Experimental evolution

Carl Zimmer writes about “Experimental evolution” in the Yale alumni magazine. As always, he writes clearly and entertainingly on a complex matter.

I was particularly intrigued to read this

“When a single phi-6 invades a host cell, it makes clones of itself. Its genetic material is inserted into the host, and the host begins producing copies of the virus's genes and pieces of the virus's protective protein shell. These chunks of genes and shell float around inside the microbe before assembling themselves into new viruses. All the new viruses are clones of the original invader, differing only by whatever mutations emerged as their genes were produced.”

I knew that viruses multiply by hijacking the host’s cellular machinery and then getting it to churn out copies of the virus, but I had no idea it worked by self-assembly of the genes and the coat. I thought viruses somehow emerged full-fledged from the host’s genes, but it’s not even a complete set of genes that is produced, but chunks of genes and viral coat that later assemble themselves.

However, I was surprised by how slowly the process works. Lenski’s bacteria took 40,000 generations to get double their rate of growth. On average, that is an improvement of just 0.0017% per generation! I thought evolution was much faster than that.

And I was momentarily defeated by this sentence

“Natural selection favored viruses that could use the proteins made by other viruses in the same cell”.

What does that mean? Once virus A and virus B have merged with the host’s genome, what identity do they retain?

I think he is saying that variants of the virus (of which A is an instance) that no longer carry the genes required to produce those “chunks of shell” have some advantage of variants that still carry those genes, but I am not sure how much of an advantage that is.