A mutation may produce a phenotype with a beneficial effect on fitness.
How might a beneficial mutation affect a population over several generations?
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Statements such as “organisms evolve in response to a change in an environment” are quite common, but such statements can lead to two types of misunderstandings.
This passage explains that there is often a misconception within the theory of evolution that individual organisms evolve intentionally. Firstly, evolution is a process that affects populations, not individuals. Secondly, evolution occurs by natural selection, where a change in the environment causes favorable phenotypes within a population for that environment to benefit from the change. Individuals with advantageous traits are then more likely to survive, reproduce, and pass those traits on. Therefore, neither the environment itself nor the individual is causing those changes intentionally.
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As explained in Determining Evolutionary Relationships, when similar characteristics occur because of environmental constraints and not due to a close evolutionary relationship, it is an analogy or homoplasy. For example, insects use wings to fly like bats and birds, but the wing structure and embryonic origin are completely different. These are analogous structures (Figure 20.8). On the other side, the bird and bat wings are homologous because the bones are inherited from a common ancestor, while the wings themselves are analogous as they evolved independently.
This passage explains the difference between homologous and analogous traits in evolution. Similar characteristics in different organisms do not always indicate an evolutionary relationship. When similar characteristics arise from a common ancestor, they are called homologous structures. However, when similar characteristics arise due to environmental pressures, they are called analogous structures. A structure can be homologous in its origin but analogous in its function if the adaptation occurred independently. For example, in bird and bat wings, although the bones are homologous because they are inherited from a common ancestor, the wings themselves are analogous because the ability to fly evolved independently in each lineage.
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