25. Phylogeny

Which of the following statements about homologs are true?

I) Phylogenetic trees of different taxa can be made using orthologs but not paralogs.

II) A gene family is a group of paralogs within a genome.

III) Orthologs are homologs that are found in different species.

The different genes that code for the photoreceptive proteins that function in the vertebrate eye are all believed to be paralogs. Which statement below is most consistent with this idea? 

What is one challenge to trying to date a speciation event using a molecular clock?