A few posts back, I commented on some experimental genetic results that throw a real monkey wrench into the generally-accepted view of the evolution of gender. Part of that post dealt with the concept of X-inactivation, the process by which one of a woman’s X chromosomes is inactivated so that she has no more functional X chromosomes than a man. A commenter known as jsilverheels then asked an excellent question, which I attempted to answer.
All of that discussion related to chromosomes, dominance, recessiveness, etc. got me thinking about sex-linked inheritance. It’s a common subject taught in high school biology, and it is something I discuss in my biology textbook. However, X-inactivation seemed (in my mind) to contradict something that is routinely taught in most high school biology courses. I searched the web for an answer to this apparent contradiction, but to no avail. No matter what kinds of keywords I used, I couldn’t find an article that addressed this particular problem.
As a last resort, I ended up E-MAILing my sister-in-law. My wife is brilliant, and she comes from a family of brilliant people. Her oldest sister is not only an accomplished molecular biologist, she is also a dedicated college professor. I knew she would have the answer to this question, but I hate to bother people who are busy doing such productive things. Nevertheless, I really wanted an answer, so I broke down and sent her the question. Not surprisingly, she answered it straightaway. I thought I would blog about it, mostly so I would remember it later on.
WARNING: If you thought what I have posted before was geeky, you probably won’t like what appears below the fold!
An infrequent but enjoyable commenter (Black Sheep) recently asked a question that I thought I would answer with a post. The relevant portion of the comment is:
A friend and I are always puzzled by the way our bodies, or rather minds, react after a run. This really only tends to happen when we do longer runs, like when we were training for a half marathon. After we finish, whether it be a race or just a training run, we feel completely stupid…why the diminished mental capacity?
Some might argue that a person who wants to do such a long run is actually starting out with a diminished mental capacity. However, there is actually a very good reason for why you can feel stupid after a long run, even if you started with a keen mind. You will find the answer below the fold.
A student recently sent me a question based on a statement made in Dr. Jerry Coyne’s book, Why Evolution is True. Since there doesn’t seem to be much written about it for a general audience, I thought I would summarize the issue. Here is Dr. Coyne’s statement:
One of my favorite cases of embryological evidence for evolution is the furry human fetus. We are famously known as “naked apes” because, unlike other primates, we don’t have a thick coat of hair. But in fact for one brief period we do – as embryos. Around sixth months after conception, we become completely covered with a fine, downy coat of hair called lanugo. Lanugo is usually shed about a month before birth, when it’s replaced by the more sparsely distributed hair with which we’re born. (Premature infants, however, are sometimes born with lanugo, which soon falls off.) Now, there’s NO NEED for a human embryo to have a transitory coat of hair. After all, it’s a cozy 98.6 degrees Fahrenheit in the womb. Lanugo can be explained ONLY as a remnant of our primate ancestry: fetal monkeys also develop a coat of hair at about the same stage of development. Their hair, however, doesn’t fall out, but hangs on to become the adult coat. And, like humans, fetal whales also have lanugo, a remnant of when their ancestors lived on land.1 (emphasis mine)
Note the strong words by Dr. Coyne. Embryos have “no need” for such hair, and thus its presence can “only” be explained as a remnant of our primate ancestry. Not surprisingly, Dr. Coyne is wrong on both counts.
I recently got an E-MAIL from a student who heard a “university professor” say that the human and chimpanzee DNA are 99% similar. She asked whether or not the professor was correct and, if not, how similar is human DNA to chimpanzee DNA?
Well, the answer to her first question is quite easy. The professor was horribly wrong. The nonsensical idea that human and chimp DNA are 99% similar comes from misinterpreting a 1975 paper by Mary-Claire King and A. C. Wilson. 1 This groundbreaking (for its time) article compared several proteins in chimpanzees to their equivalent proteins in humans.
In case you don’t know, proteins are complex molecules that are composed of many smaller molecules (called amino acids) linked together. The primary structure of a protein is simply the order in which its amino acids link up. King and Wilson showed that in many, many proteins, the difference in the primary structures of chimpanzee and human proteins was about 1%. Since DNA determines the order of amino acids in each protein an organism makes for itself, they made the reasonable inference that for the portions of DNA that code for those proteins humans and chimpanzees are 99% similar.
However, the genes that code for these proteins make up a tiny, tiny fraction of the human or chimp genome, and only SOME of those proteins were studied. Thus, the idea that one can extend that number to the entire genome and say that human and chimp DNA are 99% similar is just absurd.
Josiah, a frequent commenter on this blog, asked an excellent question in a post on my previous entry. I started to reply to his question, but I realized the answer would make a good blog entry. Josiah asked whether or not “cooperative relationships in the animal world” are a problem for evolution. He doesn’t think so, but the author of one of his homeschool books thinks it is. What do I think?
Well, let’s start with the terminology. A relationship between two or more individuals from different species is called symbiosis. However, that word has grown to refer to different kinds of relationships. It can refer to a relationship in which all participants benefit, a relationship in which only one participant benefits but the others are not harmed, or a relationship in which one benefits and another is harmed. Thus, to specifically talk about cooperative relationships, we use the term mutualism, which refers specifically to a relationship in which all participants benefit.
Josiah mentioned a couple of examples of mutualistic relationships. One was the tick bird, which eats ticks off a rhino’s skin. This is beneficial to the rhino, which becomes relatively “tick free,” and it is obviously beneficial to the bird, as the bird has a fairly safe place to find food. The tick birds also warn each other (and the rhinos) about any incoming danger. The other example was the single-celled protistans (flagellates) that live in a termite’s gut and digest cellulose. The termite (indeed, all multi-celled animals of which I am aware) cannot digest cellulose on its own, and since wood is 50% cellulose by mass, this would be a problem for an animal that eats wood. However, the flagellates in a termite’s gut digest the cellulose, which allows the termite to eat wood. Obviously, both participants benefit in this relationship. Are these kinds of situations a problem for evolution?