I came across this excellent article by Jerry Coyne, which is part book review, part defense of natural selection. I recommend it highly. But, in reading the article, I wondered why people are so threatened by natural selection. Because that’s not the philosophically challenging part. Unless you’re a biblical ‘literalist’, the idea of a creator dude who acts through the mechanism of natural selection isn’t too theologically challenging. After all, traits that are beneficial (at least locally and in the short term) increase, while the deleterious ones decrease. Surely, this is the best of all possible worlds.
But what will bend most people’s noodle is the neutral theory, the implications of which are gloriously pessimistic.
The neutral theory was developed to make sense of molecular variation. Importantly, it asks a subtly different question about variation. Typically, when you encounter explanations of natural selection, it’s usually along the lines of: “There are a bunch of grey ones, and a bunch of brown ones. Under the right ecological circumstances, the grey ones replace the brown ones.”
Neutral theory starts from the (obvious) assumption that a novel allele (gene variant) initially occurs as a single copy in a population. So, if we consider a population of 1,000 diploid organisms (critters that, like us, have two copies of every gene), the initial frequency of that novel allele is one in two thousand. If the allele is neutral–does not confer a reproductive or survival advantage–it will become the dominant allele (‘sweep to fixation’) with a probability of one in two thousand (I’m leaving out some math). However, since it is rare, it will most likely go extinct due to genetic drift: the odds of that allele surviving more than a few generations are very slim (in fact, it has a 37% chance of disappearing by the next generation).
In this model, selection acts as a nudge that increases the probability that the now advantageous allele will not disappear. In the same population, if the allele has a two percent fitness advantage, which is quite large for most alleles in natural populations, the advantageous allele will likely disappear…over 96% of the time.
This is not Panglossian at all. But it gets worse. An allele with a weak negative effect on fitness–it lowers fitness–can also sweep to fixation. In the same population, an allele with a cost of 0.1% has a 0.4% chance of ‘beating out’ the ‘good’ allele. That doesn’t sound so bad, until we remember that humans have around 20,000 genes (as do many other eukaryotic organisms).
Let’s throw in one more complication: effective population size. You’ll notice that I stipulated a population size of a thousand. Actually what I meant is the effective population size which considers that not all individuals reproduce, and that the breeding range of populations within a species can be quite small. For example, the effective population size of Finnish wolves is forty (even though the total number of wolves is around 250). To keep things simple, the smaller the population size, the stronger the advantage of a rare allele has to be to overcome removal by random chance (conversely, small population sizes also make it easier to fix ‘bad’ alleles). In other words, the fitness of an allele is contingent on those stupid fucking natural history facts. If volcanic eruptions subdivide a species, for instance, the likelihood that an adaptive allele will become predominant decreases–and has nothing to do with genetics.
So let’s move to the long arc of evolution. Unless selection is really strong, it will take a long time to purge a population of the less fit allele. In the case of an allele with a two percent advantage in a population with an effective size of 1,000, it will take around 1,500 generations to eliminate the less fit allele. That’s a long time for crap to hang around.
1) More fit alleles have a high chance of going extinct.
2) On occasion, less fit alleles will replace more fit alleles.
3) Whether an allele is more fit in part depends on the population size. The fitness of an allele is contingent on those stupid fucking natural history facts.
4) It takes a really long time to purge less fit alleles. Consequently, until Glorious Equilibrium Day Draweth Nigh, populations are full of a lot of genetic flotsam.
But natural selection is supposedly the theologically challenging part?
Nothing I’ve described here is news to evolutionary biologists or geneticists: this is taught in introductory evolution courses. Yet most people are completely unaware of the neutral theory, even though it was a raging debate in population genetics for decades. Here are the potential explanations I came up with for its ‘popular’ absence:
1) Math is hard and icky[/snark]. Probably not good fodder for a newspaper article. On the other hand, this doesn’t explain why it’s not discussed in books.
2) Math is hard and icky, part deux. Maybe science popularizers don’t understand it very well?
3) Perhaps there’s a concern that explaining natural selection is hard enough, and that neutral theory would simply confuse the issue.
4) The scientists who popularize science fall into the ‘selectionist’ camp, and they’re not interested in writing about this.
5) This is a population genetics approach to evolution, and that is underrepresented in popular treatments of evolution.
Other ideas? And how do we make it accessible to non-scientists? Cuz it’s kinda important.