February 15, 2007

Sex for Valentines, Evolution for Darwin

My first article of the semester (covering the purpose of sex, the Red Queen and the shrinking Y chromosome) was supposed to come out yesterday, but due to the giant snowstorm stomping across the Northeast, school was closed. Today, however, it was published:

This week marks two important days. The twelfth was Charles Darwin's birthday, Darwin Day, celebrating his conception of the governing principle of biology, evolution. Today, Valentine's Day, is the day where men everywhere strive to impress their significant other with a gaudy plumage of pretty paper, tasty treats and innuendo (a.k.a. sweet nothings).

Their intentions are dubious at best. Admit it, ladies.

Sex is the driving force behind every relationship in the natural world. The origin, evolution and persistence of sex may be a bit less exciting than the act itself, but no less interesting.


Since sex produces offspring with a more varied genome, the descendents of an infected organism have a better chance of evolving defenses against the parasites. In an infected asexual organism, the offspring will be copies of the parent, with only a relatively slim chance of mutating measures of avoidance.

In an experiment involving a type of snail from New Zealand, scientists were able to test the hypothesis. These snails, like many other organisms, can reproduce sexually or asexually, depending on the environmental conditions. When reproducing asexually, female snails of this particular species go through a process called parthenogenesis, in which they generate viable female offspring identical to themselves.


Scientists have recently traced the evolutionary lineage of the Y chromosome, leading them 300 million years into the past, to our distant ancestors. They have found evidence that the male determining Y chromosome is a mutant of the X chromosome. All humans have 23 pairs of chromosomes housing our genetic code, with the last pair determining sex. This pairing is another advantage of sexual reproduction, allowing the code within to make repairs more easily. If one is damaged, the other can provide the template.

The problem is that male chromosomes are not all paired. The Y chromosome stands alone, among the set and is susceptible to damage without a backup. Three hundred million years ago, the Y chromosome had some 1,000 genes; today the chromosome is down to a few dozen.

I just arrived in San Francisco after eight hours of planes, airports and grumpy people. It might be time for a little snooze. I'll probably be back tonight talking about the tropical dry forests as part III of the biome basics posts.

1 comment:

  1. Nice. There are some complexities that arise in there, as you likely know, like genetic defect expurgation due to unpaired Y. Some theorists suggest that males are a sacrificial sexual "caste" for getting defects more rapidly out of the germline (male mammals, some female birds). We burn fast and bright and spread our seed, then drop dead of a heart attack. I think I'll make myself a drink...