Developmental processes constrain evolution because all evolutionary innovations are modifications of previously existing structures. An adaptation can evolve only if the fitness benefits it confers exceed the fitness costs it imposes.
learning outcomes
You should be able to:
Formulate a hypothesis to explain why potentially highly advantageous phenotypes might not evolve.
Describe an example illustrating a macroevolutionary pattern that is not easily explained by micro-
445
The largest living adult insects weigh less than 75 g (less than the weight of a relatively small songbird, such as a starling). Many flying birds are much larger than any insect, so clearly flight alone does not constrain the current size of insects. Formulate a hypothesis for why there are no giant insects presently roaming Earth.
Insects may be physiologically constrained by their system of respiration, which may not be able to support as large a body as that of birds. This is an example of an historical constraint that limits insect body size.
Natural selection results in gradual changes within species over long periods of time. Describe an event that could result in significant and sudden changes across many species simultaneously.
Anything that resulted in sudden, widespread environmental change would be likely to affect many species simultaneously. Examples include a meteorite or comet impact, widespread volcanic activity, or sudden climatic changes.