An Excerpt from Jim
of October 27, 2013
issued from the Frio Canyon Nature Education Center in the valley of the
Dry Frio River in northern Uvalde County, southwestern Texas,
on the southern border of the Edwards Plateau, USA
SHEEP, FLEABANES & A COLD NOSE
Many, maybe most, retrovirus insertions on sheep and human genes were placed there so long ago that those retrovirus strains have gone extinct, just as dinosaurs did, through the usual processes of chance and evolution. Since retroviruses are little more than encapsulated genetic information, our genes' "fossil" retrovirus coding can be studied just like fossils in rocks. The longer a certain host species has existed, the more retrovirus coding its genes are likely to carry. If the genes of two host species carry the same fossil retrovirus coding, it can be deduced that once the two species shared a common ancestor.
Thinking like this brings to mind the phenomenon of "ploidy," mentioned last week when we looked at our local Plains Fleabane, which is a "polyploid." Polyploids are organisms with extra sets of chromosomes. Very rarely when sex germs are formed, regular meiosis fails to split the genes' chromosome pairs, resulting in a sex germ carrying double the number of chromosomes it should. The embryo formed from that sex germ then has "too many" chromosomes, and the individual developing from that embryo may be unable to mate with individuals of the parent's species, because of the mismatch in chromosome number. However, it can mate with other polyploid individuals produced by the same process. In other words, through ploidy, very rarely a whole new species can arise.
Often polyploids are more robust than their parents. D.A. Lavine in a 2000 paper in Journal of Ecology, entitled "The origin, demise, and expansion of plant species," suggests that over 70% of all flowering plants have ancestors who arose through ploidy. Ploidy is less common among animals than plants.
We didn't know all this when I was in college. Having this new-to-me information profoundly colors how I think of the natural world. Though since I was a kid Nature has awed me with its diversity and knack for "doing things with flair," this new information heightens those insights. More vividly than ever now I see how fervently Nature ("The Creator") spews all kinds of life into the Universe, sometimes even instantaneously creating species, and She is so pleased with her creations that She honors such hardly visible, debatably alive entities as retroviruses with fossilization, their own evolution, and the ability to sicken or even kill humans, as they do when they cause HIV-AIDS and certain cancers.
Lately our nights here shimmer with moonlight and insect calls, cricket tintinnabulations oceanic like the moonlight itself, and I lie with my nose in a window crack as nippy, explosively fresh air drifts in, tree silhouettes outside, deer and armadillos moving about, and I think of retrovirus fossils and ploidy. I visualize all around me a Creation effervescent with old and new things, everything forever changing, so little of it understood, really nothing absolutely understood, and what a wonder that within all this surging blossoming beginning with the Big Bang, now, here, there's a little sanctuary where I can look around and take account and wonder about it all, where I can lie with a cold nose in a window crack as crickets sing. .