Question

Chordates are characterized by having a dorsal hollow nerve chord, a notochord, and a tail. Some years ago, researchers discovered that in Tunicates, a single mutation in one gene caused the nerve chord, the notochord, and the tail to all disappear. A writer for Science magazine concluded from this that all three definitive chordate traits must have evolved at the same time, with the appearance of this gene. Explain the flaw in this reasoning.

Answer 1

Tunicates or Urochordates in their larval stage possess a dorsal hollow nerve chord, a notochord, and a tail but adults or chordates do not possess any such characteristics. If a single mutation is thought to be responsible for the dissaperance of all three definitive chordate traits, it is obvious that this mutation arose earlier than the evolution of the traits. This is because in order to completely evolve into a different clade, species undergo a lot of selection pressure. A single mutation would most likely change the protein sequence, however for a species to evolve into Tunicates these traits must be selected for by selection pressure. Thus, mutation in one of the chordates over time has been selected to be passed on to the subsequent generations allowing the species to evolve. It does not happen at the time of evolution.