Blending In

In nature, it often pays to blend in to your background, especially if you’re a prey species like the deer mice used in this study. (Image Credit: David Cappaert, CC BY 2.0)

Linking a mutation to survival in wild mice (2018) Barret et al. Science, 363, p. 499-504.

The Crux

A big part of ecological studies involves investigating how certain traits or behaviors work (adapted) or don’t work (maladapted) in a specific environment, while scientists who study genetics may investigate specific parts of the DNA that are under selection for specific values of a given trait. Surprisingly, not many studies investigate these two aspects of natural selection simultaneously, instead they will attribute selection to a specific trait value without knowing the genetic mechanisms behind it.

The authors of this study used a well-studied model system of deer mice (Peromyscus maniculatus) to link these two aspects of ecology together, tying a mutation in a gene that codes for coat color into selection in the wild. The study took place in the Sand Hills of Nebraska, a relatively young region (in geological terms) where these mice are expected to have recently adapted to the environment due to strong selection for traits that promote their survival.

What They Did

The authors collected 481 mice from both “dark” and “light” habitats. The dark habitats are ancestral, it is the kind of habitat where these mice originally came from, while the light habitat is a recent habitat for the mice. The dark and light mice were then introduced in equal proportions into field enclosures, which were either the ancestral dark or derived light habitats. These enclosures kept out terrestrial predators, but allowed avian predation (hawks, owls, etc.) which would impose the chief selective pressure.

They were interested in two things: how the populations changed over time (did they get lighter, darker, or stay the same on average?) and if a specific gene or genes were under selection.

Did You Know: Selection vs. Evolution

Even people who don’t study science have heard the terms “natural selection” and “evolution”, but what some don’t know is that these are not the same thing. Natural selection is the process by which the environment that an organism lives in exerts some kind of “pressure” on the organisms; promoting survival for organisms that have traits or behaviors beneficial to that environment, while organisms that have traits or behaviors that increase their chances of being killed or dying are selected against.

Evolution, on the other hand, is the process by which a population of individuals changes over time in response to natural selection. This is defined by a change in allele frequencies, or how much the average genetic makeup of a population changes. When a population evolves in response to natural selection, the next generation (or generations, if the selective pressure remains constant) will have a higher proportion of individuals with genes that promote survival in that environment.

What They Found

In both the light and dark enclosures, mice that had a coat color that matched their habitat had higher survival than the mice with the opposite coat color. This was attributed to how the mice with the “wrong” coat color stood out against the ground, making it easier for their avian predators to see and catch them. On the genetics side, the authors found that mice in the dark enclosure had significant changes in allele frequency towards lighter coat color, indicating that natural selection is operating on this specific trait in the dark enclosures. There was no indication that selection was operating on this allele in the light enclosures.

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Predators like this Red-Tailed Hawk exert strong selective pressures on their prey, but other important drivers of selection are parasites, competitors, and even the environment itself (Image Credit: Keenan Adams, CC BY 2.0)

Problems?

This paper has gotten a lot of attention for showing evolution in real time, and the authors themselves state that they have shown “the process of evolution by natural selection”. While they did show natural selection in real time, and supplement the phenotypic data with corresponding genetic data, this was not an example of evolution.

As mentioned in the box above, evolution involves a change in allele frequencies in a population over time following natural selection. Basically, if the next generation has a higher percentage of alleles that promote survival in their environment, they are evolving to be more adapted. This study showed changes in coat color demographics and allele frequencies over time, but all of it happened within one generation. Individuals cannot evolve, populations do. This is a great example of natural selection against indivuals, but it is NOT evolution of populations.

So What?

Studies such as this one are immensely important for the scientific community. They link multiple aspects of an organisms biology, genetics, and ecology to tell a great story about how a mutation is allowing this species to survive in habitats where it would normally be at an huge disadvantage. It takes a lot of work to collect all of this data, much more than what was shown in this paper, but having a dedicated research program focusing on one model system can not only tell the researchers a lot about that system in particular, but about certain aspects of ecology in general.

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