8. Phylogenetic tree construction has been used to identify the major branches o

Question

8. Phylogenetic tree construction has been used to identify the major branches of the olfactory receptor (OR) gene family. Each of these branches has been assigned a Greek letter – including the , , , , , and branches. Briefly discuss the reasons for believing that each of these branches originated in a single gene duplication and divergence event, from which all subsequent genes of that branch were descended. [Hint: your answer should include phylogenetic tree construction methods such as neighbor joining, and molecular gene duplication mechanisms, and molecular clocks.]

9. With respect to your answer in question #8 above, species comparisons indicate that the and branches expanded specifically in tetrapods, while the older , , , and branches expanded primarily in fish and have been nearly lost from most tetrapod genomes (except amphibians). Why did this happen? Was this caused by: (i) natural selection, (ii) mutation bias, (iii) both, or (iv) neither? Why?

Explanation / Answer

8. Phylogenetic tree is used to find out the evolutionary relationship between among the group of organisms. Method of phylogenetic tree creation can be broadly divided into two groups: (i) Distance based method (ii) Character based method.

Distance based method include neighbour joining and UPGMA whereas character based method include maximum parsimony and maximum parsimony.

Branch in phylogenetic tree represent how different species are originated from a common ancestor. In our question, five gene family have been evolved from a single gene family and represented by , , , , , and . Gene duplication is the most important mechanism for the evolution of new species, gene family. We can assume that, each of these branches originated in a single gene duplication because molecular clock are used by evolutionary biologist to determine the divergence of the species. Molecular clock is useful for predicting the common ancestor of the gene family and in our case, the commons ancestor is same using the molecular data like DNA. In this case, either neighbour joining or UPGMA method has been used as these are distance based. Hence, we can say that five gene family have been evolved from a single gene family.

9. The answer is (iii) option as the branch length is more in tetrapods means more evolution have been occurred in tetrapods and this evolution might happen either due to natural selection or due to mutation. Since, branch length is proportional to the amount of evolution have been occurred.

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