The echidna is an egg-laying mammal whose body is covered in…

The echidna is an egg-laying mammal whose body is covered in quills for protection from predators.  The porcupine’s body also is covered in quills for protection, although porcupines are not closely related to echidnas and most other mammals do not have quills. What is the best explanation for presence of quills in these two species?

A tsunami kills most of a hermit crab population but there a…

A tsunami kills most of a hermit crab population but there are a few survivors.  Several of them have unusually wide claws on their front legs.  Several generations later, the hermit crab population has recovered but claw width is larger than in the original population.  Which of the following best describes this scenario?