Added What We Learned and more

- added "most" to recent common ancestor in def of orthologs
- mainworded "gene family"
- reworded last sentence (to acknowledge that it is already defined) in
  p1 of 5.1: "In the context of this lecture, two genes are orthologs if
  their MRCA is a speciation event; two genes are paralogs if their MRCA
  is a duplication event."
- Removed unused headings because they are misleading