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Understanding Homology vs. Analogy in Evolutionary Biology

Explore the concepts of homology and analogy in biological classification, distinguishing between shared traits from common ancestors versus convergent evolution. Learn about the methods of phenetics and phylogenetics to decode evolutionary relationships and build phylogenetic trees.

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Understanding Homology vs. Analogy in Evolutionary Biology

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  1. Homology: a character shared between two or more species that was present in their common ancestor. Analogy: “shared” character not present in common ancestory (I.e., the character evolved twice, independently): convergent evolution.

  2. Classification of Biological Diversity: • Phenetics • Phylogenetics (Cladistics)

  3. PHENETICS • Classic approach (Linnaeus) • Group species based upon phenotypic similarity • Use ALL traits

  4. PHYLOGENETICS • New (Hennig, 1966) • Based on patterns of ancestry (species are groups based on most recent ancestory) • Uses only shared, derived characters (i.e., derived homologues) • Therefore, it aims to reval actual evolutionary history (the “real” phylogeny; “the tree”)

  5. “The Actual Events” Horse Beetle Butterfly

  6. Horse Horse Beetle Beetle Butterfly Butterfly Phenetics Phylogenetics

  7. “The Actual Events” Dragonfly Lizard Bird

  8. Phenetics Phylogenetics Dragonfly Lizard Bird Lizard Dragonfly Bird

  9. Equivalent Trees

  10. Characters Used

  11. Species: Character State: 1 2 3 4 Ancestral state Analogy In Sp. 1 and 3 is an analogy; ‘red’ arose independently twice

  12. Species: Character State: 1 2 3 4 Ancestral state Ancestral Homology In Sp. 1, 2 and 3 is an ancestral homology; It’s ancestral because it’s present at the base of the tree.

  13. Species: Character State: 1 2 3 4 Ancestral state Derived Homology is homologous in Sp. 1 and 2 and it’s derived (i.e., there has been a transition from the “basal” state).

  14. Building a “tree” (aka phylogeny or cladogram) • Obtain data (shared derived characters)

  15. Building a “tree” (aka phylogeny or cladogram) • Obtain data (shared derived characters) • Determine ancestral state (“0”)

  16. Building a “tree” (aka phylogeny or cladogram) • Obtain data (shared derived characters • Determine ancestral state (“0”) • Assign derived states (“1”, “2”, “3”), which defines the character matrix

  17. Building a “tree” (aka phylogeny or cladogram) • Obtain data (shared derived characters) • Determine ancestral state (“0”) • Assign derived states (“1”, “2”, “3”), which defines the character matrix • Build trees (many are possible)

  18. Building a “tree” (aka phylogeny or cladogram) • Obtain data (shared derived characters) • Determine ancestral state (“0”) • Assign derived states (“1”, “2”, “3”), which defines the character matrix • Build trees (many are possible) • Pick the “best” (i.e., most parsimonious) -- fewest evolutionary steps (fewest convergences and reversals)

  19. Convergence: independent origin of trait (i.e., two lineages independently evolve similar phenotype).Reversal: origin and subsequent loss of character (i.e., reversion to ancestral state).

  20. Sunfishes

  21. Sunfish Character Matrix

  22. LMB BC WM BG RE b-1 a-1

  23. LMB BC WM BG RE d-1 c-1 b-1 a-1

  24. LMB BC WM BG RE g-1 f-1 e-1 d-1 c-1 b-1 a-1

  25. 7 Steps LMB BC WM BG RE g-1 f-1 e-1 Note: characters f-1 and g-1 are not shared: i.e., they are non-informative d-1 c-1 b-1 a-1

  26. LMB WM RE BC BG f-1 c-0, d-0, e-0, g-0 g-1 e-1 d-1 c-1 b-1 a-1 Another possible tree: 11 Steps (4 reversals)

  27. X X X X X O Y Z

  28. Y X X X X Z O X

  29. O X Y Z b-1 a-1

  30. O X Y Z d-1 c-1 b-1 a-1

  31. O X Y Z e-1 f-1 e-1 g-1 f-1 g-1 d-1 c-1 b-1 a-1 10 steps (3 convergences)

  32. An Alternative Tree

  33. O Z X Y g-1 f-1 e-1 b-1 a-1

  34. O Z Y X d-1 d-1 c-1 c-1 g-1 f-1 e-1 b-1 a-1 9 steps (2 convergences)

  35. O Z Y X Four of the possible trees d-1 d-1 c-1 c-1 g-1 f-1 e-1 b-1 9 steps (2 convergences) a-1 O X Y Z e-1 O X Z Y O Z X Y f-1 e-1 e-0 d-0 g-1 f-1 f-0 c-0 g-1 g-0 g-1 d-1 d-1 f-1 c-1 c-1 e-1 d-1 g-1 c-1 b-1 f-1 b-1 e-1 9 steps (2 reversals) 10 steps (3 reversals) a-1 b-1 a-1 a-1 10 steps (3 convergences)

  36. Fitting Linnaean systematics into phylogenetics C1 Class: O1 O1 Order: F1 F2 F3 F4 Family: G1 G2 G3 G4 G5 G6 Genus: A B C D E F G H I J Species:

  37. A monophyletic group:is a group of taxa that includes all descendants of the most recent common ancestor of the group members

  38. An example of a non-monophyletic group: Reptiles Lizard Snake Croc. Fish Amphib. Turtle Mammal Bird The most recent common ancestor of reptiles also gave rise to mammals and birds

  39. An example of a non-monophyletic group: Fish Sturgeon X Sharks + Rays Coelacanth Teleosts lungfish X = all other vertebrates (e.g., mammals, birds, amphibians, reptiles)!

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