The Rock Pocket Mouse: A Model for Natural Selection

I thought it would be helpful to use a specific example of natural selection for those having issues understanding how it works. Coat color in pocket mice is one of those examples. The paper I will be working from is found here:

First, let’s look at the environment where these mice are found. They are found in the southwest deserts of the US, and the vast majority of their range is made up of light brown dirt and rocks. However, there are isolated areas where recent volcanic eruptions have created areas with dark black basalt rock. Interestingly, when we look at the distribution of coat color in pocket mice it matches the environment they are found in:

The black mice are found in islands of black basalt rock, and brown mice are found in the sea of brown desert between those islands of volcanic rock. I don’t think it is a stretch to state that camouflage is playing a strong role in the distribution of coat color alleles.

The scientists in the paper also looked at interbreeding between the black and brown mice. Sure enough, there was free intermingling between the populations. However, no black mice are seen in the sea of brown desert spanning the two black volcanic rock islands. They also found that the allele for black fur is dominant, meaning that a mouse only needs one copy of the allele to have black fur.

So why don’t we see an even distribution of black and brown mice across the entire range of these mice? Why do we only see black mice associated with these black volcanic rock islands in the sea of brown desert? Why is there selection for or against specific alleles depending on the background color of the environment they find themselves in?


Many genes have very little impact like the ones you describing with genes that drive color. The purifying selection could be simply a change in the sequence is fatal due to its internal function.

If the sequence were fatal then there wouldn’t be any black mice. There are black mice, and they are healthy. Therefore, the mutation isn’t lethal.

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In this specific case I agree with you.

Wow, I just saw this thread. As I am writing this I am literally sitting in front of my evolutionary biology class. They are taking their second exam. One of the questions on the exam asks them to discuss how this example is an example of natural selection. They have to point out all the things that are necessary for natural selection to take place and relate them to this example. One of their study aids was to watch this wonderful HHMI video about the rock pocket mouse. YouTube


In this specific case, the black mice stick out in the light brown desert, so the allele is selected against in that environment. The brown mice stick out on the black volcanic rocks, so that allele is selected against in that environment. That’s negative selection, and it is dependent on the environment.

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Glad to see that it is being used as a teaching aid. It is one of the better examples I have seen. The most interesting parts is that two black mouse populations appear to have different beneficial mutations in different genes which illustrates the contingent nature of evolution.


Back in the 1980’s I did much of my MS Thesis field work (geology) in the Armendaris Ranch area. I did not notice many pocket mice, but I was very aware of rattlesnakes, particularly the Blacktails (an aggressive type). In areas underlain by gray limestone the snakes were also gray, but the guys living on the Abo and Yeso formations (clastic redbeds) were quite distinctly more reddish. I noticed the same pattern on the horn toads.


As someone who spent a lot of my career immersed in mouse coat-color genetics, the plasticity is truly amazing, as is the applicability to human genetics, particularly neurogenetics, as pigment cells come from the embryonic neural crest.

A corollary of that is that the practice of categorizing humans by their pigmentation is biologically unsupportable.

The paper in the OP does mention this in passing:

For the purposes of this model, there are ~80 possible targets for coat color to evolve. Even within the same gene there are probably numerous different mutations that result in the same phenotype. This is why it is so cool to see different mutations in different black mouse populations, as we would expect if this trait arose independently by random mutations.

And those numbers lead to the estimate that a population of humans with the darkest skin, moved to Scandinavia, would be white in only 40 generations–and vice versa. I have forgotten where I saw that.

The pocket mouse model also allows us to look at the relationship between gene expression and mutations.

Here is the signalling axis for human melanocortin-1-receptor (mc1r):

Mc1r is a hormone receptor that interacts with other molecules in the cell, ultimately resulting in an upregulation of genes responsible for melanin production. Mutations in mc1r have the potential to change gene expression in terms of both basal activity and activity in response to hormones. In the mouse, mc1r is involved in the timing of eumelanin production in hair shafts. The mutations carried by black mice in mc1r upregulates the genes responsible for eumelanin production through the entire cycle of hair growth, resulting in black hair.

If adaptations come about due to changes in gene expression this doesn’t exclude mutations and changes in protein sequence. The pocket mouse models demonstrates this beautifully. Mutations in a hormone binding protein result in changes in gene expression downstream of that receptor in the hormone signalling axis. Depending on the environment specific changes in fitness, these function altering mutations can be selected for or against.