The seminar that I give most often when I am invited to speak at other universities begins with a brief introduction to genomes, sets up some comparisons between bacteria and eukaryotes, and then moves into a short overview of bacterial genome size evolution before spending the remainder of the time on genome size diversity and its importance among animals. The main things that I have to say about bacterial genomes are:

1) Unlike in eukaryotes, bacterial genome size shows a strong positive relationship with gene number (in other words, bacterial genomes contain little non-coding DNA).

 

I first became interested in genome size because of its tie-ins with important evolutionary questions in which I was (and still am) interested, such as punctuated vs. gradual patterns, levels of selection, and adaptive vs. non-adaptive processes. What I didn't realize was that one component of the question, the quantity of DNA that is non-functional (but not necessarily inconsequential) with regard to the phenotype of the organism, is such a hot-button issue. I had vague inklings at first that young-earth creationists would object to the idea of non-functional DNA -- because God, as they say, don't make no junk. (Why intelligent design proponents, who purport to take a strictly scientific view of the question, also assume that non-coding DNA cannot be non-functional remains unstated). And of course there has always been a persistent undertone in biology that non-coding DNA must be doing something or it would have been deleted. This latter view, which derives directly from a hardcore adaptationist approach, destroys the argument by creationists that "Darwinism" has prevented researchers from considering functions for non-coding DNA.

I recently did an interview with New Scientist for what, I am happy to say, was one of the most reasonable popular reviews of "junk DNA" that has appeared in recent times (Pearson 2007). My small section appeared in a box entitled "Survival of the fattest", in which most of the discussion related to diversity in genome size and its causes and consequences. It even included mention of "the onion test", which I proposed as a tonic for anyone who thinks they have discovered "the" functional explanation for the existence of vast amounts of non-coding DNA within eukaryotic genomes.

In November 2007, Donald Kennedy, then-editor of the prestigious journal Science, announced that for the next five issues, each of the research articles would include a brief "author's summary" written in plain language.

The new journal Evolution: Education and Outreach is now available online and free to download. My contribution to the first issue is "Evolution as fact, theory, and path". Feel free to distribute this and any other papers from the journal as widely as you like, but please link to the journal website rather than re-posting papers.

There are now several available articles that discuss this important subject:

A recent issue of Genome Research contains a report of the cat genome sequence (Pontius et al. 2007), adding Felis catus to the rapidly growing collection of animal genome sequences. One of the reasons that the number of mammal sequences is increasing so quickly is that there have been reduced standards for sequence coverage. To wit, the cat is one of 24 mammal species approved by NHGRI for "low redundancy" sequencing, meaning that the sequence will be covered only 2-fold (vs. up to 7x coverage in dog, chimp, human, mouse, and rat).