Homology Vs Homoplasy
Homology is similarity between trait share from common ancestor, most of the homologous characters are recently derived shared characters.
Homoplasy is the similarity that are most likely superficially similar (identical on the surface but there is no genetic relation) due to independent evolution and cause misleading of the phylogenetic inferences.
The reasons of the homoplasy
- Convergent evolution: evolve similar traits in unrelated organisms due to similar environmental pressure. E.g. – wing of the bats and insects.
- Parallel evolution – two species are evolved at the same time in similar eco-space and grain similar characters.
- Reversal – lost of derived features; e.g. – snakes lost their legs.
Different descriptive statistics have been devised to demonstrate the degree of homoplasy in a cladogram.
- Tree length
- Consistency index (CI)
- Retention index (RI)
Tree length
Trees minimum length is considered to have less homoplasies and these trees considered as the best fit trees.
Consistency Index (CI)
This is a measurement of the amount of the homoplasy on a tree.
It measures the parsimony fit of a character to a tree, i.e data (character) fit the tree under the parsimony principle which means tree that having fewest evolutionary event (trees that having shortest branch length.)
High value of consistency index means low level of homoplasy.
- If CI = 0, Same character change many times on the tree.
- If Cl = 1, One character change only once.
Retention Index (RI)
It measures the amount of homoplasy.
- G – Maximum No. of steps (changes) on the tree.
- S – Observed No. of state changes on the tree.
- M – Minimum No. of state changes on the data.
If higher the RI trees are less homologous
