© 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.