Questions: The protostome developmental sequence arose just once in evolutionary history, resulting in two main subgroupsLophotrochozoa and Ecdysozoa. What does this finding suggest? Division of these two groups occurred before the protostome developmental sequence. Division of these two groups occurred after the protostome developmental sequence appeared. The lophotrochozoans are monophyletic. These two subgroups have a common ancestor that exhibited deuterostome development.

The answer is the second one: Division of these two groups occurred after the protostome developmental sequence appeared.

Explanation for each option:

1. Division of these two groups occurred before the protostome developmental sequence.

   • This is incorrect because the protostome developmental sequence is a defining characteristic that arose once and then led to the divergence of the two subgroups. Therefore, the division could not have occurred before this sequence appeared.

2. Division of these two groups occurred after the protostome developmental sequence appeared.

   • This is correct because the protostome developmental sequence arose once and then led to the divergence into the two main subgroups, Lophotrochozoa and Ecdysozoa.

3. The lophotrochozoans are monophyletic.

   • While this statement may be true, it does not directly address the implication of the protostome developmental sequence arising once. The question is about the timing of the division of the two groups relative to the appearance of the protostome developmental sequence.

4. These two subgroups have a common ancestor that exhibited deuterostome development.

   • This is incorrect because the common ancestor of Lophotrochozoa and Ecdysozoa would have exhibited protostome development, not deuterostome development.

Summary: The finding that the protostome developmental sequence arose just once in evolutionary history suggests that the division of the two main subgroups, Lophotrochozoa and Ecdysozoa, occurred after the protostome developmental sequence appeared.