Thus the latest evolutionary change appears postpartum. The fetus may have characteristics, like the gill slits in mammals, that are entirely maladaptive after birth, but as long as they cause no serious problems for the fetus and are lost before birth, they can be retained. Our gill slits are vestiges not of ancient fish but of ancient fish embryos. Many new organ systems develop not by the addition and preservation but by the modification of older systems, as, for example, the modification of fins to legs, and legs to flippers or wings; or feet to hands to feet; or sebaceous glands to
...more
