Last week I went with a group of postdocs and grad students to teach a lesson on Phylogenetics to local high school freshman. I had the foresight to do a pre-assessment and post-assessment (those results will be coming soon), but I wanted to start by sharing how this experience made me think about how we introduce the concept of evolution.
During one of the breaks, I spoke with the students' teacher. I told him how, to me, one of the neatest things about studying evolution is understanding the tremendous effects of genetic drift. It amazes me that so much of the natural variation we observe within and across species is due simply to stochastic processes in the population. Selection doesn't need to enter the picture. It does, of course, most popularly through positive selection acting to increase the frequency of beneficial alleles, and also (and perhaps more often) through purifying selection acting to remove deleterious alleles, or through balancing selection to maintain a balance of alleles that might be harmful under some conditions and helpful under others. And, all of this natural selection can affect the frequency of linked neutral alleles.
But, a large proportion of natural variation doesn't result from natural selection. It can just accumulate and drift to high frequency or fixation through neutral processes. This could be as populations separate geographically from each other, or if one population experiences a severe reduction in size, or any number of scenarios that change the history of the population. How cool is that?!
Later the teacher went through their evolution lesson. I am, first of all, very excited that they have a whole unit on evolution. They spend several classes introducing the concept of natural selection, giving examples of different island populations of lizards adapting to their new environments, and learn to build phylogenetic trees using the physical features of the lizards, and then analyze DNA sequences from the lizards. Cool!
But, the whole lesson focused on the small part of evolution that is positive natural selection. Sure, positive selection is the cool kid on the block, but I think it would be very instructive, and perhaps even more convincing to also introduce purifying selection (because, hey, there are a lot of sequence/functions/features conserved across any chosen clade), and the awesomeness that is neutral evolution (because otherwise we're training students to
look for zebras see function everywhere they look).
I guess I shouldn't be surprised by this when, at the NESCent Catalysis meeting today, very qualified evolutionary biologists suggested that one of the primary topics journalists should know about science is natural selection, and then gave a detailed example of positive selection acting on a population.
Maybe natural selection is a good place to start to introduce evolution. It is tangible, easy to understand, and, there are very accessible examples of positive selection. But, there is so much more to evolution that positive selection. I hope educators can see the importance of reaching beyond positive selection.