Titan wrote:The book begins with the history of evolution. It does talk about directional selection and disruptive selection but in the context of natural selection (my book is on
www.biology.com, if you wanted to see it, it is the fifth edition). They talk about point mutations in relation to DNA resequencing along with a few other things.
If it's Neil Campbell's book, then it's as good as they come. Even so, I'd approach it the other way around (in fact, I do). I think we need to start with the molecules, and make it really, really clear how mutations alter proteins, and how proteins alter phenotypes. Without this, the whole natural selection thing is fuzzy. I'd come back to the history, and ask how Wallace and Darwin figured it out with so much less evidence. I came up with this sequence, by the way, in teaching non-majors, who had biology between 4 and 8 years before, hated it, and didn't remember very much. It's a fun challenge.
Titan wrote:I've always had a problem with embryology because I would expect them all to look similar. Think about it, the organism needs to develop certain organs in order, they are living in a fluid filled sack so they need gills. The thing that I find fascinating is that if they are so similar then why doesn't a bird embryo become a human once in a while. I realize that this sounds inane, I think so to. But you have to understand that differences in DNA are still fairly big. Look at the difference between man and monkeys. We have a high rate of protein similarities but we are still quite different.
First, the gills. Mammals, at that stage, aren't breathing at all, whether by gills or anything else. They're getting everything from Mom, and using Mom's system to take in oxygen and get rid of CO2. In birds, and reptiles, they manage gas exchange through the eggshell (permeable to O2 and CO2, but not to H2O). Exactly how they handle it, I'm not sure. We should check with some chicken physiologists. I'd guess that their circulatory system manages to get close enough to the shell to manage gas exchange, but exactly where, I can't say. I'd better look it up, I guess.
You've hit on an interesting puzzle, there, with the similarity of embryos. The YECs like to say that Haeckel fudged his drawings, so it's not true, but the real embryos really are remarkably similar. At that early stage, all of our body plans are pretty much the same. What differs is basically the relative proportions of the parts that form later. The plans for those parts are set up when the organ first starts forming, and is really tiny. At that size, diffusion of small molecules, and of short-range protein hormones, along with cell contact, determines the relative numbers of cells that will form each part.
We all use pretty much the same genes to do these things. You know, the Hox genes, Sonic Hedgehog, etc. The differences are partly small differences in the sequences of the proteins themselves, but mostly differences in the patterns of expression of the genes. It doesn't take much to change the sensitivity with which gene-expression control mechanisms respond to the concentrations of small molecules. Nor does it take much to cause a particular developmental pathway to reiterate itself. Compare us to snakes, and count the rib-bearing vertebrae. Snakes just repeat the rib-production developmental pathway more than we do. Compare broccoli and cabbage to their wild cabbage ancestor (all the same species). Broccoli has a mutation that results in reiteration of the flower-development pathway. Cabbage reiterates the leaf initiation pathway. So, I'd say: look at the final animals, and think about how easily you could morph one into the other. All of the parts are basically the same, just somewhat different sizes and shapes--and that's determined by the patterns of expression of developmental control genes.
Titan wrote:Another oddity. The horse shoe crab has evolved very little over the span of many generations. Why is that? Shouldn't the organisms that are evolving more win out in the end? Shouldn't the horse shoe crab have to constantly adapt?
Fossil horseshoe crabs are not identical to current horseshoe crabs, so it's clear that evolution has occurred. The reason that the body plan has remained so similar is that
it works. There may be various species that are offshoots from the horseshoe crab lineage, but these guys are so successful in their environment that they've out-competed any newcomers. I guess we'd call it "stabilizing selection," which has worked against significant changes in the species.
