I believe pretty strongly in evolution. I’m quite skeptical of people who argue against it. But if you asked me to lay out the strongest case for evolution, until recently… I’d be pretty hard-pressed? I’d probably mumble something about fossils and then get kind of embarrassed.

If I can’t make the case, then why do I believe it so strongly? I think the honest answer is that I find the people who argue for evolution much more credible than the people who argue against it. It seems pretty clear to me that evolution is a widely accepted scientific consensus, and I trust scientific consensus unless I have a really really good reason not to. And yes, I’m sure you can find a good scientist who rejects it – but you can find good scientists with almost any view you want if you look hard enough. Knowing why X is true vs. thinking X is true because a smart person you know believes X is what I’d call “first order knowledge” vs. “second order knowledge”. I think good second order knowledge is a totally valid reason to believe something. Second order knowledge is kind of fascinating in its own right, but let’s leave that for another post.

Historically that’s why I’ve believed in evolution and I think that’s a fine reason, but… I kind of want to be able to make the case myself? I want to be able to make the case for evolution from first principles.

So I read The Greatest Show on Earth in which Dawkins argues that evolution is undeniable (although it’s denied every day, a fact which clearly irritates him to no end). Here’s my attempt at summarizing his 500 page book into a very short blog post so that my future forgetful self can remind himself of the main argument from time to time.

The logical argument

You only need to accept two very plausible sounding premises for evolution to become almost a logical inevitability.

  1. Some amount of random variation is introduced during reproduction.
  2. That variation is hereditable.

If there is variation among individuals within a species, then surely that variation will have some impact, however small, on the likelihood of reproduction. If the variation is hereditable, then the variation which improves the likelihood of reproduction will be passed on more than than the variation that decreases the likelihood of reproduction.

If you let this process iterate for a really really long time – an unfathomably long time – you’ll end up with things as different as birds, sharks, and humans all from a common ancestor.

Evidence that the premises are true

I think the best evidence is that we can directly observe both these premises on human timescales. Consider selective dog breeding, or banana farmers selecting for larger bananas. In a few hundred years you can end up with a drastically different gene pool for dogs or bananas. In those examples, one can observe both the fact that variation is introduced from generation to generation, and that the variation is hereditable via selective breeding.

In the cases where we can directly inspect DNA, we can observe mutations from one generation to the next (presumably random, although that seems hard to prove). And we can observe that those mutations are decently likely to be inherited by the next generation.

Arguments against

It seems exceedingly unlikely

In order to evolve from a single celled organism to a human you need countless mutations, each of which is incredibly unlikely. If you multiply those tiny probabilities together, the probability of this happening is essentially zero.

The counter

First off, the timescale of evolution is hard to fathom.

Second, saying the probability of our particular sequence of mutations is essentially zero feels similar (to me) to drawing a number between 1 and a billion, getting 487223, and saying the probability of getting that particular number was essentially zero. Any time you observe one particular outcome out of an immense set of possibilities, the probability of that particular observation a priori will be very small, but something had to happen. If you draw a number of between 1 and a billion, you will - with 100% probability - observe an event which was only 1 in a billion to happen. There’s a sort of meta perspective which makes it unsurprising to observe such a “surprising” event.

Our particular sequence of mutations was exceedingly unlikely – granted! – but it didn’t have to happen that way. This is made clear by the fact that apes went through a different sequence of mutations, as did sharks, as did mushrooms.

If both humans and whales evolved from the same common ancestor, where are the whale-people? Or where are the whale-people fossils? How come you can’t find any?

The counter

This is just a straight up misunderstanding. Evolution does not predict that there will be species A and B and then every variation in between. What it predicts is that there will be some common ancestor of A and B that lived probably a super long time ago that also probably doesn’t look anything like either A or B does today!

For A and B consider humans and whales. According to google, the last common ancestor of humans and whales was a small land-dwelling shrew-like creature. That’s nothing like a either a human or a whale. It’s not that humans evolved from whales or that whales evolved from humans, it’s that we both evolved from shrews.

So you shouldn’t expect to see - even in the fossil record - some whale-human fossil. You might expect to find something that’s kind of between a whale and a shrew, and something else kind of between a human and a shrew, but even that is probably very oversimplified.

Other interesting ideas from the book

Naming is hard

Biologists like classifying stuff. Animals are in a different “kingdom” than plans or fungi. Within the animal kingdom, animal species are separated into different “phylum”, e.g. spiders are in the “arthropod” phylum and tigers are in the “chordata” phylum.

So what phylum was the most recent common ancestors of spiders and tigers in? It’s really hard to say. It’s probably not even well defined.

I suspect this is NOT true, but let’s say the most common ancestor of tigers and spiders looked almost identical to a spider. Let’s just say it was a spider, species-wise. You trace the lineage between that common ancestor and some particular tiger today. At what point did the species change from spider to tiger? More realistically, that lineage went through many different species, so let’s just ask: what was the first common ancestor that wasn’t a spider?

Isn’t the definition of species “a group of organisms that can reproduce naturally with one another”? Do you think there was ever a particular generation which was so different from the last that it couldn’t interbreed? I suspect not. So… that means that we can create a chain from spider to tiger where every link in the chain should be considered the same species as the last, and yet… we end up with a different species?

The answer is that all this is fuzzier and more continuous that biology class might make it seem sometimes. Classifications are useful in practice but the idea that you can cleanly divide all living things (including common ancestors) into different groups based on the ability to interbreed is just false.

You sculpt the gene pool, not the genes

Dawkins like the analogy of sculpting, because a sculptor – unlike other artists – is mostly in the business of subtracting. A sculptor starts with a solid block of marble and gradually cuts away pieces until a statue remains.

Evolution uses a similar process. It gradually cuts away genes that are not advantageous (via making reproduction less likely) from the gene pool. Any given animal/gene is not directly affected by evolution, but by genes being either more or less likely to be passed on via reproduction, evolution sculpts the gene pool over generations.

Simulating evolution

Dawkins described a pretty simple way to conceptualize evolution in terms of a simple computer program.

Let’s assume we start with a population has a uniform distribution over some attribute - say height. At each time step, we simulate reproduction (asexual, to keep things really simple) and we give a slight reproductive advantage to members of the population with height above some arbitrary threshold. Here is one possible outcome of such a simulation: