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© 2020 NoWorriesBiology All rights reserved.
WORKSHEET FOR THE VIDEO: JOSHUA TREES AND EVOLUTION
Topic: Evolution
Notes/Vocabulary:
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This worksheet accompanies the video “Joshua Trees and Evolution”, which is available for free on YouTube. (https://youtu.be/mibrOhcuqDM)
1. Joshua trees (Yucca brevifolia) belong to the group of monocotyledons (monocots). Complete the comparison of monocots and dicots in the following table.
flowering plants
Monocotyledons (Monocots) Dicotyledons (Dicots)
# of species living today approximately 60 000 species approximately 200 000 species
Age up to 150 million years multiple groups evolved independently at
different times
Characteristics seedlings possess one embryonic leaf (cotyledon)
seedlings possess two embryonic leaves (cotyledons)
Monocot Seedling (onion)
Dicot Seedling (tomato)
Examples:
lilies, tulips, orchids, grasses, Joshua trees, …
tomatoes, roses, sun flowers, peas, …
© 2020 NoWorriesBiology All rights reserved.
WORKSHEET FOR THE VIDEO: JOSHUA TREES AND EVOLUTION
Topic: Evolution
Notes/Vocabulary:
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2. While Joshua trees inhabited a significantly larger area in the past, today they can only be found in small patches in the southwestern US. Explain which factors contributed to the shrinkage of the Joshua tree habitat.
The climate in the southwestern United States has
changed since the Pleistocene. The previously much more
humid climate allowed the Joshua trees to grow in places
where they can no longer occur due to the lack of water.
In addition, increasing competition is another reason why
the distribution of the Joshua trees has shrunk
considerably. A number of plant species that compete with
the Joshua trees today didn’t exist in the Pleistocene, or
they weren’t as numerous as they are today.
Distribution of Joshua Trees Picture altered from: https://commons.wikimedia.org/wiki/File:US_map_-_states-ca.png
public domain
3. Give a description of what the term ‘coevolution’ refers to.
Coevolution is a process during which two or more species undergo interlinked evolutionary change. These changes or
adaptations causes these species to become more specialized in their mutual interactions.
4. At the time signature 3:30, the video „Joshua Trees and Evolution“ contains a brief intermission. It asks you to pause
the video for a few minutes to work on this task: Form a hypothesis on how the relationship between Yucca moths and Joshua trees might have evolved.
[different answers possible]
assumed distribution of Joshua trees during Pleistocene
occurrence of Joshua trees today
© 2020 NoWorriesBiology All rights reserved.
WORKSHEET FOR THE VIDEO: JOSHUA TREES AND EVOLUTION
Topic: Evolution
Notes/Vocabulary:
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5. Describe and explain the hypothesis on the course of coevolution between Yucca moths and Joshua trees presented in the video.
The hypothesis presented in the video assumes that there was a time when the flowers of the ancestors of our Joshua
trees today were less specialized than they are today. This enabled them to be pollinated by a number of different
insects, which placed their eggs into the flowers of the Joshua trees. Just like the larvae of the yucca moth, the larvae
of these other insects fed on the seeds of the Joshua trees.
Joshua trees that had somewhat narrower flowers than others had an evolutionary advantage, as their flowers were
accessible to fewer insects due to their narrower structure. With fewer eggs deposited into their flowers, a smaller
number of their seeds got eaten by insect larvae. As a result, they had more offspring and could pass on the
characteristic of the tighter flower structure to the next generation more often.
At the same time, the insects competed with each other for flowers to lay eggs into. The ancestors of today's Yucca
moths that were able to use flowers that were inaccessible to other insects due to their narrow structure had an
evolutionary advantage. Features such as a lean body shape or a longer ovipositor (organ for laying eggs) increased the
fitness of these individuals because they increased the likelihood of successful reproduction.
In successive generations, the ancestors of today's yucca moths had the advantage that their physical characteristics
gave them exclusive access to Joshua tree flowers to deposit their eggs into.
In summary, the ancestors of today's Yucca moths and Joshua trees changed in adaptation to each other creating an
exclusive relationship between them.
© 2020 NoWorriesBiology All rights reserved.
WORKSHEET FOR THE VIDEO: JOSHUA TREES AND EVOLUTION
Topic: Evolution
Notes/Vocabulary:
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6. Give reasons why the exclusive relationship between Yucca moths and Joshua trees also poses risks for their survival.
The specialization of these two species on one another that arose from their coevolution poses a risk, because
both species are now dependent on one another. Neither species can reproduce without the other. The Joshua
tree can only be pollinated by the Yucca moths, and the larvae of the Yucca moth need the seeds of the Joshua
tree for feeding.
If one of the two species goes extinct, the other species can no longer reproduce and will, therefore, perish too.