The response is derived from web search results. Evolution by natural selection and intelligent design are two different explanations for the origin and diversity of life on Earth. Here are some of the main differences between them:

- **Evolution by natural selection** is a scientific theory that proposes that living organisms change over time through random genetic mutations and differential reproductive success. Natural selection is the process by which organisms with traits that are better adapted to their environment tend to survive and reproduce more than those with less favorable traits. Evolution by natural selection explains how complex structures and functions can arise gradually from simpler ones, without the need for a designer¹²³.
- **Intelligent design** is a philosophical argument that claims that some features of living organisms are too complex, intricate, or purposeful to have evolved by natural selection. Intelligent design proponents infer that such features must have been designed by an intelligent agent, such as God or an alien. Intelligent design does not offer a testable mechanism for how the design was implemented, nor does it identify the nature or identity of the designer¹²⁴.

Some of the examples that intelligent design proponents use to challenge evolution by natural selection are:

- **The eye**: Intelligent design proponents say that the eye is an irreducibly complex system, meaning that it consists of several interdependent parts that are all necessary for its function. They argue that the eye could not have evolved gradually from simpler structures, because removing any part would render it useless. Therefore, they conclude that the eye must have been created fully formed by a designer¹².
- **The bacterial flagellum**: Intelligent design proponents say that the bacterial flagellum is another irreducibly complex system, meaning that it is a molecular motor that propels bacteria through liquid environments. They argue that the flagellum consists of several protein components that are all essential for its function, and that none of them have any other use in the cell. Therefore, they conclude that the flagellum must have been designed as a whole by a designer¹².

However, evolution by natural selection can provide plausible explanations for how these complex systems evolved from simpler ones, without invoking a designer. For example:

- **The eye**: Evolution by natural selection can explain how the eye evolved gradually from light-sensitive cells to more complex structures with lenses, irises, and nerves. There is evidence of intermediate stages of eye evolution in living organisms, such as flatworms, mollusks, and vertebrates. Moreover, there is no reason to assume that all parts of the eye are indispensable for its function, as some organisms have eyes with fewer or different parts than humans¹²³.
- **The bacterial flagellum**: Evolution by natural selection can explain how the bacterial flagellum evolved gradually from simpler molecular structures that had different functions in the cell. There is evidence that some of the protein components of the flagellum are homologous to those involved in other cellular processes, such as secretion and transport. Moreover, there is no reason to assume that all parts of the flagellum are indispensable for its function, as some bacteria have flagella with fewer or different parts than others¹²³.

Therefore, evolution by natural selection is a more parsimonious, explanatory, and predictive theory than intelligent design for understanding the origin and diversity of life on Earth.

Source:
1. [Evolution vs. Intelligent Design: 6 Bones of Contention](https://www.nationalgeographic.com/culture/article/evolution-intelligent-design)
2. [Evolution - Intelligent design and its critics](https://www.britannica.com/science/evolution-scientific-theory/Intelligent-design-and-its-critics)
3. [Intelligent Design vs. Evolutionary Theory: A Brief Comparison](https://umso.wordpress.com/2008/04/19/intelligent-design-vs-evolutionary-theory-a-brief-comparison/)
4. [The Flaws in Intelligent Design](https://www.americanprogress.org/article/the-flaws-in-intelligent-design/)
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