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Lecture 21: Macroevolution

Lecture 21: Macroevolution. Last class: 1) Peramorphosis: add’n of extra stages a) Hypermorphosis : dev’t extended from  to  1.  1. Descendant Ancestor. . - same allometry (relationship of y to x) - early start of y means greater y (not x) at maturity . log y.  1. .

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Lecture 21: Macroevolution

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  1. Lecture 21: Macroevolution Last class: 1) Peramorphosis: add’n of extra stages a) Hypermorphosis: dev’t extended from  to 1

  2. 1 Descendant Ancestor  - same allometry (relationship of y to x) - early start of y means greater y (not x) at maturity log y 1  log x b) Predisplacement: y starts growing early rel. to x in descendent vs. ancestor

  3. c) Acceleration • faster growth of y rel. to x in descendent vs. ancestor Descendant Ancestor 1  log y Larger (or more dev’d) y (not x) at maturity  log x

  4. 2) Paedomorphosis • retention of juvenile features in adult A) Progenesis B) Neoteny C) Postdisplacement

  5. Ancestor Descendant  1 Smaller y, smaller x at maturity vs. ancestor - Allometry unchanged - Compare: hypermorphosis log y  log x a) Progenesis • dev’t stops early

  6. Ancestor Descendant  1 log y - Smaller or less developed y rel. to x at maturity  log x b) Neoteny • slower rate of growth of y rel. to x in descendant vs ancestor

  7. c) Postdisplacement • y starts growing late rel. to x in descendant vs. ancestor Ancestor Descendant  1 log y - same allometry - late start of y means smaller y (not x) at maturity  1 log x

  8. Evolutionary Significance of Heterochrony? 1. Large changes in phenotypes easily accomplished - mutations at one or several loci may be involved 2. Likely important in speciation • gene pools w diff. heterochronic mutations  repro. isol’n

  9. 3. May release lineages from phylogenetic constraints - e.g. paedomorphosis: descendant no longer passes through the same develop’l stages as ancestor - can “free” the sp. from the constraint imposed by that structure - only affects existing structures.

  10. Genetic Basis of Heterochrony Homeotic (Hox) genes: • 1st discovered in Drosophila spp. • involved in gross alterationsin phenotype • Affect develop’t of cuticular structures from imaginal disks • in allanimal phyla • share # of common characteristics • e.g. antennapedia

  11. Hox Genes 1. organized in gene complexes - probably involves gene duplication 2. spatial &temporal collinearity: - 3' end expressed anterior; 5' end expressed posterior - 3' end expressed earlier in dev’t than 5' end

  12. Hox Genes cont’d 3. contain highly-conserved 180 bp region - involved in binding Hox genes are regulators - control timing and expression of other genes e.g. Ubx (ultrabithorax) in Drosophila: controls expression of 85 - 170 genes

  13. Type of Heterochronic Process? Axolotl vs. Tiger Salamander • failure to metamorphose • [thyroxine] : can be exp’tally induced • external gills in adult (juvenile morphology)

  14. So what’s going on? • not postdisplacement : age at maturity ≈ other salamanders • not progenesis : body size at maturity ≈ other salamanders (progenesis tiny adult) • Neoteny: somatic dev’t slows & is overtaken by normal sexual maturity giant juvenile

  15. D’Arcy Thompson • early 20th century • comparative anatomist • “On Growth & Form”: transformation grids: explain changes in shape & determine allometric growth • measurements made & plotted on rectangular coordinates • same measurements made in a related organism or a different stage in dev’t • shown as deformations of grid system • now : partial warp analysis

  16. Hatchetfish Wrasse & Angelfish Skulls of Human, Chimp & Baboon

  17. Evolution of Higher Taxa (Gould) • new groups often arise from neotenic or progenetic ancestors • e.g. flightless birds • e.g. insects: from larval form of millipede-like ancestor? • e.g. chordates larval cond’n of tunicates?

  18. Saltationists • distinctive features of higher taxa arise through “systemic mutation” (complete reorganization) • Argument: - few intermediates among higher taxa - little selective advantage to incipient structures - results in dramatic, discontinuous effects

  19. Neodarwinists Counter-argument: - characters of higher taxa evolve mosaically - many intermediate forms e.g. Archaeopteryx, Lepidoptera - early stages of complex structures selectively advantageous - mutations with disruptive pleiotropic effects usuallyfatal (no change in rate)

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