November 28, 2017 at 12:47AM

Today I learned: 1) Tardigrades (water bears) aren't a species -- they're a freaking *phyla* with over a thousand known species. 2) There are a lot of models of evolution out there. Today I learned a bit about three classes of evolutionary model -- additive, multiplicative, and stickbreaking. These different model classes have to do with the effects of multiple mutations on an organism's fitness. It's relatively easy to understand what a single mutation might do to fitness -- if it's good, then fitness goes up, and if it's bad, fitness goes down, and if it doesn't affect the organism's ability to reproduce at all, fitness stays the same. But what if you have *two* mutations with a fitness effect? How do you combine the effects? Do you add their individual fitnesses together? Do you multiply them together? Do you write out a table where every pair of possible mutations has a unique fitness with no particular pattern? In additive models, as you might guess, the fitness effects of multiple mutations are added together to get the total fitness effect of the mutations. That means that a mutation has a property like "this mutation increases fitness by 2". In multiplicative models, the fitness effects of multiple mutations are multiplied together to get the total fitness effect of the mutations. That means that a mutation has a property like "this mutation doubles fitness". The two models are actually quite similar -- they're identical to within an exponential transformation, so you can always do the math in whatever model you want and then transform the results to see what happens in the other model. Moreover, as fitness effects become smaller and smaller, the additive and multiplicative models become identical. For continuous evolution with extremely fine-grained fitness differences, they're the same. I'm not sure if the same is true for the stickbreaking model -- it's a *little* bit different. In the stickbreaking model, each mutation moves you a fixed *fraction* of the distance from the current fitness to the maximum fitness, so each mutation has a property like "this mutation moves you halfway to the best possible fitness". This model has the somewhat different property that it makes fitness converge to a maximum, which may be a more realistic representation of physical constraints. What's the "best" model to use in an evolutionary simulation or analysis? That's still up for some debate. The Lenski long-term evolution experiment* has some relevant data -- as of a few years ago, there were several phenotypic traits for which fitness could be tracked as the population evolved. The majority of those fit the stickbreaking model best, but there were clear examples of additive and multiplicative processes as well. It sems that it depends very much on the mutations. * If you don't know about the Lenski experiment, I highly recommend looking it up. It's one of the most well-known and, I think, envied experiments in Biology. 3) There's too much naval cargo shipping capacity in the world right now! According to FreightHub's 2017 report on global shipping capacity (http://ift.tt/2k3h0ZR), about 10% of the world's shipping ships are sitting idle. This is not a new phenomena, and persists despite widespread ship-scrapping. Another fun fact -- there are between 2,000 and 3,000 known cargo ships in 2017.