crosscalc

Beach mouse evolution: How mice evolved sand-colored coats

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Table of contents
  1. Two kinds of oldfield mice
  2. The genes behind the pale coat
  3. Mc1r: a receptor that works less well
  4. Agouti: more of a signal that lightens fur
  5. The two genes interact
  6. Same look, different genes
  7. Testing whether color really matters
  8. Rock pocket mice: the same gene, the opposite direction
  9. A worked example: a dominant dark allele
  10. Common questions
  11. Did the mice change color to match the sand?
  12. Is the pale coat caused by one gene?
  13. Do mice that look alike have the same genes?
  14. Are beach mice the same species as lab mice?
  15. Practice
  16. Where the model stops
  17. Related
  18. Sources

Level: High school to college · You’ll need: dominant and recessive alleles and the idea of natural selection · Part of mouse genetics

Beach mouse evolution is one of the clearest cases of natural selection traced all the way to genes. Oldfield mice living on the white sand dunes of Florida’s Gulf Coast have pale, sand-colored coats, while their relatives a few miles inland are dark brown. Researchers found that much of the difference comes from changes in two pigment genes, Mc1r and Agouti, and showed that matching the background protects mice from predators.

These are wild deer mice and their relatives (the genus Peromyscus), not the house mouse used in labs, but the coat genes are the same ones.

Two kinds of oldfield mice

Mainland oldfield miceGulf Coast beach mice
WhereFields and forests inland, on darker soilCoastal dunes and barrier islands, on white quartz sand
CoatDark brown back, light gray bellyPale back with much of the body white
SpeciesPeromyscus polionotusThe same species: several pale beach subspecies

The barrier islands and their dunes formed within roughly the last 6,000 years, so the pale coats evolved quickly. Because the two forms are the same species, they can be crossed in the lab, which is how the genes were found.

The genes behind the pale coat

Mc1r: a receptor that works less well

Mc1r (melanocortin-1 receptor) sits on pigment cells. When it’s active, the cells make dark eumelanin; when it’s weak, they make paler pigment. In the Santa Rosa Island beach mouse, a single DNA change swaps one amino acid in the receptor (arginine to cysteine at position 65), and the receptor works less well.

In lab crosses, this Mc1r change explained about 10% to 36% of the variation in different parts of the color pattern. A big effect, but not the whole story.

Agouti: more of a signal that lightens fur

Agouti makes a protein that blocks Mc1r, pushing pigment cells toward lighter pigment. Beach mice make more Agouti in their skin than mainland mice do. The protein is unchanged; it’s the amount that differs.

The two genes interact

The Mc1r change only shows its lightening effect in mice that also carry the beach form of Agouti. When one gene’s effect depends on another gene, it’s called epistasis. It’s a reminder that “one gene, one trait” is a simplification even for coat color.

Same look, different genes

Beach mice on Florida’s Atlantic coast are pale too, but they don’t carry the Gulf Coast Mc1r change. Their pale coats evolved separately, using different genetic changes. When different populations reach the same solution by different routes, it’s called convergent evolution.

Testing whether color really matters

Does a pale coat actually help a mouse survive? In 2010, researchers put out hundreds of plasticine mouse models, light and dark, in both beach and inland habitats, and counted how many were attacked by predators such as foxes, coyotes, owls and herons.

Models that didn’t match their background were attacked more often, in both habitats: dark models on the pale dunes and pale models on dark inland soil. That’s evidence that predators that hunt by sight select for a coat that matches the background, though the authors note their sample of attacks was small.

Rock pocket mice: the same gene, the opposite direction

Rock pocket mice (Chaetodipus intermedius) live in the deserts of the southwestern United States and Mexico. Most are sandy-colored and live on light rock, but some populations living on black lava flows are almost black.

  • In one lava population in Arizona, the dark coat comes from four changes in Mc1r, the same gene as in beach mice. Early lab tests suggested they make the receptor more active, the opposite of the beach-mouse change.
  • The dark allele is dominant: mice with one copy are as dark as mice with two.
  • A dark population on a different lava flow, in New Mexico, showed no link to Mc1r, so dark coats evolved more than once there too, through different genes.

The rock pocket mouse story is the subject of a widely used free classroom film from HHMI BioInteractive, The Making of the Fittest: Natural Selection and Adaptation.

A worked example: a dominant dark allele

Use D for the dark Mc1r allele in rock pocket mice and d for the light allele. D is dominant.

Dd
DD/D darkD/d dark
dD/d darkd/d light

Two dark heterozygotes (D/d × D/d) are expected to give ¾ dark, ¼ light offspring. Now add selection: on black lava, light mice are spotted and eaten more often, so fewer d/d mice survive to breed, and the D allele becomes more common over generations. On light rock, the opposite happens. Genetics tells you what offspring to expect; selection tells you which ones survive.

Common questions

Did the mice change color to match the sand?

No. Individual mice don’t change. Mice that already matched the background survived and bred more, so their alleles became more common.

Is the pale coat caused by one gene?

No. At least two interacting genes matter in beach mice, Mc1r and Agouti, plus smaller effects from others.

Do mice that look alike have the same genes?

Not necessarily. Atlantic and Gulf Coast beach mice look alike but evolved pale coats through different genetic changes.

Are beach mice the same species as lab mice?

No. Beach mice and deer mice are Peromyscus, rock pocket mice are Chaetodipus, and lab mice are Mus. They’re different animals that share the same pigment genes.

Practice

  1. In rock pocket mice, a dark mouse (D/d) mates with a light mouse (d/d). What fraction of the offspring are expected to be dark?
  2. A dark population on one lava flow carries Mc1r changes; a dark population on another lava flow doesn’t. What does that suggest?
  3. In the plasticine-model experiment, why were models placed in both beach and inland habitats?
Answers
  1. ½. The offspring are ½ D/d (dark) and ½ d/d (light).
  2. Dark coats evolved independently in the two places, through changes in different genes (convergent evolution).
  3. To show that it’s the match with the background that matters, not light or dark color itself. Mismatched models were attacked more in both habitats.

Where the model stops

  • Coat color in wild mice is polygenic. Mc1r and Agouti have large effects, but other genes add smaller ones.
  • Dominance can differ between populations and genes. The dominant dark allele describes one rock pocket mouse population, not every case.

Across species: MC1R and agouti everywhere. MC1R is the gene behind red hair in people and chestnut coats in horses, and the agouti gene decides tabby or solid in cats and makes lethal yellow mice yellow. The cat’s famous orange color is an exception: it comes from a different gene on the X chromosome (see calico and orange cats).

Sources

Try it: work out any cross with the mouse coat color calculator, or a general one with the Punnett square calculator.