A, T, G, C, M & M

 How is the theory of neutral evolution different from the theory of evolution by natural selection? How are they the same?

What's up, science people?! Who wants to talk about ~neutral evolution~?! 

...Yeah, me neither. But I don't make the assignments. Let's get into it. 

Neutral evolution is a big fancy way to describe a weirdly specific situation. Basically, genes mutate all the time. A lot of those mutations don't actually affect anything beyond the DNA sequence. These harmless or "silent" mutations occasionally become fixed into the genome - and they do or do not become fixed at a relatively constant rate. 

This sounds a lot like evolution by natural selection. But it can't be natural selection, since the mutations are fixating at random (and not because they're more or less advantageous). It's evolution done by drift. 

That's a lot of science jargon. I'm gonna translate it into M&M jargon instead. 

Let's say someone is putting five M&Ms of the same color in your bowl every minute. Sometimes they're all red, or all green, etc. But you're also eating a random assortment of 5 every minute. By chance, you might end up with way more green M&Ms over time. It doesn't matter to you, because they all taste the same; you just happened to not grab a bunch of green ones. That's neutral genetic drift. 

Now if you do this experiment over and over again eventually you'll wind up with a bowl full of every M&M color. That's fixation of neutral mutations by genetic drift. It doesn't matter to you that the colors have changed, that doesn't affect how you eat them, but they've still changed. And you'll get solid-colored bowls at roughly a constant rate, with some inputs (those 5 identical M&Ms) sticking around and others not sticking around. Since who stays and who goes is random, it happens at a constant pace. 

Do this experiment enough times, and you should see fixation for each color of M&M at roughly equal rates. 

Now pretend that there are only 4 types of M&Ms, and they're called A, T, C, and G. 

Boom. Neutral evolution via genetic drift. 

The theory of evolution by natural selection is almost identical to the M&M scenario described above. But this time, instead of eating a random 5, you target M&Ms of a certain color. Maybe you're trying to eat all the green ones, or you're trying to eat ever color but the green ones. Either way, since the change is no longer random, it's going to reach "fixation" (elimination of one or most colors) much faster. 

In the real world, this difference between randomly eating 5 M&Ms and choosing a color to target is analogous to neutral evolution and evolution by natural selection, but it can also be pictured as silent and non-silent genotypic change. Remember, natural selection can only act on phenotype; it needs to see the color of the M&Ms its eat. But neutral evolution works even on mutations that don't cause a phenotypic change; thus, it doesn't need to see the color of M&Ms. So while both cause change, one cares about the phenotype and works towards a fitter organism, while the other doesn't care about the phenotype and fixes alleles at random.  

Strangely, because drift fixes alleles randomly, it actually does so at a predictable rate. We can't tell which mutations will be fixed, but we know that some changes will stick around purely by chance. You can think of it as you doing the M&M experiment with your friend. 

Let's say you and your friend work together for five bowls. Then they get greedy and start doing their own experiment for five more bowls. If this scenario is replicating neutral evolution, you'll both be fixating into solid-color bowls at about the same rate. You could then look back on the number of bowls and their colors to determine how long ago you guys separated. If you have five sets of differently colored bowls, that means you separated five bowl-generations ago - five ticks of the metaphorical "clock". 

That's how we use neutral evolution to determine temporal distance between lineages. 

Of course, we're talking candy here, so the analogy isn't perfect. Real-world analysis doesn't care about the "color" of the mutation so much that it happens at a relatively constant rate. If you already know two lineages are related, you can tally up the number of changes between them to get a pretty good idea of how long ago they separated and started neutrally evolving on their own. Scientists are also super smart and know how long it takes for one mutation to fixate, allowing them to literally estimate the years between two lineages. Which is a lot more applicable than counting in bowl-time, let me tell you. 

So go share some M&Ms! Talk to your friends about neat science concepts! Pavlov train them to associate your science ramblings with candy! 

Now if you'll excuse me, I'm going to go ask my roommate to proofread this and thank her with candy.