complex humans, proportional equations. If an evolutionary change is made over time, it has to follow the rules or it is just a dead end mutation. Just aman
Radical Edward Posted December 13, 2002 Posted December 13, 2002 I think the mathematical element comes down to the fact that the stuff that the organisms are made out of is inherently geometrical. for example the arrangement of seeds in a sunflower couldn't follow any other geometry, or else it would fall apart. The same goes for eggs, and, I suspect, shells.
Giles Posted January 9, 2003 Posted January 9, 2003 Whales may not have had the opportunity to develop gills; they've only been down there about 60 million years. bear in mind how long it took to get out of the water in the first place. brains are very expensive organs (yours uses about 20% of your oxygen supply). patterns can emerge in design due to developmental mechanisms as well - each cell just following a simple local rule. It's easy to imagine a spiral forming by this sort of mechanism.
Skye Posted January 26, 2003 Posted January 26, 2003 Could make it more controversial, if convergent evolution happened, how come we haven't all converged upon The Ultimate Lifeform? But moving right along... The main reason is that evolving to reach a certain form requires of random mutation an unbroken chain of advantageous forms to get there. Convergence leads to competition. If you take an ecological view of it two species can't occupy the same niche in the same area, so either one goes extinct through competition or they diverge. This affects the function of the organism as a whole not the individual parts though. A whale would be in competition with pretty much the same species it is now if it had gills, species that share it's nutrient sources. Mutations are a positive and negative force, in the sense of evolving to a certain fixed aim. The shape of the fin in both fish and whales is very efficient and deviations from mean are probably going to be less efficient. Despite this humpback whales have enough variation between individuals that they can be identified by their flukes (tail fins). So whales don't have perfectly shaped flukes, the nicely shaped flukes carved out by natural selection are constantly being bent out of shape by random mutations. Mutations that are beneficial don't always become fixed in a population or species. A whale might be born with a fully functioning set of lungs but, with tragic nobility, die in the name of (culinary) science. Sexual selection can also be a factor, mutations just might be unattractive. Attractive mates might be different from the convergent aim, creating a drive away from convergence. Since sexual selection applies more strongly to males you can also get an assymetry of forms between the sexes that could be seen as non-convergent... And.....gills would probably make sonar and communication impossible for whales so they might not be advantageous. By the way, can you really argue against the plausibily of something whilst admitting it exists?
Giles Posted January 26, 2003 Posted January 26, 2003 Both gills and lungs evolved from a small flow-through system with some dead ends, probably with some pumping characteristics (see modern arthropods). Gills still are flow-through, whereas lungs are in-then-out pumping. It's hard to see how you cold go from one to the other without returning to the intermediate, which is simply inadequate for large organisms. Or, in general, varying characteristics may be a more highly adapted form of a common intermediate ancestor; returning to the ancestral form would be maladaptive to the enviroment and/or other characteristics of the organism, so it cannot be done.
fafalone Posted January 26, 2003 Posted January 26, 2003 Humans have the gene for gills, but it's not activated. One day, I'm going to activate it and see what happens
Giles Posted January 26, 2003 Posted January 26, 2003 'Gene for gills' hmmm. Inactive genes tend to decay in any case.
Radical Edward Posted January 26, 2003 Posted January 26, 2003 Originally posted by Giles Whales may not have had the opportunity to develop gills; they've only been down there about 60 million years. bear in mind how long it took to get out of the water in the first place. I don't think it's so much an opportunity, than no real driving force. a whale would only develop gills if each stage of quasi-gilled-whale had some sort of evolutionary advantage overa whale with lungs, and seeing as whales, dolphins (and humans- though I never knew this until very recently) have certain physiological responses which facilitate deep diving, I can't really see any quasi-gills having any significant advantage and driving a whale to evolve gills. incidentally, some people do have little structures on their nexks which some people think might be gill remnants. they aren't much cop though, except they are good at getting infected.
fafalone Posted January 26, 2003 Posted January 26, 2003 Infection is one of the primary reasons I suspect the gene became deactivated as lungs became to handle it all on their own.
da1999 Posted March 12, 2003 Posted March 12, 2003 Originally posted by blike Why do we have larger brains? Our brains are big because memes replicate more and better in big brains as opposed to small ones.
Deslaar Posted March 12, 2003 Posted March 12, 2003 Originally posted by blike Its about right time to stir up some controversy Alright, given that convergent evolution happened, what mechanisms put restraints on convergence? I understand that different environments play a role; I don't need fins. Likewise, a fish wouldn't have much use for my legs. I'm asking what puts constraits on organisms that live in the same environment as one another. Someone mentioned whales (previously land mammals) developing fins, much like fish. What kept the whales from developing gills, eliminating the need to surface? Why do we have larger brains? I'm sure all of us would agree that larger brains is advantageous; why doesn't my dog have a larger brain? It's caused by stablizing selection and results in genetic homeostasis. Here is enexplantion that I got from here - http://library.thinkquest.org/19926/text/tour/12.htm?tqskip1=1&tqtime=0311 Stablizing selection - Phenotypic traits, such as height, weight, number of offspring, and life span, typically show greater numbers of individuals with average characteristics and fewer and fewer toward the extremes. A situation such as this, when individuals with intermediate phenotypes are favored and extreme phenotypes are selected against, is referred to as stabilizing selection, also normalizing selection. The range and distribution of phenotypes then remains approximately the same from one generation to another. Stabilizing selection is very common because the individuals that survive and reproduce more successfully are those that have intermediate phenotypic values. Mortality among newborn infants, for example, is highest when they are either very small or very large. Infants of average size are generally more likely to survive. As a result of stabilizing selection, populations often maintain a steady genetic constitution with respect to many traits. This attribute of populations is called genetic homeostasis.
blike Posted March 12, 2003 Author Posted March 12, 2003 Perhaps currently stabilizing selection is putting limits on diversity, but environmental factors around early ancestors of whales were obviously favoring diversifying selection. In such a scenario, what is to prevent whales from developing some sort of gills, especially when such diversity is being encouraged environmentally?
Deslaar Posted March 12, 2003 Posted March 12, 2003 Originally posted by blike Perhaps currently stabilizing selection is putting limits on diversity, but environmental factors around early ancestors of whales were obviously favoring diversifying selection. In such a scenario, what is to prevent whales from developing some sort of gills, especially when such diversity is being encouraged environmentally? No immediate pay off. Gills developed in proto-fish incrementally becuase they offered the organism an immediate pay off. Whales have lungs that have devoloped of millions of years. They do the job just fine. Nature can't backtrack like that and certainly can't do it on that scale. Also, the whales physiology is based on lungs, if you magically gave it gills they'd die.
Guest Eli_Cash Posted November 2, 2003 Posted November 2, 2003 I think the answer is inevitably probability. Considering the proposed mechanism for evolution, there is no reason why absolute convergence is not possible. In other words, there is no reason why two very different species could not evolve into the same species over time, except that we find this intuitively improbable. However, if we tried to codify this by assigning probabilites to certain kinds of changes, the entire theory of evolution, and especially the theory of a naturalistic origin to life, would be in serious trouble.
Radical Edward Posted November 23, 2003 Posted November 23, 2003 Eli_Cash said in post #19 :I think the answer is inevitably probability. Considering the proposed mechanism for evolution, there is no reason why absolute convergence is not possible. In other words, there is no reason why two very different species could not evolve into the same species over time, except that we find this intuitively improbable. However, if we tried to codify this by assigning probabilites to certain kinds of changes, the entire theory of evolution, and especially the theory of a naturalistic origin to life, would be in serious trouble. the thing that would stop them is the design problems. remember that evolution progresses up a firness peak. for something like a horshow crab to develop fins, it would need to go down the peak and back up a different one, and that won't happen evolutionarily, because any crabs "evolving" down the peak will be outcompeted and die.
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