Evidence and exasperation

“Isn’t it amazing how everything seems to provide evidence for evolution? The brain shrinks in some form of pygmy homo erectus. Thats evolution! Ancient genes survive millions of years unchanged. That’s evolution?! Women have orgasms. That’s evolution! Although not all women have orgasms and they still manage to reproduce hmm luckily with the right spin…That’s evolution! We live in a civil society with people working for cooperative goals. That’s evolution! Unfortunately some people murder and rape. Just an unfortunate side-effect, but that’s evolution.

“Not only is everything evidence for evolution but evolution explains everything! No its not circular reasoning its Evolution! Thank goodness we don’t need to resort to God to explain the world around. Now we have Evolution! Its the all-encompassing answer to the ultimate question (I always thought it was 42). The evolutionist has reached the omniscient nirvana. maybe we should start meeting at the biology lab on Sunday mornings. We can sing some Evolution Hymns. Do they exist? Don’t worry they’ll evolve. I’ll just start selectively pressing some keys on the organ and type a few letters while blindfolded. Okay I’m getting a little carried away…chalk it up to evolution.” [sic]

I find that in situations like this, it helps to step back for a moment from evolution and look at the other major scientific theories of the past couple centuries that explain a lot about the natural world. You could translate Doug’s complaints about evolution into complaints about any of them.

Take the theory of plate tectonics. According to this theory, the Earth is covered in plates of crust. Each plate grows along one margin with molten rock that rises from the Earth’s interior. The margin on the other side of the plate is cold and sinks down into the interior, where it is remelted and mixed up with the rock down there. Continents ride on top of these plates. In some cases they crash into each other, such as India and Asia, forming mountains. In other cases, a new rift splits a plate apart, pushing continents away, as with Africa and South America.

From the 1920s to 1960s, geologists put together this theory as a way to explain patterns on the Earth. They couldn’t actually see the continents crash into each other like bumper cars, because the process takes millions of years. Instead, they had to develop hypotheses that they could then test by looking at the Earth. For example, they calculated the age of rocks around mid-ocean ridges. The rocks closer to the ridges were younger than the ones further away. Years of studies both in the field and in the lab have strengthened the theory, but they’ve also led scientists to expand it from its original form. The original theory didn’t account for what was driving hot rock up from the interior in the first place, for example. Yet new ideas for these sorts of things do not invalidate the realization that the continents move.

Now imagine a blog about plate tectonics (I wish there was one). The blog is dedicated to new research into how all the dizzying variety of landscapes on the planet, from jagged cliffs to undersea volcanoes, are produced by the Earth’s geological engine. It could even have a few posts about how plate tectonics helps explain some things you might never expect geology to explain, such as why it is that some animals in Africa and South America are surprisingly similar. Answer: their common ancestors lived at a time when the two continents were still joined together.

Imagine the sort of exasperated comments such a blog would get:

“Isn’t it amazing how everything seems to provide evidence for plate tectonics? Continents split apart. That’s plate tectonics! Continents crash into each other. That’s plate tectonics?! Plates sink under other plates. That’s plate tectonics. Although some plates actually slide past each other. That’s plate tectonics. Not only is everything evidence for plate tectonics, but plate tectonics explains everything! No it’s not circular reasoning, it’s plate tectonics! Thank goodness we don’t need to resort to God to explain the world. Now we have plate tectonics!”

Any theory that would explain the Earth’s landscape has to be able to account for a huge variety of features. The same goes for any theory that would explain the Earth’s biological diversity. Just consider fish. There are fish with eyes and fish without. Most fish only swim, but some fish can fly and some can crawl on dry land. A theory that could only shed light on one kind of fish wouldn’t be much of a theory at all.

The theory of evolution explains this variety, but not in an arbitrary way. Fish descend from a common ancestor, and along the way they have been modified, primarily through natural selection, into different forms. Flying fish do not have wings made out of balsa wood. Their wings are actually modified fins. The fins that some fish use to crawl on land are also clearly modified from the fins other fish use to swim. Fish without eyes still retain the genes required to form eyes, but they have been modified so that the eyes never fully develop. If these fish really did evolve from a common ancestor, you’d expect that their DNA would reflect this common kinship. And it does. If these fish really did evolve from a common ancestor, you’d expect that the fossil record would be consistent with their descent. And it is.

As a result, the specific examples that Doug brings up are not circular, but rather are particular cases of well-studied patterns in evolution.

Dwarfing is not an idea that someone came up with when Homo floresiensis was discovered. It’s been documented in many animals. Is there a compelling explanation for how full-sized elephants come to islands and then become the size of cows other than evolution? Let’s hear it.

The genes Doug refers to are the ones found in jellyfish and humans. As animals, we descend from a common ancestor. We have lots of genes in common with jellyfish—genes for building cells, proteins, and DNA, for example. Now it turns out that some body-building genes are also conserved in humans and jellyfish. But these genes are not carbon copies of one another. They have been modified in each lineage, just as you’d expect if life did indeed evolve.

Doug’s example of female orgasms raises another important point: an overarching theory about the history of life or the Earth does not automatically give you all the answers about that history. How did the Andes Mountains form? If a geologist simply says, plate tectonics, that’s not a very satisfying answer. Yes, plate tectonics were involved, but how? It turns out that the best explanation geologists have is a staggeringly complex interplay of continental collision, flowing rivers, and climate change. But the issue is still very much in debate. Orgasms are also an open question, as are the precise evolutionary origins of many things in nature. Natural selection may well turn out not to have much to do with human female orgasms. We’ll see.

If a scientific theory can explain an aspect of the natural world, withstand scrutiny, and lead to important new insights into how the world works, we really shouldn’t hold its success against it. No one’s asking for evolution hymns—certainly no more than they’re asking for gravity hymns or hymns to the periodic table of the elements.