Almost all mammals have seven cervical vertebrae. There is a fitness disadvantage to mammals that have a different number of cervical vertebrae; for example, in humans, fetuses with an abnormal number of cervical vertebrae are often stillborn, and such children are more prone to pediatric cancers. This is an example of
A) convergent evolution.
B) phenotypic plasticity.
C) constraining evolution.
D) pleiotropy

Question

Quizplus · Accepted Answer

This is an example of constraining evolution, where certain evolutionary pathways are limited or "constrained" by previous adaptations, in this case, the standard number of cervical vertebrae in mammals. The fitness disadvantages associated with deviations from this norm illustrate how certain traits can become fixed within lineages, limiting the range of viable evolutionary changes.

Almost All Mammals Have Seven Cervical Vertebrae