MattofVA

Many traits appear to have evolved that certainly require more than one genetic change-and have done so in relatively short amounts of time. Take bipedalism, for example, having arisen in us in just 5-10 million years which is a relatively short time given our relatively advanced age at reproduction. So, hoe does this occur? Bipedalism requires more than one trait-wider hips, bigger knees, and probably a couple other things I am failing to think of. In short, dozens of genes. So, how does this occur, since it is a collection of traits rather than really just one. How do multiple traits evolve in a certain direction when chance would seem to indicate they should reach a certain point-less than halfway-and then stop, especially when we are dealing with a species that diverged from the rest of its order in half a million generations at most? I suppose the same goes for sensory organs and such since they are all dictated by much more than one gene. These are examples of evolution moving in particular directions that could reasonably indicate to some people the appearance of a teleological process. If I don't get some good replies, I'm afraid I'll have to become a deist or something

MattofVA

Any takers? Come on guys-the pattern of mutations in short time periods is rather bothersome.

Origian

The progression to bipedalism is one that did indeed require many new characteristics, such as a centred foramen magnum, 'S' shaped spine, a more bowl-shaped hip bone, and the valgus angle for instance. The thing to remember is that the development of these would have been as with all natural selection where individuals are selected for the natural variations in a characteristic that may be present in a poplulation - mutations would not have to have been the only way to achieve these new characteristics. For example, an individual with a more pronounced valgus angle would have found it a LOT easier to escape predators and therefore would have been selected for, producing a poplulation of individiuals with greater valgus angles. As this continued, the point may have been reached where the angle in an individual was too large, resulting in not being able to stand or walk easily - this would be selected against. In the same way it can be seen that all of the characteristics I listed above could easily have been developed within 500000 generations (quite a few if you ask me), as many could have developed in parallel with each other.

Then again, I am no Biologist - this is just what I remember!

Undercurrent

Not really. Chimps can be bipedal for short stints without any of these. A change in hip shape and locking knees just make it easier to be habitually bipedal, and I don't think either of them are terribly major mutations.

The thing about our larger hips doesn't have anything to do with bipdealism (and, in fact, inhibits it somewhat). It has to do with our big heads that need a wide birth canal to be born through. Lucy was perfectly bipedal with narrow hips -- her species had much smaller heads. Similarily, a downward-pointing foramen magnum would take some stress off the neck, but isn't really nescessary for a bipedal animal with a small head.

pz

No, it has nothing to do with teleology.

Your root problem here is that your question is so wrong, so incomprehending of the actual state of affairs, that it's hard to answer it. You've got too many misconceptions.

First off: ditch that idea of mosaicism in evolution. There are no genes "for" wider hips, bigger knees, bipedalism, foramen magnum position, etc. Those things emerge as part of a process that involves genes and cells interacting with one another and the environment, and you can't simply say X features translate into X genes.

Richard Dawkins made an informative analogy to illustrate this:

"... If we follow a particular recipe, word for word, in a cookery book, what finally emerges from the oven is a cake. we cannot now break the cake into its component crumbs and say: this crumb corresponds to the first word in the recipe; this crumb corresponds to the second word in the recipe, etc. With minor exceptions such as the cherry on top, there is no one-to-one mapping from words of recipe to ?bits? of cake. The whole recipe maps onto the whole cake."

The other important consideration is developmental regulation. Since genes interact with each other and the environment in order to generate the organism, a change in one factor causes changes in many others.

For instance, look at Abby and Brittany Hensel:

They are conjoined twins, with two heads sharing one body. It's an amazing example of how an organism regulates to account for radically different conditions in development -- there are an immense number of structural internal differences that you can't even see internally. The circulatory system is radically changed to allow a single heart to supply a pair of heads. The CNS had to fuse at the spinal cord. Everything functions, but no, there were not a whole bunch of discrete genetic changes to allow this to happen. Instead, you had a single, drastic environmental change, and a stock human genome interpreted that information to assemble an integrated whole.

I imagine bipedalism arose in a similar way. It didn't require 50 discrete mutations all assembled at once in a single individual. It required a smaller number of changes that could happen sequentially; a shift in the timing of hindlimb development, for instance, would lead to a cascade of developmental changes that worked together (because that's what regulatory genes of the limb do, is work together) to build a limb with a subtly different morphology. A series of these kinds of mutations would eventually lead to the leg we know and love...but it is simply a property of the developmental system that the intermediates would each form a functioning limb.

ps418

A nonsequiter. It may or may not be the case that between-species differences in these traits requires changes in dozens of genes. At this point there is no reason to rule out a simple model in which only a single gene involved in development, maybe a transcription factor, accounts for much of the between-species difference in locomotion-relevant anatomy. Another possibility is that the relevant genetic differences are in gene promotor regions rather than genes themselves. In general I would caution against the temptation to attribute too much of the Chimpanzee/Human differences to gene differences per se rather than to altered expression of the same set of genes due to promotor region differences.

A collection of traits can evolve just as surely as single traits can evolve, provided there is some selection. A hypothetical model in this case would be that there was a single mutation that allowed for more efficient faculatative bipedal locomotion (for instance, one affecting development of pelvic morphology and the femoral bicondylar angle) which, while still far from human bipedality, conferred a slight adaptive advantage relative to conspecifics lacking that mutation. Subsequently there could have been further selection for other traits that improve bipedal locomotion (e.g. foot morphology). Its not necessary for every trait associated with bipedal locomotion to spring into existence all at once, exactly as we see them today.

Obviously, if traits evolved only "by chance," then there were no selective pressures at all. If there was any selection at all, then the expectations of the "chance" hypothesis are not relevant. If the evolution of bipedal locomotion occurred over several steps, and each step represented some small advantage, then the evolution of multiple traits "in a certain direction" need be no mystery at all.

Patrick