Welcome to week 5 of Science Friday! Last week, we talked about the battery, a marvel of technology which is ubiquitous in the 21st century. This week, let’s talk about the theory of natural selection by evolution. Evolution, being the central tenet of biology, is not a topic I can cover entirely in one week. The theory of evolution by natural selection is perhaps the most poorly understood scientific phenomenon, right next to climate change. Much of the controversy that this has brewed is a direct result of this poor understanding. This week, I hope to clear up some misconceptions. Charles Darwin proposed the theory of evolution by natural selection in the mid-ninteenth century. He made his conclusions about the nature of change in organisms by making keen observations of different animals around the world, especially in how they differed from one another. Darwin came up with two basic facts: one, species are not immutable, that is, they are subject to change over time. Second, the principal mechanism by which species change is [b]natural selection[/b]. But what is natural selection? Natural selection is the differential contribution to future generations by various genetic types. In other words, it is a description of the observation that individuals with certain traits survive better than individuals with different traits. Natural selection acts on the level of the phenotype, the physical characteristic, but the process of evolution is contingent upon the genotype, the alleles that code for the trait. A population is said to be [i]evolving[/i] if individuals survive and reproduce at different rates as a result of differences in genotypes and phenotypes. This is crucially important. This means that [b]only populations evolve[/b], individuals [b]do not evolve[/b]. As a result of natural selection, [i]populations[/i] develop [b]adaptations[/b], which are traits that suit their physical environment better. Adaptations, however, can only develop as incremental changes to the existing gene pool. This is one of the important constraints of evolution to understand. Let’s take a look at a concrete example. Giraffes, at some point in the past, likely did not all have extremely long necks. It is most probable that, in a given population of giraffes, the neck length varied just as human-height may vary. Over the course of many generations, it probably became more advantageous to have longer necks. Longer necks most likely meant easier access to food. As a result, giraffes with longer necks in the population tended to survive better and reproduce more successfully than giraffes with shorter necks. They, in turn, produced offspring more rapidly than giraffes with shorter necks. This type of natural selection is called [b]directional selection[/b]—the selection of one extreme trait over the average trait or other extreme trait (i.e., normal necks and short necks). Three other types of selection include stabilizing, disruptive, and sexual selection. The evolutionary change exhibited by giraffe populations likely took many millennia to manifest itself. Evolution is not necessarily fast, but it is, without a doubt, a fact of nature. We observe it in the way harmful bacteria develop resistances to our antibiotics; we see it in how one species can branch off into many more (stay tuned!); and we see it in the vibrant ecosystems of plant life. Amazingly, natural selection is just one of many mechanisms by which evolution operates. Mutations, genetic drift, gene flow, and nonrandom mating also play smaller, although still pivotal, roles in the change in composition of the tremendous diversity of life we see on Earth. I hope you enjoyed this week of Science Friday. You can, as always, check out my previous postings by clicking on the #sciencefriday tag on this thread. Note, you will have to change the filter settings there to see all threads. If you have any questions or comments about my content or format here, please do not hesitate to comment. Next week, I will most likely continue to talk about evolution, but move on to the discussion of speciation—how common ancestors may branch off into different independent species.