Using Local Street Trees
to Teach the Concept
of Common Ancestry
by Yael Wyner
M
ost students do not notice the trees that they walk
past, much less appreciate them as examples of the
biodiversity that surrounds them. Trees that line
sidewalks and fill public spaces provide a local context
for student learning about biodiversity and evolution,
even in an urban setting. This article describes a portion of a newly developed curriculum that encourages
middle school students to explore the trees around
them and develop an understanding of their evolutionary history. The curriculum, piloted by 15 New York
City middle school teachers, is appropriate for use in
an urban or a nonurban context. The full curriculum is
freely available online (see Resource).
USING LOCAL STREET TREES TO TEACH THE CONCEPT OF COMMON ANCESTRY
Curriculum goals
The goal of this curriculum is to help
middle school students learn about the
street trees they see daily and use their
new knowledge of local trees to further
their understanding of the common ancestry of all trees. Although students
walk past trees and other plants every
day, many cannot identify these plants.
Implementing this curriculum helps students develop observation skills to:
FIGURE 2
A portion of a worksheet used to familiarize
students with characteristics useful for tree
identification
•notice and identify local trees;
•group local trees based on relatedness;
• understand that related taxa in a
group are similar because they share
characteristics resulting from common ancestr y, or shared evolutionary history;
•understand evolutionar y constraint, meaning that organisms
cannot change characteristics based
simply on need;
• learn that most street trees have
flowers and fruits, even if students
do not notice them; and
• learn that flowers become fruits and that flowers and fruits are reproductive structures for
fertilization and seed dispersal.
FIGURE 1
20
The Leafsnap app
To mirror the life cycle of local trees, the curriculum is divided into a 14-lesson fall unit and an eightlesson spring unit. The fall curriculum focuses on
identification, grouping, common ancestry, and fruits.
The spring curriculum focuses on grouping, common
ancestry, and flowers.
The curriculum moves students from being everyday observers of local trees to observing trees scientifically. When they finish the curriculum, students are
able to recognize variations among tree species, such
as leaf shape and arrangement and different fruit and
flower structures. During curricular activities, students
use their new observation skills to collect data to use
as evidence to identify and group trees by relatedness.
These curricular activities help students learn what it
means to be related, from an evolutionary perspective.
Students learn that related trees are similar because
they share traits inherited over generations from a
common ancestor that lived long ago.
Framing biodiversity in the patterns of evolution
is an important concept highlighted in the Next Generation Science Standards (NGSS) (NGSS Lead States
2013). Charles Darwin contributed two major themes
to our understanding of evolution: the unity of life and
natural selection, a mechanism for the evolution of life’s
diversity. An almost exclusive emphasis on natural se-
USING LOCAL STREET TREES TO TEACH THE CONCEPT OF COMMON ANCESTRY
lection has led most students to have only a limited understanding of the relatedness of all life (Catley 2006).
To counteract this deficiency, the NGSS incorporate
a new emphasis on the unity of life through common
ancestry (see p. 26 for full standards alignment). This
curriculum is an attempt to take some of the basic
themes of common ancestry, relatedness, and shared
history to the middle grades, a place where these principles are largely absent (Catley 2006).
experience, about 20% of students can identify a maple
or an oak based on leaves or fruit. Students are not,
however, able to identify specific species such as a red
maple or a pin oak.
Building student understanding of naming,
grouping, and common ancestry
Before students can contextualize common ancestry
using local tree diversity, they must first understand
how to name and group trees by relatedness. They
start by building vocabulary for a set of tree characteristics that differentiate leaf types (simple, lobed, compound), leaf margins (smooth or serrated), leaf shape
(e.g., oval, oblong), leaf arrangements (alternate or opposite), and fruit type (e.g., acorn, samara). Students
become familiar with these characteristics through
participation in a 40-minute activity in which they clas-
Curricular activities
To implement the curriculum’s activities that focus on
student understanding of naming, grouping, and common ancestry, students use a paper field guide or a freely available iPad app called Leafsnap, which is an electronic field guide that uses pattern-recognition software
(Figure 1). Like paper field guides, Leafsnap includes
images of tree leaves, leaf arrangements,
bark, fruits, and flowers. The app is optimized for the East Coast of the United
FIGURE
States, but students in other locations or
who cannot access the technology can
use paper field guides instead.
3
The teacher’s version of the tree-identification
hypothesis worksheet
Engaging students in local trees
and assessing student prior
knowledge
To introduce this unit, display a picture
of a tree that grows in front of your
school and ask students to name its
species. They will most likely not know.
This presents a good segue into discussing plant blindness, the tendency to see
plants only as part of a setting and not as
living organisms themselves. To learn
more about students’ thinking, ask them
what they notice about trees. We found
that prior to the curricular activities, almost all students noticed evolutionarily
uninformative differences among trees,
such as size, shape, and sometimes the
animals that live in trees. We found that
students did not notice evolutionarily
informative characteristics, such as leaf
arrangement and fruits. To determine
the ability of students to identify trees
based on leaf and fruit characteristics,
ask them to identify maples and oaks
using pictures of their leaves and fruits
(pictures are available from the online
curriculum; see Resource). From my
F e b r u a r y 2 0 16
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USING LOCAL STREET TREES TO TEACH THE CONCEPT OF COMMON ANCESTRY
sify plant parts arrayed around the classroom. Students
must match plant parts arrayed around the classroom
to written and visual depictions of these parts (e.g.,
opposite leaf arrangement is written and shown in a
line drawing) to build knowledge of key plant features
without overloading students with the new, technical
vocabulary (Figure 2). The visual cues are important
scaffolding techniques to support students, especially
those with special needs.
To implement this plant-part classification activity,
diverse plant parts must be collected immediately prior
to the activity. Teachers can prepare by scouting the local area for different leaf shapes, margins, and arrangements (e.g., maple leaves are lobed and opposite, buckeye leaves are compound, magnolia leaves are simple
and alternate). Fruits begin to appear in the spring and
grow on trees throughout the summer and fall. Collecting fruits opportunistically during the course of the day
is a simple and stress-free way to amass an extensive
collection. Most tree fruits are easily stored at room
temperature and, to avoid mold, perishable fruits can
be stored in the freezer. Avoid hickory nuts and black
walnuts if you have students who are allergic to nuts in
your classroom.
• a signal to listen and be quiet;
• a clear boundary for where students are allowed to
be—this is particularly important in an urban setting, in which students may not be in an enclosed
schoolyard;
• instructions to avoid areas with undergrowth that
may contain poison ivy;
• rules against climbing trees;
• precautions to avoid triggering student pollen allergies; and
• a limit on how many leaves students can extract
from a tree—students should learn about trees,
not damage them.
FIGURE 4
Naming
I think it’s cool knowing the name
of a tree. Because I’m not really into
nature, I’ve never really been a nature
person so I don’t know a lot about that
really. Now I know something about
trees and I guess it’s pretty cool just
walking by and seeing a tree that I
would know the name of.
—Elijah, sixth grade
After students complete the plant-part classification activity, they will have enough
familiarity with tree characteristics to
begin tree identification. They can now
go outdoors for a 40-minute class period,
taking with them a paper field guide or
the Leafsnap app to name street, park, or
schoolyard trees. Leafsnap and traditional
field guides support student learning by
requiring students to consider multiple
candidate species in order to identify their
trees. Students, working in pairs, compare
pictures of tree parts to actual trees, reflecting upon tree characteristics and using them as evidence to support or reject
the tree-identification hypotheses they de-
22
velop (Figure 3). Pairs discuss their observations with
each other and arrive at conclusions based on the tree
evidence.
Safety note: Making outdoor activities safe, productive, and fun requires preparation. To invest students
in the rules for a safe outdoor experience, ask them to
brainstorm a list of rules. They may consider:
A portion of a worksheet asking students to
determine whether the redbud is a legume
USING LOCAL STREET TREES TO TEACH THE CONCEPT OF COMMON ANCESTRY
Finally, I recommend that students know their tasks
before leaving the classroom. It is much easier to listen
and ask questions in the classroom than outdoors.
“We walked all the way down the street talking
about the trees and ‘Leafsnapping’ them, and then
on the way back, I could point to one and they
would yell out the name, so proud that they knew—
they could recognize something. … It was a great
moment in sixth grade, just the enthusiasm that
they showed that they had never shown towards
trees probably in their whole life.”
—a sixth-grade science teacher
Grouping by relatedness
After students gain a basic familiarity with trees and
tree characteristics from the naming activities, they
FIGURE 5
use this information to define tree groups based on relatedness. In a set of worksheets (available online; see
Resource) students explore three questions (Figure 4):
• Is the willow oak a willow or an oak?
• Is the London plane a maple?
• Is the redbud a legume?
An efficient way to implement this activity is to have
students work in groups of three, with one student completing each worksheet. When the worksheets are completed, students can share their results with the rest of
the group. Alternatively, to facilitate in-depth discussion,
students can also complete the activities in pairs. Another differentiation strategy is to ask students to complete
at least one worksheet in an allotted amount of time. Students who complete the first worksheet can complete
additional worksheets for extra credit.
Students use descendant circles to predict the appearance of the most recent common
ancestors until they determine the appearance of First Circle, the common ancestor to all circles
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USING LOCAL STREET TREES TO TEACH THE CONCEPT OF COMMON ANCESTRY
FIGURE 6
FIGURE 7
24
Students revisit their previous analysis of the
legume group to predict that the common ancestor
to legumes had alternate leaves and pod fruits
What does it mean to be
related? Common ancestry
The prior activities help students
develop a sense of the characteristics that are useful for determining
related groups, but they still do not
understand what “related” means or
why some characteristics are more
meaningful for grouping than others.
Students begin this unit not knowing why related organisms are similar. To understand the evolutionary
underpinnings of relatedness, they
must learn that related organisms
look similar because they share a
common ancestor from a long time
ago. That common ancestor passed
down the characteristics that related
Four of the 24 tree cards that students must sort into ash, buckeye, legume, maple, oak,
and willow groups
USING LOCAL STREET TREES TO TEACH THE CONCEPT OF COMMON ANCESTRY
FIGURE 8
A portion of the worksheet asking students to apply what
they learned about common ancestry in plants to animals
organisms share today. These characteristics are inherited from parent to offspring over generations, just
as characteristics of people in the same family are inherited.
Students begin this 40-minute lesson by thinking
about inheritance in their own, friends’, and celebrities’ families (e.g., I have my father’s nose, and he
inherited his nose from his mother). Photographs of
celebrity families, which can be found online through
the search term “celebrity parents and children,” are
most useful for classroom discussion. For example,
a photo of Will Smith with his son Jaden makes it
obvious that Jaden inherited his ears from his father.
Probing questions lead students to recognize that
Will Smith must have inherited them from a parent
and that Jaden and Will share a common ancestor,
Jaden’s grandparents. To develop this idea further,
students use the appearance of fictional, oddly shaped
circles to predict the appearance of the circles’ common ancestor, “First Circle”(Figure 5) (to facilitate
differentiation, three versions of the worksheet, with
varying levels of difficulty, are available online at the
full curriculum website; see Resources). Students
then apply what they learned in the context of the
fictional circles to the tree groups they determined
in the “grouping by relatedness” activity. For example, students previously determined that redbuds are
legumes, because like all legumes, they have seed
pods and an alternate leaf arrangement. Students can
now use this knowledge to predict that the common
ancestor to legumes had pods and alternate leaves
(Figure 6).
Bringing grouping and
common ancestry
together
Once students learn about
the role of inheritance in
common ancestry and the
role of common ancestry in
related organisms’ shared
characteristics, students apply the concept of common
ancestry to making groups
of most closely related trees
in a sorting activity. Students
in groups of six sort cards
of different tree species into
groups based on common
ancestry (Figure 7) (student
group size can vary depending on how many versions
of tree cards you make, but
larger groups may be too big to involve every student;
cards can be laminated for reuse). Each tree card
shows the leaves, leaf arrangement, fruit, and a picture
of the whole tree that students can use for grouping.
Students must choose which characteristics to use to
make their groups. They then must argue for why they
made their specific groupings. My students became
invested in and enthusiastic about using evidence to
justify their claims to each other.
Once students learn about tree flowers, they participate in a similar sorting activity using cards that also
include flowers. Again, students use evidence to justify
their groups, and they use the characteristics present
in today’s plant groups to predict the appearance of the
common ancestor of each group. Students then apply
this same technique to determine the appearance of
the common ancestor of animal groups such as vertebrates (Figure 8).
Conclusion
The described activities show how teachers can create an evolutionary context for understanding trees
that students see daily. The aim is for students to notice the unobserved trees that surround them and to
begin to think about these trees and their characteristics as evidence for the relatedness of all life. Pilot testing revealed that middle school students enjoy learning about local trees and applying their observational
skills. Understanding of the common ancestry of local
flora engenders students’ appreciation for the continuity and unity of all life on Earth. ■
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USING LOCAL STREET TREES TO TEACH THE CONCEPT OF COMMON ANCESTRY
Acknowledgments
Thanks to Janice Koch for helping envision and design the
curriculum and for help with the manuscript, Sarah Seiter
and Jessica Genter for developing and revising curricular elements, J. Doherty for analyzing data to determine how students think about trees, and all the teachers who piloted the
curriculum in their classrooms. This project is funded by the
National Science Foundation (DRL-1221188). Any views are
those of the author and not the NSF.
References
Catley, K. 2006. Darwin’s missing link: A novel paradigm for
evolution education. Science Education 90 (5): 767–83.
National Governors Association Center for Best Practices and
Council of Chief State School Officers (NGAC and CCSSO).
2010. Common core state standards. Washington, DC:
NGAC and CCSSO.
NGSS Lead States. 2013. Next Generation Science Standards:
For states, by states. Washington, DC: National Academies
Press. www.nextgenscience.org/next-generation-sciencestandards.
Resource
Unifying Life curriculum—www.ccny.cuny.edu/education/
unifying_life_site
Yael Wyner ([email protected]) is an associate professor at the City College of New York
in New York, New York. She previously taught
environmental science for seven years at Hunter
College High School, a school for gifted learners
in New York City.
Connecting to the Next Generation Science Standards (NGSS Lead States 2013)
Standard
MS-LS4: Biological Evolution: Unity and Diversity
www.nextgenscience.org/msls4-biological-evolution-unity-diversity
Performance Expectation
The materials, lessons, and activities outlined in this article are part of a set of activities designed to reach these
performance expectations:
MS-LS4-2: Apply scientific ideas to construct an explanation for the anatomical similarities and differences among
modern organisms and between modern and fossil organisms to infer evolutionary relationships.
Dimension
Name or NGSS code/citation
Matching student task or question
taken directly from the activity
Science and
Engineering Practices
Analyzing and Interpreting Data
Students are required to use evidence to
identify their tree. They must use careful
observation to develop a tree identification
hypothesis. They then must compare their
tree characteristics to their field guides to
support or reject their hypothesis. They
must support and explain the rationale
for their tree-identification claim based
on their tabulated leaf, fruit, and flower
evidence (Figure 3).
Engaging in Argument from Evidence
26
Disciplinary Core Idea
LS4.A: Evidence of Common Ancestry and
Diversity
• Anatomical similarities and differences
between various organisms living today
and between them and organisms in the
fossil record, enable the reconstruction of
evolutionary history and the inference of
lines of evolutionary descent.
Which trees belong together and what
did their common ancestor look like (e.g.,
Figures 4, 6, and 7)?
Crosscutting Concept
Patterns
Sorting tree cards to make evolutionary
groups based on plant characteristics
(Figure 7).