New Mexico over the past one or two million years. There are two
color varieties of the rock pocket mouse, a sandy-colored variety
that lives predominantly on sandy-colored rock and a black variety
that lives predominantly on black basalt (Figure 2). The difference
in color has been linked to variation in a single gene, the melano-
cortin-1 receptor gene (Mc1r), which is one of several genes
involved in the synthesis of pigments in the melanocytes, specialized pigment-producing skin cells (Hoekstra & Nachman, 2003).
The wild-type, sandy-colored mice produce more pheomelanin, a
lighter-colored pigment, than eumelanin, the darker pigment.
Mutations in the Mc1r gene result in the production of more eumelanin in the black mice.
The mutated form of Mc1r that produces black mice is maladaptive if the mice live in the sandy-colored desert, but it provides an advantage if the mice happen to live on the black rock.
The mice are eaten by various predators, including owls, foxes,
and coyotes, that rely predominantly on sight to detect their prey.
Mice whose fur color does not match the substrate are at a considerable disadvantage when it comes to avoiding predators. Predators will eat either sandy-colored or black mice, but given the
conditions on the lava flow, black mice are more likely to avoid
predators; while on the sandy-colored desert, sandy-colored mice
are more likely to avoid predators. Mutations occur randomly,
providing the ultimate source of genetic variation. But more
importantly, genetic recombination (random assortment and
crossing over) shuffle the existing variant alleles in different combinations, resulting in the observed phenotypic variations. Natural
selection then preserves those phenotypic variations that are
advantageous. Once the lava flows had produced the black rock,
a population of predominantly black mice could have evolved
from an ancestral sandy-colored population in < 100 generations
(HHMI BioInteractive, 2005).
An HHMI BioInteractive activity suitable for middle school or
high school provides students the opportunity to analyze data and
use it as evidence to construct an explanation for the two color
varieties in the rock pocket mouse (HHMI BioInteractive,
2015a). Students first watch a short film that tells the story of
selection and adaptation in the rock pocket mouse (HHMI BioIn-
teractive, 2011). After the video, they are presented with four sets
of images like the one shown in Figure 3 and asked to place them
in a logical sequence based on information from the video. The
sets of images are snapshots showing rock pocket mouse popula-
tions at two locations over four different times. Location A is a
sandy desert and remains so through all four snapshots. Location
B starts out as a sandy desert but changes to dark black following
a lava flow. Students sequence the images based on data collected
by counting the number of mice of each color variety at each loca-
tion and using information from the video. Location A has pre-
dominantly sandy-colored mice and there is little change across
all four snapshots. However, when the snapshots are sequenced
correctly, Location B starts out with predominantly sandy-colored
mice (10 sandy, 2 black) but ends up with predominantly black
mice (10 black, 2 sandy).
After sequencing the images, counting the mice, and recording
the data in a table, students construct graphs like those shown in
Figure 4. Working in small groups using the I2 strategy on their
graphs, students first identify the trends they see, then write “What
I see” statements and “What it means” statements. Figure 5 provides examples of what students might write as they annotate the
Once students have completed their analysis of the graphs, a
whole-class discussion gives them the opportunity to share what
they’ve found and argue for a particular interpretation of the evidence until the class arrives at some agreement on the relevant
Figure 2. Used with permission from the Howard Hughes
Medical Institute, © 2005.Allrightsreserved.ht tps://www.hhmi.
org/biointeractive. Original source: Nachman et al. (2003).
Figure 3. Used with permission from the Howard Hughes
Medical Institute, © 2015.Allrightsreserved.ht tps://www.hhmi.