August 22, 2016 at 04:36AM

Today I Learned: 1) In evolutionary biology, a commonly-calculated number is N_e, or the effective size of a population. N_e comes from the fact that most of the math in evolutionary biology is based on models of "ideal populations" of organisms, with properties like random and independent assortment, constant population size, and others. Real populations, however, are rarely ideal. For example, many species have males and females in unequal number, or aren't monogamous, which means that some individuals may not have a chance to reproduce. That means that the number of breeding individuals is less than the total population size, so all the math has to be adjusted. In this example, and many others, you can simply calculate an "effective population size", or N_e, and plug it into all the existing math for ideal populations (in this case, N_e is the number of breeding individuals). 2) Speaking of effective population size, today I learned that when talking about *bacterial* evolution, the effective population size is quite, quite different than the actual population size. This basically has to do with linked mutations... say a bacteria happens to acquire a mutation that's slightly beneficial, by random mutation. Unfortunately, there's a good chance that it also happened to pick up another random mutation somewhere else in its genome, and *that* mutation is likely to be bad for it (because many, many mutations are). Now this bacteria, and its descendants, have *two* mutations, and the net effect of those on fitness is probably dominated by the deleterious mutation. Unfortunately, that means that this bacteria's genome will be selected against, *even though it has a novel beneficial mutation*. This is the problem of linkage, and in eukaryotes, linkage is ameliorated by recombination -- sex is really good at shuffling around alleles, so at a population level, any particular mutation quickly gets tested against a relatively diverse genetic background, which lets beneficial mutations accumulate more easily (specifically, with a lower required benefit threshold). Bacteria, however, largely lack recombination, so only mutations with particularly large benefit (about 10^-7 higher probability of survival, it turns out -- still small, but much larger than a naive analysis of bacterial genetics would predict). This can be corrected for by adjusting the effective population size (N_e) of a population of bacteria. 3) Whale sharks reproduce continuously, not discretely. Whale shark reproduction is pretty mysterious, but from what we know, it appears that a whale shark female mates once, then uses the sperm from that mating to slowly, continuously fertilize its eggs over the rest of its lifetime. The young develop inside the mother's body, hatching free when they can survive on their own.