This week, we dive into evolutionary theory. This has been a controversial topic ever since Charles Darwin first published On the Origin of Species in 1859. Early on, some Christians felt there was a possibility of harmony between evolution and the Bible, such as the noted botanist Asa Grey.

Agreeing that plants and animals were produced by Omnipotent fiat does not exclude the idea of natural order and what we call secondary causes. The record of the fiat—“Let the earth bring forth grass, the herb yielding seed,” etc., “let the earth bring forth the living creature after his kind”—seems even to imply them.
Asa Grey (1876)

Other prominent theologians at the time also seemed willing to grapple with the evidence while remaining adamant that a purely naturalistic view of evolution was obviously incompatible with scripture.

‘Evolution’ can in no case be accepted as the formula of all that is; we must in any case rise above it to the higher formula of ‘God’—who is more than evolution, who indeed works in evolution, but also out of it… We would not willingly drag behind the evidence, indeed—nor would we willingly run ahead of it.
— Benjamin Breckinridge Warfield (1895)

Still other theologians were more pointedly critical…

We have thus arrived at the answer to our question, What is Darwinism? It is Atheism. This does not mean, as before said, that Mr. Darwin himself and all who adopt his views are atheists; but it means that his theory is atheistic; that the exclusion of design from nature is, as Dr. Gray says, tantamount to atheism.
— Charles Hodge (1874)

Or as Lewis Berkhof critiqued in his Systematic Theology

Theistic evolution is really a child of embarrassment, which calls God in at periodic intervals to help nature over the chasms that yawn at her feet. It is neither the biblical doctrine of creation, nor a consistent theory of evolution.
— Lewis Berkhof (1932)

Clearly, there was a lot of back and forth within the Christian community on the topic ever since evolution was more formally described by Charles Darwin. Regardless of how the Christian community has responded to this theory over the past 150 years, it behooves us to understand it correctly so that if we critique it, we do so accurately, not ignorantly.

putting the fossils in order

One of the key pieces of evidence for evolution is the fossil record. The mere fact that there are some species which appear in very old fossil layers but not more recent ones (such as dinosaurs) and other fossils which appear in very recent ones but do not appear in older ones (such as modern birds) suggests there has been some significant change in the composition of life on Earth over time.

**The principles of stratigraphy help us understand the relative age of rock layers.**
© 2013 Nature Education All rights reserved.

The first step toward understanding this evidence is having a decent way to estimate a relative date for various fossil layers. That is, we need a way to make an educated guess as to whether a given fossil layer in one region is older or younger than a comparable layer in a different region. The techniques developed for this process are called stratigraphy. The principle assumptions of stratigraphy are:

Original Horizontality: Layers of rock are deposited horizontally at the bottom of a lake.
Superposition: Younger layers are deposited on top of older layers.
Cross-Cutting Relationships: Layers that cut across other layers are younger than the layers they cut through.

Another important step toward estimating the age of a fossil layer is to compare the fossil composition across many different layers. There are some fossils which only seem to appear in a narrow range of layers. These are called “index fossils,” and they can help provide a means of estimating the age of the fossil layers in which they are found.

**An illustration of how index fossils can help provide an estimate of a fossil whose age might otherwise be difficult to determine.**
© 2013 Nature Education All rights reserved.

Looking at this figure, we can see that if you found one of the yellow-colored sea shells in the column on the right in the same fossil layer as one of the red crabs on the left, then you can date that sea shell to be a relatively recent fossil since the crab fossils shown here do not appear in older layers.

The Age of the layers

These two techniques, stratigraphy and the use of index fossils, only help us get a relative date for a fossil layer. They cannot help us sort out the absolute age of a fossil layer, nor do they tell us anything about the overall age of the Earth. In fact, using just these two techniques, it would be possible to piece together a picture in which all of the fossil layers gradually change over the course of just a few thousand years. However, there are a few separate lines of evidence that suggest that is not the case.

The clearest and most commonly cited evidence for estimating the age of a rock is radiometric dating. This technique utilizes a few key assumptions:

Some, special crystals in igneous rock purge certain forms of Lead or Argon from them as they begin to cool from their molten state but allows traces amounts of radioactive Uranium or Potassium to remain trapped in the cooled crystals.
Thus, any of that Lead or Argon found in those crystals must have appeared due to radioactive decay of the Uranium or Potassium.

By measuring the relative amounts of Uranium and Lead or Potassium and Argon, we can estimate how much time has elapsed since the crystal first cooled. This measurement is typically made using a device called a mass spectrometer.

Schematic of a mass spectrometer and a mass spectrum.
**Source**: OpenStax College, *Chemistry*

When using a mass spectrometer, a rock sample (e.g., one of those special crystals) will be extracted and prepared by crushing it into a fine power. After rock powder is inserted into the mass spectrometer, it is vaporized and ionized by heating it and bombarding it with electrons. The two circular plates with a horizontal slit (on the left of the diagram) accelerate the ions using an electric field. The moving ions pass through a magnetic field, which causes their path to curve. The heavy ions are harder to turn, so they follow a broad arc while the lighter ions follow a tighter arc. Finally, the detector at the far right of the schematic is used to count the number of ions which follow a given arc, which corresponds to the number of ions with a given mass.

The graph to the right of the mass spectrometer diagram shows an example mass spectrum illustrating the relative abundance of different isotopes. In this diagram, only different isotopes of Zirconium are shown, but the mass spectrometer is able to measure the relative amount of all kinds of isotopes, including those of Uranium, Lead, Potassium, and Argon.

natural selection

Evolutionary theory is often misunderstood on a number of levels. One common misunderstandings is that evolutionary theory claims that all the biodiversity on Earth is the result of random, effectively chaotic interactions. It is true that modern, “neo-Darwinian” evolution claims that random genetic mutations are responsible for producing gradual changes in a given species. However, this is only part of the picture.

It may be helpful to consider an analogy here. The molecules in the air you’re breathing right now are moving randomly. In fact, that type of “thermal motion” is one of the best examples of randomness we can find in nature. However, each time you exhale a breath, your diaphragm moves upward, applying pressure to your lungs, and that pressure in your lungs causes the air to flow uniformly out of your lungs in a nice, predictable way. In this case, the random motions and collisions of the many air molecules in your lungs experience an organized, average motion from a region of high pressure (inside your lungs) to one of lower pressure (outside your lungs). The claims of evolutionary theory are a bit like that: a random underlying process (random mutations) is organized by external forces (natural selection) to cause the average population of a species to gradually change over time.

The following figures and animations illustrate this process for a pair of species in a predator-prey relationship. In the first case, shown here, I consider just one key characteristic: the speed of the animals. In this case, the fast predators can catch enough prey to eat and survive, and the fast prey can escape from the predators and survive.

**An illustration of two populations: predators and prey.**

In this example. I have shown two colored regions indicating the number of predator and prey with a given speed. The predator indicated at the red dot is fast enough to catch the prey inside the blue box. In my simulation, a predator who randomly encounters a prey slower than them is assumed to catch and eat that prey.

Next, I simulated the predators going on a number of hunts throughout the year and randomly encountering a potential prey. In each instance, if the predator is faster than the prey, then it catches it and eats it.

A simulation of what happens to the predator and prey populations over the course many hunts.

After many hunts, if a predator has not caught enough prey, then it starves and dies. Over the course of the year, the slower prey are killed off, so the slower predators also run out of food and begin to die. The net effect of this is that the faster prey and the faster predators both are more likely to survive. This is “survival of the fittest.” Most people seem to understand and agree with the basics of this since that by itself doesn’t really say anything about the evolutionary history of life on Earth.

Finally, I simulated some kind of preproduction for both the predators and the prey. Since all the slowest animals died, leaving only the faster animals, the typical speed of the animals which reproduce is faster the average we started with at the beginning. If this increase in average speed of the surviving animals is somehow passed along to the next generation, then we would expect the next generation of animals (both predator and prey) to be slightly faster than the previous generation. That process is what is shown in the video here.

A simulation of how natural selection would be expected to impact the average speed of the predator and prey species over many generations.

It is important to note: this is not actually “evolution” in the sense that most people usually mean it. This is just the predators (e.g., lions) and the prey (e.g., gazelles) getting faster over time. This doesn’t cause lions to transform into something other than lions. It just makes a slow group of lions transform into a faster group of lions. They may have slightly longer legs or larger muscles as a result, but typically, this wouldn’t really be considered an adequate account of “the origin of species.” For that we need some mechanism that will cause lions to become something other than lions, which is what we look at next.

How evolution works

To see how natural selection might make it possible to cause one species to diverge into two, we need to consider a similar process to the one above, but imagine at least two key characteristics for the animal (or plant, or fungus, etc.).

**An example of a predator and a prey species with their relative speeds and camouflage (prey) or visual acuity (predator).**

I have illustrated this with the image here, considering both the speed of the animal, and the camouflage (which enables the prey to hide) or visual acuity (which enables the predator to see well hidden prey). The red blob in the center of the image indicates the distribution of predator speeds and visual acuities while the blue blob (partially hidden by the red) shows the distribution of prey speeds and camouflage.

A predator with a speed and visual acuity corresponding to the red dot is fast enough and has good enough sight that it can find catch and eat any of the prey in the blue box.

Once again, I simulated what this would look like if we allowed all of the predators to head out on 50 hunts over the course of a year, randomly encountering prey as they go. For each hunt, if the predator is fast enough and have good enough eyesight, then they catch and kill the prey they encounter. You can see that illustrated in the video below.

A simulation of what happens to a the predator and prey populations over the course of a single generation.

Once again, as the easy prey are killed, the slower predators with poor eyesight run out of food and die. This leaves only the faster predators with better eyesight at the end of the year. However, we can see from the final frames of this animation that the prey have two successful paths for survival: either run faster or hide better. It is this effect that evolutionary theorists claim is responsible for the divergence of one species into two or more.

To help illustrate what this might look like, I produced one final animation illustrating the predator prey relationship over many generations in two geographically disparate regions (Europe and Africa). The point of putting them in different regions is

The species in Europe won’t breed with the species in Africa, so any changes in the average genetics of one group won’t be blurred out by cross-breeding.
The two different groups will experience slightly different environmental pressures.

In this illustration, I have highlighted the possibility that in Europe, the prey may be trying to hide in a densely forested area, so running fast may not be effective (it is too difficult to navigate all the brush and trees), but it may be very advantageous to hide in the bushes.

At the same time, I have considered a case where the species are hunting or being hunted on the open plains of Africa. In this context, there’s not as much hiding to do, so the best strategy is to run as fast as you can! You can see the simulated effects of this in the video below.

A simulation of two different populations experiencing divergent natural selection forces over many generations.

This animation illustrates the basic idea behind evolutionary theory where these naturally occurring pressures can cause two geographically separated populations of a single species to slowly diverge to form two distinct species. The European prey become very good at hiding in the forest while the predators, out of necessity, develop better eyesight. Meanwhile, the African prey become much faster as they attempt to escape from the predators (who are also become faster with each generation).

Summary

I should be clear at this point: I am not intending to affirm that this definitely does happen or that it actually is able to account for all of the biodiversity we find on Earth. I am merely trying to describe the consensus view of modern biologists as accurately as possible.

With that said, I don’t think Christians have much to fear from this one way or the other. I completely agree with many of the theologians up at the top of this page: it is possible that evolutionary theory is compatible with the Bible. However, I also agree with them that to assume that God is in no way involved is tantamount to atheism and not something the Bible can be twisted to affirm.

Regardless of how exactly, these difficult questions get sorted out, the basic point of Genesis remains: there is one God who created everything in heaven and Earth, seen and unseen. Those things which he created should worship him now and forever. In the famous words of St. Francis of Assisi

All creatures of our God and King,
lift up your voice and with us sing,
"Alleluia! Alleluia!"

Amen.

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