Many people assume that natural selection depends on something outside the plant in the environment affecting the plant, but is that true? Wood is always inside the plant. It is never in contact with the environment. Can the phenotype of wood affect a plant’s fitness? Its survival (Figure 17-9)? If a mutation caused a plant to not be able to make either vessels or tracheids, just fibers, in its wood, do you think that would affect the tree’s fitness compared with all of the other trees in the population?

Figure 17-9

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(A) (B)