This document discusses molecular evolution and phylogenetic relationships as seen through molecular homology and comparisons of DNA, RNA, and protein sequences. It defines key terms like homology, orthologs, and paralogs. Orthologs are homologous sequences that result from speciation events, retaining similar functions in different species. Paralogs arise from gene duplication events and may develop new functions. Comparing sequences allows scientists to construct phylogenetic trees and study how genes and genomes have evolved over time.

1. EVOLUTION
Topic:: Phylogenetic relationship-Homology,Homologous
sequence of protein and DNA, Orthologous & Paralogous
MERIN TESS ZACHARIAS
1ST MSC ZOOLOGY
NIRMALA COLLEGE
MUVATTUPUZHA
ROLL NO:4861

2. CONTENT
● MOLECULAR EVOLUTION
● PHYLOGENETIC RELATIONSHIP
● MOLECULAR HOMOLOGY
● HOMOLOGOUS
● ORTHOLOGOUS
● PARALOGOUS

3. MOLECULAR EVOLUTION
● It is the area of evolutionary biology that studies evolutionary changes
at molecular level
● Molecular evolution is the process of change in the sequence
composition of cellular molecules such as DNA, RNA & Proteins across
generation.
● It includes the study of rates of sequence change ,relative importance
of adaptive and neutral changes & change in genome structure

4. ● DNA sequence comparison can show how different species
are related
● Eg:: The amino acid sequence of cytochrome c in humans and
chimpanzee is identical ,although they diverged 6 million years
ago
- Between human and rhesus monkeys,which diverged from
their common ancestor 35 million to 40 million years ago ,it
differ by only one amino acid replacement.

5. PHYLOGENETIC RELATIONSHIP
● Phylogeny means the evolutionary
history of organisms.
● Phylogenetic relationship are the
relationships that show how far back
two species shared a common
ancestor.
● Phylogenetics is important bcoz it
enriches our understanding of how
genes,genomes ,species & molecular
sequences evolve.

6. Amino acid sequences in individual proteins can be informative about
the evolution of individual gene
● In order to overcome the phylogenetic problem, now a days
we use molecular information rather than placing exclusive
reliance on morphological characters

7. ● On molecular level ,we can obtain information by comparing
sequences of nucleotides in various DNA & RNA molecules, as well
as by comparing sequences of amino acids in different Proteins.
● Molecular comparisons transcend barriers among organism whose
relationships cannot be evaluated by traditional experimental
techniques
● This close tie to the molecular Biological has transformed evolution
from a “ theoretical explanation of historical events to an observable
& continues link among life forms”

8. MOLECULAR HOMOLOGY
● HOMOLOGY is the characteristic shared by two species that is similar
because of common ancestor.
● Molecular homology -species placed in the same taxonomic category
show similarities in DNA,RNA & Proteins.
● The molecules that carry the instructions for constructing and
running the living organisms, ie DNA & RNA shows homology across
species

9. ● By comparing the differences between sequences of same protein in
different species, evolutionary biologist can determine when the two
species split off from their common ancestor & began to evolve
independently
● The more recently two species descended from a common ancestor,
the more similar their DNA sequences.
● DNA encodes instructions for every living thing on earth .This itself
is powerful evidence of common ancestry.

10. *In molecular Biology, it is mainly genes & Proteins that are
homologized.
*Evolution oriented molecular biologist working for instance in
molecular evolution or molecular phylogeny, view the concept of
molecular Homology is derived parallel to the concept of Homology in
morphological structures.
*Their focus is on how genes evolve and how they are related.
*Genes are homologous in case they are derived from an ancestral
gene.

11. Eg: Humans and
chimpanzees share a
similarity in the
sequence of amino
acid in hemoglobin.

12. Hemoglobin comparison between humans and other vertebrates

13. HOMOLOGY
● It forms the basis of organization for comparative biology
● In genetics the term ‘homolog’ is used to refer both
homologous protein and to the gene encoding it.
● As with anatomical structures ,homolog between protein or
DNA sequences is defined in terms of shared ancestry
● Two segments of DNA can have shared ancestry because of
either a speciation event( orthologs)or duplication
event(paralogs)

14. Sequence Homology
● Sequence Homology is the sequential arrangement of
bases in DNA and RNA and amino acid in case of Proteins
● Homologs are two or more sequences that descent from a
common ancestor
● They are the results of divergent evolution

15. HOMOLOGS
ORTHOLOGS
XENOLOGS
PARALOGS
RESULT OF SPECIATION
EVENT
RESULT OF GENE
DUPLICATION EVENT
RESULT OF HORIZONTAL GENE
TRANSFER

16. ORTHOLOGS
● They are sequences which are the results of speciation
event
● Similar and identical sequences in different species
● Uses::
*construction of phylogenetic tree
*comparison of evolution of genes
*Give information about classification of organisms

17. PARALOGS
● They are sequences that arose due to gene duplication
event
● Similar sequences within same species
● Two types :::
1 ] In paralog -Genes that have duplicated after a
speciation event
2 ]Out paralog-Genes that have duplicated before
speciation event

18. Paralog sequences are important
- Help in the study of Protein evolution
- Provide useful insight into the way genome
evolve.

19. ORTHOLOGS PARALOGS

20. Homology:: Orthologs & paralogs

21. GENE PHYLOGENY AS RED AND
BLUE BRANCHES WITHIN GREY
SPECIES PHYLOGENY
TOP:: Ancestral gene duplication
produces two paralogs
A speciation event produces orthologs
in the two daughter species (human &
chimpanzees)
BOTTOM :: in a separate species
(E.coli) a gene has a similar function
but has a separate evolutionary orgin
and so is analog

22. THANK U 😊