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Ch. 25 - Phylogenies and the History of Life
Chapter 24, Problem 12

The vast majority of animals that ever existed are now extinct, but Tereza Jezkova and John Wiens wondered which variables were most important in driving the diversification of species that exist today. Why are there so many species in some phyla, such as Cnidaria (see photo), but so few in others, such as Ctenophora? Based on your reading of this chapter, propose at least five traits that you think might have been most important in triggering diversification within phyla (examples: origin of hearing, origin of internal fertilization).

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1. The first step is to understand the concept of diversification within phyla. Diversification refers to the process by which a species evolves into two or more different forms. These forms then co-exist and continue to evolve independently.
2. Next, consider the traits that could potentially trigger diversification. These traits should be significant enough to cause a species to evolve into different forms. For example, the origin of hearing could have allowed some species to better adapt to their environment, leading to diversification.
3. Another trait to consider is the origin of internal fertilization. This could have given some species a reproductive advantage, leading to diversification. Other traits could include the development of different feeding strategies, the ability to live in various habitats, or the evolution of different defense mechanisms.
4. Now, apply these considerations to the specific phyla mentioned in the question. For example, Cnidaria includes species like jellyfish and corals, which have a wide range of forms and habitats. This could be due to a variety of traits, such as the ability to sting prey or predators, the ability to live in both deep and shallow water, or the ability to form symbiotic relationships with other organisms.
5. On the other hand, Ctenophora, also known as comb jellies, are less diverse. This could be due to a lack of significant traits that would trigger diversification. For example, they might not have developed as many different feeding strategies or defense mechanisms as Cnidaria.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Species Diversification

Species diversification refers to the process by which new species arise and existing species evolve over time, often influenced by environmental factors, ecological niches, and evolutionary pressures. This concept is crucial for understanding why certain groups of organisms, like Cnidaria, exhibit a high number of species, while others, like Ctenophora, have fewer. Factors such as reproductive strategies, habitat diversity, and adaptive traits play significant roles in this process.
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Phylogenetic Relationships

Phylogenetic relationships illustrate the evolutionary connections between different species and groups, often represented in a tree-like diagram. Understanding these relationships helps explain why some phyla have more species than others, as it reveals how closely related species have diversified from common ancestors. The evolutionary history of a group can indicate the potential for diversification based on shared traits and adaptations.
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Adaptive Traits

Adaptive traits are characteristics that enhance an organism's ability to survive and reproduce in its environment. These traits can lead to increased diversification as species adapt to different ecological niches. Examples include the development of specialized feeding mechanisms, reproductive strategies, or defense mechanisms, which can significantly influence the success and proliferation of species within a phylum.
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Related Practice
Textbook Question

Use the fossil evidence shown in Figure 25.6 to determine whether flight evolved earlier in insects or in birds. Is flight an example of homology or convergent evolution? Explain.

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Textbook Question

Coral reefs are biodiverse ecosystems that provide food, income, coastal protection, and many other services to millions of people. Yet coral reefs are under threat from human impacts such as climate change—many corals cannot tolerate the warming ocean water and have suffered massive 'bleaching' events, which can be fatal. Researchers have discovered that some corals have the capacity to acclimatize to warmer water, while other corals do not. The researchers fear that because corals are long-lived colonial animals and thus evolve slowly, they may not be able to adapt to global warming fast enough to avoid extinction. Explain how the difference between acclimatize and adapt is important to the fate of corals.

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Textbook Question

The vast majority of animals that ever existed are now extinct, but Tereza Jezkova and John Wiens wondered which variables were most important in driving the diversification of species that exist today. Why are there so many species in some phyla, such as Cnidaria (see photo), but so few in others, such as Ctenophora? Draw a horizontal axis to represent the number of species within phyla using a logarithmic scale (1, 10, 100, 1000 species, etc.). Then use Table 30.1 to map seven representative phyla from small to large at intervals of about an order of magnitude on this scale.

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Textbook Question

The vast majority of animals that ever existed are now extinct, but Tereza Jezkova and John Wiens wondered which variables were most important in driving the diversification of species that exist today. Why are there so many species in some phyla, such as Cnidaria (see photo), but so few in others, such as Ctenophora? Jezkova and Wiens used a type of graph called a linear regression to find correlations between variables such as the proportion of species per phylum with legs (on the y-axis) and the diversification rate per phylum (on the x-axis). Sketch a graph to show what a strong positive correlation between these two variables would look like and what the absence of a correlation would look like.

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Textbook Question

The vast majority of animals that ever existed are now extinct, but Tereza Jezkova and John Wiens wondered which variables were most important in driving the diversification of species that exist today. Why are there so many species in some phyla, such as Cnidaria (see photo), but so few in others, such as Ctenophora? A sample of Jekova and Wiens' results is shown here. The R2 value represents the strength of the correlation (where 0.00 is lowest and 1.00 is highest). The P value represents the statistical significance. Which five traits look most important?

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Textbook Question

Which traits do not correlate strongly with diversification rate within phyla, but are likely to have been important in the original diversification of animal phyla during the Cambrian? Select True or False for each trait. T/F presence of a head T/F mobile lifestyle T/F terrestrial lifestyle T/F bilateral symmetry

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