June 15, 2016 at 10:56PM

Today I Learned: 1) Slime molds are... not a real thing. Specifically, they're not a real taxonomic group. More to the point, they are no longer considered fungi! Instead, the different species of slime mold are classified as different clades of non-fungus, non-animal, non-plant eukaryote (the "group" commonly known as "protists"). This is huge news, as far as I'm concerned. 2) Photons in sunlight carry 2 or 3 electron volts of energy. 3) ...a new way of thinking about aging, as a biological process. A reasonable way (though not the only reasonable way) to define "aging" is as an increase in rate of death and/or decrease in rate of fertility over time in an organism under ideal conditions (plenty of food, good environment, no predators or diseases, etc). Under this operational definition, many species actually stop aging after they've lived long enough. Many insects, for example, become more prone to death over time for most of their lives, but after a certain point, they stop becoming more death-prone. They still die, and at a high rate, but the *rate* of death doesn't increase. They effectively stop aging. Humans may do this too, by the way. I'm not sure exactly what the aging cutoff is, but mortality in the very elderly stops increasing at a certain age. Unfortunately, it's really hard to tell if that's "real" or just because of some cohort effect or selection process or other, secondary cause. It was curiosity about this point that prompted the aging experiments with insects that showed that they stop aging. There are also species that don't age at all, as far as we can tell. Many, but not all, species of hydra maintain a mostly-constant rate of death their whole lives, as determined by some relatively famous and quite lengthy hydra-rearing experiments. A key point is that every hydra that reproduces asexually, by budding or splitting, doesn't age. All of the ones that reproduce sexually age. Why would this be? One plausible explanation is that in sexually-reproducing species, there is inherently less selection pressure against traits that only pop up in older organisms. Consider, for example, a gene variant that is 100% lethal. If it is expressed at birth, then it will be 100% selected against, because every organism that gets that gene will die before reproducing. If it is only expressed well after the organism reaches sexual maturity, then there will be less pressure against it, because the organism may have already reproduced several times before the effects of the gene took hold. By contrast, in a species that splits symmetrically (or, to a lesser degree, in a species that buds), there is less difference in selection pressures for traits that pop up at different ages because there aren't really "children", just copies of the parent. In any case, it seems that aging is, in general, not inevitable. It is not even normal, in the grand scheme of things. It is a temporary period of declining health and fertility experienced by sexually-reproducing organisms.