3. Evolution makes sense of homologies 3. Evolution makes sense of homologies
Richard Owen (1848) introduced the And why would homologous term “Homology” to refer to structural structures be inefficient or even similarities among organisms. useless? To Owen, these similarities indicated that organisms were created following For example, certain cave-dwelling fish a common plan or archetype. Although have degenerate eyes that cannot see. each species is unique, the plans for each might share many features. Darwin made sense of homologous structures by supplying an evolutionary explanation for them:
If every organism were created independently, why are there so A structure is similar among related organisms because many homologies among certain organisms, while so few among those organisms have all descended from a common others? ancestor that had an equivalent trait.
3. Evolution makes sense of homologies 3. Evolution makes sense of homologies Example: The "Universal" Genetic Code Example: The "Universal" Genetic Code
5’ T C A G 3’ TTT Phe (F) TCT Ser (S) TAT Tyr (Y) TGT Cys (C) T TTC Phe (F) TCC Ser (S) TAC Tyr (Y) TGC Cys (C) C 5’ T C A G 3’ T TTA Leu (L) TCA Ser (S) TAA Stop TGA Stop [Trp] A TTT Phe (F) TCT Ser (S) TAT Tyr (Y) TGT Cys (C) T TTG Leu (L) TCG Ser (S) TAG Stop TGG Trp (W) G CTT Leu (L) CCT Pro (P) CAT His (H) CGT Arg (R) T CTC Leu (L) CCC Pro (P) CAC His (H) CGC Arg (R) C TTC Phe (F) TCC Ser (S) TAC Tyr (Y) TGC Cys (C) C C Basic code is universal among CTA Leu (L) CCA Pro (P) CAA His (H) CGA Arg (R) A T CTG Leu (L) CCG Pro (P) CAG His (H) CGG Arg (R) G TTA Leu (L) TCA Ser (S) TAA Stop TGA Stop [Trp] A ATT Ile (I) ACT Thr (T) AAT Asn (N) AGT Ser (S) T eukaryotes and prokaryotes. ATC Ile (I) ACC Thr (T) AAC Asn (N) AGC Ser (S) C A TTG Leu (L) TCG Ser (S) TAG Stop TGG Trp (W) G ATA Ile (I) [Met] ACA Thr (T) AAA Lys (K) AGA Ser (S) [Stop] A ATG Met (M) ACG Thr (T) AAG Lys (K) AGG Ser (S) [Stop] G CTT Leu (L) CCT Pro (P) CAT His (H) CGT Arg (R) T GTT Val (V) GCT Ala (A) GAT Asp (D) GGT Gly (G) T GTC Val (V) GCC Ala (A) GAC Asp (D) GGC Gly (G) C G CTC Leu (L) CCC Pro (P) CAC His (H) CGC Arg (R) C GTA Val (V) GCA Ala (A) GAA Glu (E) GGA Gly (G) A C Minor variants are observed GTG Val (V) GCG Ala (A) GAG Glu (E) GGG Gly (G) G CTA Leu (L) CCA Pro (P) CAA His (H) CGA Arg (R) A in mitochondrial genomes CTG Leu (L) CCG Pro (P) CAG His (H) CGG Arg (R) G ATT Ile (I) ACT Thr (T) AAT Asn (N) AGT Ser (S) T and some nuclear ones (e.g. ATC Ile (I) ACC Thr (T) AAC Asn (N) AGC Ser (S) C A Mycoplasma and Tetrahymena). ATA Ile (I) [Met] ACA Thr (T) AAA Lys (K) AGA Ser (S) [Stop] A ATG Met (M) ACG Thr (T) AAG Lys (K) AGG Ser (S) [Stop] G GTT Val (V) GCT Ala (A) GAT Asp (D) GGT Gly (G) T GTC Val (V) GCC Ala (A) GAC Asp (D) GGC Gly (G) C G GTA Val (V) GCA Ala (A) GAA Glu (E) GGA Gly (G) A Of the millions of alternatives, why do all GTG Val (V) GCG Ala (A) GAG Glu (E) GGG Gly (G) G organisms have virtually the same genetic code? [Mammalian mitochondrial code in italics.] 3. Evolution makes sense of homologies 3. Evolution makes sense of homologies Example: The "Universal" Genetic Code Example: The "Universal" Genetic Code
5’ T C A G 3’ 5’ T C A G 3’ TTT Phe (F) TCT Ser (S) TAT Tyr (Y) TGT Cys (C) T TTT Phe (F) TCT Ser (S) TAT Tyr (Y) TGT Cys (C) T TTC Phe (F) TCC Ser (S) TAC Tyr (Y) TGC Cys (C) C TTC Phe (F) TCC Ser (S) TAC Tyr (Y) TGC Cys (C) C T T TTA Leu (L) TCA Ser (S) TAA Stop TGA Stop [Trp] A TTA Leu (L) TCA Ser (S) TAA Stop TGA Stop [Trp] A TTG Leu (L) TCG Ser (S) TAG Stop TGG Trp (W) G TTG Leu (L) TCG Ser (S) TAG Stop TGG Trp (W) G CTT Leu (L) CCT Pro (P) CAT His (H) CGT Arg (R) T CTT Leu (L) CCT Pro (P) CAT His (H) CGT Arg (R) T CTC Leu (L) CCC Pro (P) CAC His (H) CGC Arg (R) C CTC Leu (L) CCC Pro (P) CAC His (H) CGC Arg (R) C C C Because the anti-codon is at the CTA Leu (L) CCA Pro (P) CAA His (H) CGA Arg (R) A Evolutionary descent from a CTA Leu (L) CCA Pro (P) CAA His (H) CGA Arg (R) A CTG Leu (L) CCG Pro (P) CAG His (H) CGG Arg (R) G CTG Leu (L) CCG Pro (P) CAG His (H) CGG Arg (R) G ATT Ile (I) ACT Thr (T) AAT Asn (N) AGT Ser (S) T ATT Ile (I) ACT Thr (T) AAT Asn (N) AGT Ser (S) T opposite end from the amino acid ATC Ile (I) ACC Thr (T) AAC Asn (N) AGC Ser (S) C common ancestor makes sense of ATC Ile (I) ACC Thr (T) AAC Asn (N) AGC Ser (S) C A A ATA Ile (I) [Met] ACA Thr (T) AAA Lys (K) AGA Ser (S) [Stop] A ATA Ile (I) [Met] ACA Thr (T) AAA Lys (K) AGA Ser (S) [Stop] A ATG Met (M) ACG Thr (T) AAG Lys (K) AGG Ser (S) [Stop] G ATG Met (M) ACG Thr (T) AAG Lys (K) AGG Ser (S) [Stop] G binding site of a tRNA and does GTT Val (V) GCT Ala (A) GAT Asp (D) GGT Gly (G) T the similarity among genetic codes. GTT Val (V) GCT Ala (A) GAT Asp (D) GGT Gly (G) T GTC Val (V) GCC Ala (A) GAC Asp (D) GGC Gly (G) C GTC Val (V) GCC Ala (A) GAC Asp (D) GGC Gly (G) C G G GTA Val (V) GCA Ala (A) GAA Glu (E) GGA Gly (G) A GTA Val (V) GCA Ala (A) GAA Glu (E) GGA Gly (G) A not interact with the binding site, GTG Val (V) GCG Ala (A) GAG Glu (E) GGG Gly (G) G GTG Val (V) GCG Ala (A) GAG Glu (E) GGG Gly (G) G there is no chemical necessity for a codon to be assigned to a The common ancestor to all known organisms had a genetic particular amino acid. code similar to what we see today.
Over the ages, the genetic code has passed unchanged (or nearly so) from parents to offspring, because mutations to the The genetic code is homologous among living organisms: it is similar genetic code would have reduced fitness (changing the amino despite the fact that there exist many equally good genetic codes. acid sequence of all proteins produced).
3. Evolution makes sense of homologies 3. Evolution makes sense of homologies Example: The plasma membrane Example: The Pentadactyl limb Frog Lizard Bird “Five” “Digit” All tetrapods (= four legged) have The plasma membranes of all limbs with five digits, at least at organisms, eukaryotic and some stage in development. prokaryotic, are structurally similar, consisting of a phospholipid bi- Certain tetrapods lose some of Human Bat layer. these digits during development, Cow Whale as in the bird wing shown here.
But if the bird wing does not need The similarity of the plasma membrane suggests that all living five digits, why do five initially cells have descended from an ancestor with a similar develop in the growing embryo? membrane structure. 3. Evolution makes sense of homologies 3. Evolution makes sense of homologies Example: The Pentadactyl limb New structures evolve from pre-existing structures Frog Lizard Bird Even when two species function in The five digits are not functionally completely different ways, they necessary, but they represent a often use homologous structures genetic artefact inherited from the to carry out those functions ancestors of birds. (e.g., wings of birds and bats). Human Bat Cow Whale Similarly, the stinger of wasps and bees is a modified ovipositor, not an entirely new structure. Homology explained by descent from (Explaining why only females sting!!) 4HE ORGAN