Evolution of the Beach Mouse

(Peromyscus polionotus) is a rodent that lives in the South West United States. It has a great many predators including weasels, herons, owls, snakes, foxes, domestic cats and dogs.

In order to avoid these predators it lies up in its burrow during the day and usually comes out to forage for food at night. It is most active foraging for food on moonless or cloudy nights.

oldfield mouse with brown fur walks on a log

The Oldfield mouse has brown fur on its upper body with an underside that is pure white.

It is well camouflaged in its natural habitat which includes anywhere where the soil is brown such as hedgerows, corn fields, woods (‘timber tracts’) and cotton fields.

Being well camouflaged can be a great advantage!

Liu Boilin camouflaged against backdrop of Great Wall of China

Evolution from ‘Oldfield’ to ‘Beach’ mouse

We will now consider events of 6000 years ago when, among the population of brown furred Oldfield mice, was born a few mice with lighter colored upper coats. 

Oldfield mice were all ‘meant’ to have brown coats but by complete chance a ‘genetic accident’ led to some individuals being born with lighter colored coats.

6000 years ago world sea levels rose quite dramatically following the end of the last ‘Ice Age’.Sand dunes on what is now called the ‘Florida Panhandle’ became cut off from the mainland forming so called ‘barrier islands’. You can see one of these ‘barrier islands’, the Santa Rosa Barrier Island, in the satellite image below:

santa rosa barrier island from space

 The arrow shows the location of the Santa Rosa Barrier Island on this image of the South West United States:

satellite image of the Florida Panhandle with arrow indicating santa rosa barrier Island

After the rise in sea levels both dark and light furred Oldfield mice became marooned on these white sand ‘barrier islands’,unable to return to their natural brown soil habitats. There was no escape!

Natural selection in action-what happened next?

How did the survival rates of brown and white colored mice compare in this new sandy habitat in which both brown and lighter colored mice were forced to live?

If you play this educational game from the Natural History Museum you will see ‘natural selection’ in action! The game uses green and brown ‘bugs’ instead of mice with brown and lighter colored fur -but you can extrapolate the results to fit in with what would have happened 6000 years ago to the populations of ‘darker’ and ‘lighter’ colored Oldfield mice marooned together on barrier islands.

  • How many green bugs can you spot near the beginning of the game?

Evolution Game showing more blue bugs than green bugs surviving

  • How many green bugs can you spot at the end of the game? What has happened to all the blue bugs?

Natural History Museum game showing far more green bugs than blue bugs

So what is ‘natural selection’?

The minority of Oldfield mice born on Santa Rosa Barrier Island with ‘light’ pigmentation genes stood a greater chance of not being seen and eaten by predators. Many more of the mice with ‘light pigmentation genes’ survived and reproduced and were therefore able to pass their own ‘light pigmentation genes’ onto their offspring.

Beach mouse scuttling across white sand

The gene which creates lighter colored coats in mice (mc1r) is the identical gene that creates people with ginger colored hair!

An Uyghur girl in Kashgar, China's Xinjiang region, with red hair

In the context of living on a sandy barrier island, the genes which lead to some mice have white coats are ‘good’ genes. They are ‘good’ genes because they lead to higher rates of survival and reproduction.

The evolutionary pressure is for each successive generation to produce lighter and lighter colored coats until all the mice living on barrier islands are well camouflaged and living in total harmony with their environments.

beach mouse watching for predators sitting on top of  burrow

In contrast ‘dark pigmentation’ genes for mice living on barrier islands are ‘bad’. Darker colored mice stand less chance of surviving and reproducing.

Oldfield and Beach mouse side by side so their different coloured coats can be compared

Random mutations in context

Accidental mutations, like a change of coat color, occur regardless of whether the mutation is useful or not. Sometimes mutations are useful and sometimes they are not. It is ‘natural selection’ that determines whether a mutation is useful or not.

With regards to the Beach mice it is the predators, the birds and snakes, that ‘select’ which coat color predominates. That is ‘natural selection’ in action! It is the eating habits of predators that has led to the evolution of the lighter colored coats!

  • If an Oldfield mouse living in a ‘brown soil’ habitat were born with the ‘light pigmentation gene’, would this be a ‘good’ or a ‘bad’ genetic mutation?

Oldfield and Beach mice today

Oldfield mice are very numerous and can be found over a wide geographical area in the South West United States.

In contrast Beach mice are critically endangered and found on only very few barrier islands, nearly all in Florida. You can see all the sub species of beach mouse in this diagram.

The Santa Rosa Beach mouse has the lightest color of all the sub species.

map showing geographical location of different beach micembe.oxfordjournals.org

The Beach mouse is critically endangered as a result of a variety of factors. These include humans building houses in their habitats, results of predation in populations that are already critically low and the ravaging effects of tropical hurricanes and storms.

This image shows the effects on a section of Santa Rosa Barrier Island following a hurricane. Notice the sea water flowing across the island.

Section of Santa Rosa Barrier Island taken from the air showing seawater flowing across the island after hurricane Ivan

Fortunately large numbers of Beach mice are now being protected on Barrier Island National Parks on which human development is forbidden. The outlook for many of these sub species looks ok..for now!

Meme showing Beach mouse poking nose out of burrow with words 'protect our habitats, no building houses on barrier islands

What is a ‘gene’ and how do ‘genetic mutations’ occur? (additional information )

This is where it starts to get a little bit (too) technical! Remember in the introductory article we discussed how nearly all cells in a living organism have 46 DNA molecules in their nuclei?

A ‘segment’ or ‘sequence’ of each DNA molecule is called a ‘gene’.

double helix dna molecule with gene sequence shown

Changes in the sequence of DNA occur as mistakes (mutations) when the DNA molecules are copied and reproduced. Copying and reproduction of DNA occurs when cells are reproduced at the time when the mouse foetus is developing inside a doe’s (female mouse’s) womb.

  • Compare these two ‘identical’ DNA molecules. Can you see a ‘genetic mutation’ in a ‘base pair’ of ‘nucleotides’ in the molecule on the right hand side? Apart from this alteration of one ‘base pair’ both molecules are identical. The molecule on the right is a ‘mutation’ of the one on the left.

Double helix dna molecule compared, showing before and after mutation has occurred

Fun Stuff

Beach mouse attacks a crab at night!

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