Students essentially begin their modeling by proposing explanations of the phenomenon of the different climates (here defined by
average precipitation and temperature over a year) in tropical and
temperate regions. Although instructed to focus on temperature,
they also attempt to explain precipitation differences. Students are
required to create a drawing that illustrates their global climate
model. With assistance from instructors using probing questions
to activate students’ thinking, after about 10 minutes of discussions
in groups and using their prior understanding of seasons and climates, students are able to construct a physical representation of
their mental models similar to that shown in Figure 2.
Most student groups correctly deduce that the equator
receives more “direct” sunlight throughout the year, or more sun
energy per unit area, which helps them explain the temperature
patterns noted in Figure 1. In addition, a few students may be
able to explain that heavier rainfall patterns in the tropics are
due to rising warm air. If not, instructors support this understanding through discussion that Earth’s climate is also based on
the location of hot and cold air-mass regions and the atmospheric
circulation created by warm equatorial air and trade winds. Warm
air masses rise at the equator, and this rising air is replaced by
trade winds north of the equator blowing in from the northeast,
and by trade winds south of the equator blowing in from the
southeast. The instructor can detail how the trade winds of the
two hemispheres meet near the equator, while the warm equatorial air rises and cools, causing clouds and rain to develop, creating tropical rainfall conditions near the equator. After these
discussions, alongside their prior basic understanding of evolutionary processes, students have climate-specific understandings
needed to engage with pertinent data and begin to model the
greater biodiversity in the tropics.
The Main Biological Phenomenon of Focus:
Latitudinal Diversity Gradient
Next, students work with data from Fischer’s classic paper on latitudinal variation in species richness. Fischer’s (1960) paper is data-rich
and provides specific data on a wide variety of organisms, including
ants, nesting birds, snakes, and corals (for examples of data included
in this article, see Figure 3). Data can be presented on handouts or
Relying on their global climate model and other relevant prior
knowledge (e.g., regarding natural selection), students make observations about the data and pose possible scientific explanations for
the species diversity. After allowing groups ample time to complete
this work (about 30–45 minutes), the class discusses possible
explanations (models). Students offer a wealth of explanations,
most of which (as they will soon come to realize) the scientific
community has also offered historically. Students’ explanations
mainly match one of the following two (not mutually exclusive)
The “living is easy in the tropics” hypothesis:
• In the tropics, the climate is gentler compared with temperate
• More organisms survive to and through reproductive age.
• More reproduction begets more organisms of the same species,
some with additional random genetic advantages, potentially
conferring additional advantages to their own offspring and
begetting more diversity.
• More surviving organisms (within and between species) means
more competition, which increases selection for niche specialization, begetting more diversity.
• A relatively stable climate makes being a generalist less favorable than being a specialist.
The “more diversity begets more diversity in the tropics” hypothesis (a
“circular” or tautological argument):
• More favorable conditions for the lowest trophic levels directly
influence the diversity of higher trophic levels (e.g., the abundance
of plant species in the tropics – perhaps due to faster recycling of
nutrients or the abundance of water – favors more specialization
on the part of organisms that consume them, thereby lessening
competition between consumers and allowing more consumers
to survive; similar patterns occur with trophic levels up the chain).
• Because of the wider variety of ecosystems than in temperate
regions, there are more niches available in an area of the same size.
With more niches, there is greater capacity to support a higher
diversity of organisms.
• More species exist that are able to crossbreed, which leads to formation of new species. More diversity begets more diversity.
The various models offered by students are summarized, and the
instructor explains that they will be investigated further before the
next activity begins.
Further Progress in Historical Modeling via
Engagement with Primary Literature
Next, students read Fischer’s (1960) seminal work and see how
Fischer identified an ultimate cause of the greater species richness
in the tropics as the intertwined processes of evolution of species
Figure 2. Students’ initial model depicting the different
climates in the tropical and temperate regions of Earth.