The term polymorphism is used when there are two obviously different groups of a species that exist while still being the same species. This apparent separation of different phenotypes usually occurs when there is a sudden environmental change. Random mutations that may have existed as a trace populous could increase while the latter decreases, as in the case of industrial melanism.
Industrial melanism, simply put, is melanism induced by industry. Melanin is a black pigmented protein, so melanism is the occurrence of this black pigment in species. The most common example of this is the peppered moth example.
In regions of industry, melanism occurred in populations of nearby peppered moths, meaning more moths were found to be black in color rather than speckled and gray. Black residues from industries covered the bark of trees, making them black in color. This black color was not helpful to the peppered moth population, because it disallowed the peppered moths to be camouflaged, or blended in with their surroundings, and so the frequency at which they were eaten by predators increased dramatically. The population of the oddball moths that were melanic, however, had a dramatic increase in their population, because they were then camouflaged with their surroundings.
In recent years, the population ratio has been turning around; this can be explained by increased efficiency in industries, industries that are less pollutant to the environment.
Dr. Nachman's research group collected rock pocket mice at four sites to determine the genetic basis of melanism, a change in the coloration of mice resulting in dark mice. Two of these sites, Armendaris and Pinacate, had patches of lava rock on which dark colored mice lived. The paper "The genetic basis of adaptive melanism in pocket mice" discusses the genetic changes in the melanocortin-1-receptor gene in the Pinacate mice which leads to melanism.
Three sites in Arizona and one in New Mexico were used for specimen collection. The lava flows were furthest from each other with light mice captured on light substrate at two locations in between, Avra Valley and Portal.
The Pinacate Biosphere Reserve is six hundred square miles residing largely in northwest Mexico and consisting of lava formations, volcanic cones, and the Sonoran Desert (1). Pinacate is composed largely of circular Maar craters, which are craters formed by an explosion caused by the contact of rising lava with groundwater (2). The site at which specimens were collected was near to Yuma, Arizona.
The Pedro Armendaris lava field was formed 760,000 years ago and covers approximately 200,000 acres (3). The lava field exists in the western part of New Mexico.
Specimens were taken off lava flows in Armendaris and Pinacate. They were also taken approximately three kilometers from the site on the lava flow.
Genetics