The Origin of Species: The Beak of the Finch
Film Guide
Educator Materials
Current Galápagos finch species tend to keep to themselves, but some are still capable of exchanging genes. In
rare instances, the Grants have shown that a cactus finch might mate with a medium ground finch on Daphne
Major and produce fertile offspring. This might happen, for example, when a male finch learns the wrong song
from a neighbor. Hybrids can survive and reproduce under some circumstances, specifically if there is a rich
supply of intermediate-size seeds suitable for birds with intermediate-size beaks.
• In an upper level biology course, students may learn about allopatric, parapatric, and sympatric speciation.
These are the three major mechanisms for speciation described in textbooks. Allopatric speciation takes place
when two populations are completely geographically isolated from each other and no gene flow occurs
between them. Genetic changes in one population are never shared with the other population. When the
founding population of seed-eating birds arrived on the early-forming Galápagos Islands, it became isolated
from its mainland population. However, allopatric speciation does not explain the origin of all new species.
For example, in parapatric speciation, genes occasional flow between two populations that have adjacent
geographic ranges. Genes may still flow intermittently between populations of finches on nearby islands, but
it is rare. In sympatric speciation, gene flow is common because the two populations live in the same
geographic area but occupy different parts of the same habitat. When speciation occurs in sympatric
populations, one or more traits that are responsible for interbreeding are selected against. Such traits may
include ones that affect appearance, the ability to tolerate certain environmental extremes, or the timing of
reproduction for the two populations (e.g., spring or summer). In the case of the Galápagos finches, most
researchers think that species originally formed following geographic isolation (or in allopatry). They then
remained distinct species primarily due to behavioral isolating mechanisms that kept members of different
species separate if and when they came into contact.
• The film’s narrator explains that a species is defined as a group of organisms whose members do not breed
with members of other groups. This is often called the biological species concept, first introduced by
evolutionary biologist Ernst Mayr, and is the definition most often taught to students. This concept
emphasizes reproductive isolation. However, because the ability to mate and produce hybrid offspring is
often difficult to test, biologists focus on size, shape, and color to group individuals into distinct species.
• The narrator of the film reveals that the average beak depth of the medium ground finches increased by over
4% from the time the 1977 drought began until after it ended. This change is nearly undetectable to the
human eye. However, stress to students that for a bird who is already close to starving and whose only means
to get food is to pick up seeds, maneuver them so they are positioned correctly, and crack them open, even a
4% larger beak may mean the difference between life and death. Perhaps have students imagine that their
only route to survival is being able to pick up a basketball by palming it with one hand. Fingers that are 4% to
6% longer than average could be quite an advantage. You might also consider doing a nut-cracking
demonstration with a couple of different-sized pliers to show how even small amounts of mechanical
advantage can have a huge impact. Also point out to students that 4% is an average number. Individuals with
beaks that are 1% smaller are at a small disadvantage, whereas those with beaks 8% or 10% smaller are at a
strong disadvantage. Survival is probabilistic; the probability of survival for birds with beaks that are 10% or
more smaller than the average is extremely low, whereas for birds with beak sizes near the average the
probability is high and even higher for those with above-average beak sizes.
• Students might be confused as to whether the changes in beak size described as a result of the drought
provide an example of speciation. The medium ground finches that survived the drought and their offspring,
which inherited larger beaks, are still all medium ground finches. These changes in beak size represent
adaptations in response to natural selection. If changes like these occur over thousands of generations and
for different traits, including traits involved in reproductive isolation, then a population might be on the path
to becoming a new species.