- Some complex adaptations could not have arisen gradually, like wings or the eye.
- The eye is often invoked as an example of a complex structure which supposedly could not have evolved slowly --- what good is half an eye? But this kind of thinking is wrong-headed. Genetic variation and natural selection do not operate by piecing together parts. Rather, structures can very easily begin simply and end up very complicated. So we do not expect to see half an eye; rather we expect to see primitive eyes, which is, by the way, exactly what we see.
I should also mention that 'gradual' is a relative term. Really, in Evolution Theory it depends on how large the effects that random mutations in the DNA can have. It turns out that small mutations can produce quantum effects; Rennie (2002) gives the following example:
Mutations that arise in the homeobox (Hox) family of development-regulating genes in animals can also have complex effects. Hox genes direct where legs, wings, antennae and body segments should grow. In fruit flies, for instance, the mutation called Antennapedia causes legs to grow where antennae should grow. These abnormal limbs are not functional, but their existence demonstrates that genetic mistakes can produce complex structures, which natural selection can then test for possible uses.
Please return to Empirical Doubts.
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